Prepared by RDH Building Science Inc. for FortisBC and BC Hydro

February 2017

Window Energy Performance

A Reference Guide for Energy Advisors

11233_000 2017 03 23 AJ Reference Guide for Energy Advisors FINAL.docm

Contents

Introduction 1

Terminology Used in this Guide 1

1 Windows Have Come a Long Way 2

1.1 The Worst Performing Elements of the Building Enclosure 2

1.2 Today’s Windows 2

1.3 Energy Savings vs. Comfort 3

2 Anatomy of a Modern Window 5

2.1 Frame Materials 5

2.2 Glass Technology 6

3 Window Energy Performance Properties 12

3.1 Mechanism of Heat Transfer 12

3.2 U-Value: the Rate of Heat Loss 14

3.3 Solar Heat Gain Coefficient (SHGC) 14

3.4 Visible Transmittance (VT) 15

3.5 Air Leakage 16

3.6 Energy Rating (ER) 17

4 Today’s Window Performance Levels 18

5 Windows and HOT2000 21

5.1 Example: Fixed Window for BC Climate Zones 4 & 5 24

6 Windows and Comfort Considerations 25

6.1 Windows and Cold Window Surface Discomfort 25

6.2 Windows and Overheating Discomfort 25

6.3 Overheating Discomfort in ESNH Homes 25

7 Window Energy Standards, Labeling and Certification 27

7.1 Energy Performance Rating Standards 27

7.2 U-Values Determined Under Standard Conditions 27

7.3 Energy Performance Labeling 29

Page 1

Introduction

Energy Advisors are expected to have a sound, working understanding of the energy

performance characteristics of buildings and building materials, including those of

windows, doors and skylights. The energy performance characteristics of these

fenestration products are not well understood in the construction industry, even though

they play a major role in affecting both the energy use of buildings and the comfort of

building occupants.

This guide provides an overview of these energy performance issues to help builders,

building designers, and energy advisors to confidently discuss fenestration energy

performance with their clients and to make more effective product choices.

This reference guide was developed by RDH Building Science Inc. for Fortis BC and BC

Hydro to complement the information presented in a recorded Webinar titled “Window

Energy Performance for Energy Advisors”, originally presented on February 1, 2017.

Terminology Used in this Guide

In this document, the term U-value has the same meaning as U-factor, without reference

to a specific system of measurements. U-value is often represented in the following

abbreviated forms:

USI represents the U-value in Canadian “système intenationale” metric units of W/m2·K.

RSI represents the metric R-value, the inverse of USI.

UIP represents the U-value in Imperial/inch-pound units of Btu/h·ft2·F.

RIP or R-value represents the inverse of UIP.

Page 2

1 Windows Have Come a Long Way

1.1 The Worst Performing Elements of the Building Enclosure

It is often said that from an energy performance viewpoint, “windows are the worst

performing elements of the building enclosure”. The relatively poor performance of

windows, compared to walls, for example, is often demonstrated with infrared

thermography (Figure 1.1). While a conventional 2x4 stud wall with nominal R-12

insulation may have an effective insulating value of R-7, considerably lower than that of

the 2x6 R-15 to R-22-effective walls required under current building codes, a 1960s

vintage single pane aluminum window in that same house has an insulating value of

approximately R-1. Upgrading that single pane window to dual pane only increases its

insulating value to approximately R-2.

Figure 1.1 Windows still account for most of the winter heat loss in a Canadian home. Infrared thermographs show the relative differences in temperature between elements of the building enclosure

1.2 Today’s Windows

Today’s code-compliant vinyl windows have R-values in the range of R-3. While they

represent a 300% improvement over those1960s single pane windows, they still have only

1/5th the thermal resistance of the R-15 walls they are installed in. For this reason, it is not

surprising that windows account for as much as 40–80% of winter heat loss through the

enclosure of a single family home.

Windows in the Canadian ENERGY STAR® “Most Advanced” category have R-values in the

range of R-5 (UIP 0.20, USI 1.14), and represent the highest level of performance generally

available from Canadian window manufacturers. Imported Passive House certified

windows can reach state-of-the-art performance levels in the range of R-7 (UIP 0.14, USI

0.8) or higher.

Page 3

TABLE 1.1 TYPICAL WINDOW U-VALUES

Window type RIP-value USI-value (metric)

Single pane R-1 ±5.7 1

Dual pane, air filled R-2 ±3.3 2

2012 BCBC R-3 ≤ 1.80

ENERGY STAR zone A R-4 ≤ 1.60

ENERGY STAR “Most Efficient” R-5 ≤ 1.14

Passive House – (central European climate) R-7 ≤ 0.80

1 Whole Building Design Guide https://www.wbdg.org/resources/windows-and-glazing, UIP 1.3

2 Review of Standard Operating Conditions for HOT2000, Final Report, by Innes Hood Consulting

Inc., p. 11. Window USI 3.3 (RSI 0.30) derived from the EnerGuide for Homes Database for

existing homes.

So yes, windows have come a long way and are still getting better. The main barrier to

greater adoption of windows with performance better than R-3 is cost: at this time, there

is relatively little demand for R-5 products, and even less for R-7.

This will likely change as building codes become more stringent, and the home building

industry begins to supply more Carbon Neutral, Net Zero, and Passive House buildings.

1.3 Energy Savings vs. Comfort

Energy Savings

Using lower U-value windows does result in energy savings, though the savings are more

pronounced in colder climate zones. In Southern BC, consumers are not inclined to see

those savings as compelling from a cost payback viewpoint.

Figure 1.2 Predicted Energy Savings from use of lower U-value windows; USI -3.3 corresponds to an R-2 dual pane clear glass window.

-

10,000

20,000

30,000

40,000

50,000

Tota

l Ann

ual C

onsu

mpt

ion

(kW

h/yr

)

Electric Heating

Ex. U-3.3 U-1.8 U-1.4 U-1.0

- 10,000 20,000 30,000 40,000 50,000 60,000 70,000

Tota

l Ann

ual C

onsu

mpt

ion

(kW

h/yr

)

Natural Gas Heating

Ex. U-3.3 U-1.8 U-1.4 U-1.0

Page 4

A recent fenestration market study examined the potential energy savings for upgrading

older, existing windows in a 1980s-vintage home in three BC locations, Vancouver

(climate zone 4), Kamloops (climate zone 5), and Prince George (climate zone 6). The

study compared the relative energy savings for upgrades from windows common in

existing homes (RIP-2, USI-3.3), to current code compliant windows in BCBC climate zones

4 and 5 ( RIP-3, USI-1.80), and even better performing options. The results for homes

heated with electricity and homes heated with natural gas are presented in Figure 1.2.

Figure 1.3 Replacing old, single pane windows can greatly improve occupant comfort, and reduce heating energy use.

Comfort

Window manufacturers have found that “it is easier to sell comfort that energy efficiency”.

The tangible benefits that consumers can relate to, such as improvement in comfort or

reduction of outdoor noise, are more compelling, and more likely to influence a decision

to upgrade windows than reduced energy or lower heating bills.

Page 5

2 Anatomy of a Modern Window

2.1 Frame Materials

Most modern windows have frames made from low-conductivity materials such as rigid

PVC (vinyl), wood, and fiberglass. The energy performance of highly conductive aluminum

window frames is somewhat improved with the addition of polymer “thermal break” to

reduce heat flow through the frames. (Figure 2.1)

In addition to the use of low-conductivity frame materials, the synthetic “hollow” frame

designs have evolved to contain insulation, or be subdivided into multiple air-filled

insulating compartments. The conductivities of vinyl and fiberglass are essentially

identical, and the most energy efficient windows manufactured in Canada have either vinyl

or fiberglass frames.

Wood is also used for window frames, but to a much lesser extent. Wood has conductivity

similar to that of vinyl and fiberglass, but because wood windows are solid, they have

typically have higher U-values than vinyl or fiberglass windows. Wood windows are more

vulnerable to damage from moisture and temperature changes, generally require more

maintenance, and are typically more expensive than alternative materials. Wood finishes

are highly prized however, especially on the interior surfaces of windows, and wood

windows are often preferred when price or energy performance are not the main

purchasing drivers.

Aluminum window frames have all but disappeared from the residential window market,

but retain the dominant share of windows in large buildings, where very large windows

often demand stronger frames, and energy performance expectations are lower.

Page 6

Figure 2.1 Example window frames: L-R vinyl, fiberglass, wood, aluminum

2.2 Glass Technology

While frame designs have evolved to become more energy efficient, the greatest advances

in the energy performance of windows have come from innovations in glass technology:

multiple-pane insulating glass units with low-emissivity coatings, separated with low-

conductance spacers, and filled with low-conductance gas between the panes. Today’s

modern window has glass that is more effective in reducing heat loss than the window

frame itself.

Figure 2.2 identifies the components within an insulating glass unit (IGU).

Figure 2.2 Typical components of an insulating glass unit

IGU Components:

1 - 6 Glazing surfaces

7 Low-e coating

8 Edge spacer

9 Desiccant

10 Primary edge seal

11 Secondary seal

Page 7

2.2.1 Low-Emissivity (Low-e) Coatings

The development of low-e coatings is the most significant innovation affecting the energy

performance of glass used in windows.

A low-e coating is a transparent coating applied to glass that is designed to reflect

selected portions of the electromagnetic radiation spectrum emitted from the sun.

Depending on the intended use of the glass, low-e coatings can be optimized to minimize

or maximize solar heat gain, to reduce winter heat loss from building interiors, and to

preserve as much of the daylighting potential of natural sunlight as possible. Low-solar

heat gain coatings attempt to filter all but the visible light portion of the solar spectrum.

High-solar gain coatings admit a broader band of radiation that includes the solar infrared

range (Figure 2.3).

Figure 2.3 Transmittance ranges for low solar and high solar low-e coatings. (Source: NFRC THERM7/WINDOW7 Manual)

Low-e Coatings and Winter U-Value

To reduce heat loss from building interiors, low-e coatings are designed to reflect long-

wave infrared radiation—radiant heat—emitted from heated interior surfaces and objects.

This heat-reflective property acts in both directions, whether the long-wave radiation

comes from outdoors, or from indoor sources.

The effectiveness of the heat-reflective property of the coating is expressed with a term

called emissivity, which represents the proportion of incident long-wave radiation (heat)

that is not reflected, but is transmitted through the coating. The greater the heat

reflectivity of the coating, the less long-wave radiant energy can pass through it, and the

lower its emissivity.

Emissivity is expressed as a decimal fraction between zero and one, and represents the

proportion of heat that is transmitted through the coating to be emitted from its other

side.

Page 8

Figure 2.4 Emissivity of low-e coatings illustrated, as compared to that of uncoated glass

Uncoated glass has an emissivity of 0.84, which means it is only 16% effective in blocking

heat. A low-e coating with an emissivity of 0.20 means 80% of radiant heat is reflected

and only 20% can "escape" (be emitted from its other side). The most effective low-e

coatings have emissivities as low as 0.02, which means they reflect 98% of radiant heat,

and emit only 2% (Figure 2.4).

Low-e Coatings and Solar Heat Gain Coefficient (SHGC)

Low-e coatings can also be optimized to reflect, and in that way to reduce the

transmission of higher frequency solar radiation that contributes to overheating in

buildings. Low-e coatings can be applied to tinted glass to further reduce solar heat gain.

Low-e Coatings and Visible Light Transmission (VLT)

Low-e coatings affect the amount of visible light transmitted through coated glass, and

can affects its perceived colour tint when glass is viewed from outdoors, at an oblique

Page 9

angle. In general, the more heat-reflective the coating, the less visible light it transmits.

Visible transmittance is discussed in section 3.4.

While the coatings are designed to be transparent, when applied to clear glass they may

exhibit a neutral darkening, or a colour tint when viewed at an oblique angle from the

outdoor side. Depending on the type and number of coatings within the glass unit, the

visual effect may resemble tinted glass. (Figure 2.5)

Figure 2.5 This building features a curtainwall with clear triple-pane glass and two low-e coatings

Page 10

Low-e Coating Placement

Low-e coatings can be located on different glass surfaces within an IGU. Typical coating

positions are illustrated in Figure 2.6.

Figure 2.6 Typical low-e coating positions within IGUs

2.2.2 Low Conductance Gas Fills

Heat loss through windows can be further reduced by filling the gap between the panes of

insulating glass units with gases having a density greater than that of air. The heavier gas

reduces heat loss through the gap by slowing the rate at which heat is transferred from

the warm pane to the cold pane by convection.

The most commonly used gas for this purpose is argon. Xenon and krypton are more

effective, but are more expensive and therefore less widely used. These gases are often

termed inert gases, and are used to prevent chemical reactions that could occur between

the various components and sealants within IGUs . Such reactions are undesirable as they

could lead to visible obstructions or premature failure of the units.

Figure 2.7 Convective heat loss within insulating glass units. The optimal gap width to minimize heat loss is narrower for Krypton than for Argon or air.

Page 11

2.2.3 Warm Edge Spacer Systems

For many years, aluminum was the predominant material used for the spacer assemblies

that sealed the edges and controlled the thickness of the gap between the panes of

insulating glass units.

Aluminum was an inexpensive and easy material to work with but is also highly

conductive, and rapidly transmits heat from one pane to the next. To improve the energy

performance of windows, it became necessary to explore ways to reduce heat transfer

through spacer systems. Today there are a variety of spacer materials and innovative

design approaches used to provide better performing spacer systems.

The term warm-edge spacer is not precisely defined. It is often assumed to describe

spacers that have no metal, but is commonly applied to any spacer assembly that

conducts significantly less heat than conventional tubular aluminum spacers. For

example, stainless steel spacers with profiles designed to minimize heat transfer perform

almost as well as non-metal spacers.

Figure 2.8 Examples of “warm edge” spacers. Metal warm-edge spacers typically use stainless steel which is much less conductive than aluminum.

Page 12

3 Window Energy Performance Properties

In Canada, we use far more energy to heat our homes than to cool them. It is not

surprising then, when it comes to windows and doors, the energy performance focus is

mainly on keeping the heat in, and also on capturing some free heat from the sun.

But windows and doors don’t just affect a home’s energy performance—they can have a

big impact on how comfortable we feel, not just in winter but throughout the year. The

energy performance properties of windows can be used to make a home more

comfortable, as well as more energy efficient.

There are several ways windows and other fenestration products affect the energy

performance of a home. Windows and glass doors can:

Reduce how much heat is lost through them in winter

Capture some of the sun’s heat to reduce the need for winter space heating

Windows and glass doors can also make homes more

comfortable when they:

Block excess, unwanted heat from the sun to reduce

overheating on West, East and South facing

elevations

Have warmer glass and frame surface temperatures

in winter

Protect us from cold air drafts

How can we tell which windows will be more effective in

reducing our winter heating bills, our greenhouse gas

emissions, and in making us more comfortable? By

learning about the three key performance properties,

and how they interact:

U-value

Solar Heat Gain Coefficient

Air leakage

In addition to these three, there is a fourth energy

performance property used in Canada: the Energy

Rating.

3.1 Mechanisms of Heat Transfer

There are three principal mechanisms of heat transfer through materials, components and

assemblies which make up windows: conduction, radiation, and convection. These three

mechanisms are illustrated in Figure 3.2.

Figure 3.1 Two-dimensional simulation software can illustrate the variation in temperatures through a window frame and glass. In coloured isotherm views, red denotes the warmest temperature.

Page 13

Conduction: The transfer of energy at the molecular level through a material, and

between materials that are in direct contact. The inverse of a material’s conductance

is its thermal resistance, expressed as R-value.

In windows, heat transfer by conduction is reduced by using frames that have

lower conductivity, such as plastics and wood, or by adding low-conductance

plastic components between the indoor and outdoor surfaces of aluminum

window frames.

In insulating glass units, conduction through spacers is reduced by using non-

metal spacers or low conductance metals such as stainless steel.

Radiation: The transfer of energy in the form of electromagnetic waves through the

atmosphere, a gas, or a vacuum. Radiation emitted from a hot surface requires a clear

“line of sight” to reach a cold surface.

In insulating glass units, radiation through glass surfaces is reduced by applying

low-emissivity coatings to them.

Convection: The transfer of energy by the movement of a fluid such as air. When

optimizing window designs, measures are taken to reduce convection within the

cavities of hollow framing members and the cavities between glass panes.

In windows, convective heat loss is

reduced by subdividing large cavities in

hollow vinyl frames into small

compartments, or by filling large cavities

in hollow window frames with insulation.

In insulating glass units, convective heat

loss is reduced by using gases heavier

than air in the gap between the panes.

Figure 3.2 Mechanisms of heat transfer in windows in winter: condcution through solid materials, convection within cavities, radiation through glass

Page 14

3.2 U-Value: the Rate of Heat Loss

The U-value (or U-factor) is a measure of the overall rate at which heat escapes through a

window, door or skylight under standardized winter conditions, and includes the

combined effects of conduction, radiation, and convection.

Heat travels at different rates through the frame, the edge of glass, and through the

centre of the glass. The U-value represents the overall rate of heat transfer through all of

these components (Figure 3.3)

The U-value is expressed in either metric units (W/m²·K) or inch-pound units (Btu/h·ft2·°F).

The U-value is called U-factor in American NFRC standards.

Lower is better. Lower U-values are always desirable as they represent a lower rate of

heat loss through the window, door or skylight.

Figure 3.3 Window U-values include heat loss through the frame, the edge of glass region, and the centre of glass. The rate of heat loss differs for each of these regions.

3.3 Solar Heat Gain Coefficient (SHGC)

The Solar Heat Gain Coefficient represents the proportion of incident solar radiation

transferred through a window, door or skylight product, expressed as a decimal fraction

between 0.00 (totally opaque) and 1.00 (a hole in the wall).

Imagine a wall in which the window has not yet been installed. Through the window rough

opening, the sunlight shines onto the floor, and onto the window leaning against an

adjacent wall. The SHGC of the hole through which the sun is shining is 1.0. The SHGC of

the opaque wall surrounding this opening is zero. When the window is installed, it will

likely have a SHGC of somewhere between 0.6 and 0.2, transmitting between 60% and

Page 15

20% of the solar heat gain that would have entered the home when there was still a hole

in the wall.

The SHGC analysis is quite sophisticated, as it includes the solar heat gained through the

window frame as well as through the glass.

Solar heat gain has implications throughout the year. In winter, some solar heat gain is

desirable to reduce heating energy consumption. In other seasons, and especially for east,

south and west facing windows, too much solar heat gain can result in overheated rooms

that are uncomfortable for the occupants.

“The merits of increasing or reducing the SHGC depend on building specific design

parameters. In a suitably designed building, fenestration with high solar heat gain can

reduce the need for heating in winter. However, in many buildings, it is also beneficial

to reduce solar heat gain in summer for comfort or to reduce the need for cooling.

This balance between summer and winter priorities requires consideration of the

entire building design, including building orientation, exterior shading, window area

and glazing properties.”1

3.4 Visible Transmittance (VT)

Visible transmittance (VT), also called visible light transmittance (VLT) and sometimes

abbreviated as TVIS, refers to the proportion of visible light that is transmitted through a

window, door or skylight product, expressed as a decimal fraction between 0.00 (totally

opaque) and 1.00 (a hole in the wall). Clear glass is not entirely transparent, however, and

the visible transmittance of a clear glass dual pane unit is approximately 0.80 (80%).

Visible transmittance is not an energy property,

but the amount of light transmitted through glass

depends on the formulation of the raw glass, the

glass thickness, the number of panes, and on the

number and type of low-e coatings it has. The

amount of light entering through a window or

glass door is also affected by the width of the

framing members, and the presence of decorative

elements such as muntin bars (glass dividers).

End-users of window products, such as

homeowners, architects and designers, often wish

to maximize natural light within an indoor

building space. When considering which glass

option will admit the most daylight, it is common

to compare centre of glass VT values. When it

comes to appreciating how much of that light

actually makes it into the room, you may wish to

also consider the overall product visible

transmittance which includes the light-blocking

effects of opaque framing members.

1 Builder Insight 9,”Fenestration Energy Performance”, p. 2, Homeowner Protection Office Branch of BC Housing. http://bit.ly/2m9Y8DK.

Overall Product or Centre of Glass (COG)? Properties such as U-value, SHGC, and VT are sometimes discussed with reference to glass only. When it comes to the energy performance of windows, these values apply to the overall product, including the frame, mullions, and divider bars within or on the glass. The terms NFRC U-value (or NFRC U-factor), NFRC SHGC, and NFRC VT refer to these properties as evaluated for the overall product including frame effects, at the sizes in the NFRC standard. When discussing these values with product suppliers, make it a point to clarify whether you and the other party are talking about centre-of-glass values, or overall product values.

Page 16

Figure 3.4 Illustration showing how low-e coatings affect visible transmittance (VT) through glass.

3.5 Air Leakage

Air leakage is a measure of the rate of air flow through a window, door or skylight. It is

considered an energy performance property as it represents another mechanism of heat

transfer, as the air lost through leakage will be replaced with exterior air that needs to be

heated again. In Canadian standards air leakage is measured both inwards and outwards

through the assembly.

Air leakage is measured as a volume of air flowing per unit of time, per unit area of the

fenestration system, expressed in litres per second per square metre (L/s•m2).

For Canadian manufactured fenestration products, air leakage is reported using the A2,

A3 and Fixed ratings as defined in the AAMA/WDMA/CSA 101/I.S.2/A440-08 (NAFS-08)

standard (Table 3.1).

TABLE 3.1 CANADIAN AIR INFILTRATION-EXFILTRATION LEVELS

Canadian air infiltration/exfiltration level Rate of flow at 75 Pa

(L/s·m2) Rate of flow at 1.6 psf

(cfm/ft2)

A2 (operable products) 1.5 0.30

A3 (operable products 0.5 0.10

Fixed (non-operable products) 0.2 0.04

Page 17

3.6 Energy Rating (ER)

The Energy Rating is a Canadian measure of winter heating season window and glass

door energy performance. Though it is referenced in the ENERGY STAR® program, the

National Building Code and provincial codes, it is not mentioned in the BC Building Code

or the Vancouver Building Bylaw.

The ER evaluates the significant solar heat gained through window in addition to the heat

lost through the frame, glass, and through air leakage.

Figure 3.5 The Energy Rating recognizes that windows capture heat, as well as light from the sun, and evaluates winter solar energy gains in addition to energy losses in the rating

The ER is a dimensionless, two-digit rating computed with a formula that includes the U-

value, SHGC and air leakage as inputs.

The ER is not used for skylights or opaque doors.

The ER calculation assumes that windows are distributed evenly on all sides of a home

that has a modest window-to-wall ratio of between 10 and 20%, and windows that are not

shaded from the sun by external devices, trees, or nearby buildings. For homes to which

this rating applies, selecting windows on the basis of ER results in the lowest year-round

energy use, including energy used for both heating and cooling. For such homes,

selecting windows with a higher ER typically results in lower overall energy use. ER values

can range from 10 or less for windows with tinted glass, to as high as the mid-50s. (In the

Canadian ENERGY STAR® windows database, ER values range from 16 to 54.)

The ER is not suitable for selecting windows for homes with higher window-to-wall ratios,

windows in highly insulated homes that primarily face only one or two orientations, or

windows that are shaded from the sun.

Page 18

4 Today’s Window Performance Levels

Today’s frame and glass technologies have led to significant performance improvements

in the market average windows available today. A recent study based on interview data

with BC manufacturers showed that most windows sold in the Lower Mainland of BC today

are performing better than required by the building code in South and Central BC. This

conclusion was supported by survey data on the U-value ranges of products sold, and by a

review of thermal performance data of representative vinyl and wood framed products.

4.1.1 Market Data on Replacement Window Performance

In the replacement market, the results show that most of the respondents are supplying

products that comply with or exceed the USI-1.80 performance required under the BCBC

for climate zones 4 and 5 (Figure 4.1)

Figure 4.1 Distribution of U-values by manufacturer: Houses and Small Buildings – Replacements.

Since window replacements do not require building permits and are not subject to

inspection by building officials, this anonymously reported data represents the voluntary

behaviour of these companies. This suggests that for most window manufacturers, use of

non-metal frames, low-e glass and argon gas fill have become the standard product that is

used in all applications.

The higher than USI-1.8 were reported for replacements that comply with BCEEA2 USI-2.00

levels, or were categorized as BCEEA-compliant for large buildings where the maximum

allowable U-value is USI-2.57.

2 The BC Energy Efficiency Act (BCEEA) regulates window U-values through its Energy Efficiency Standards Regulation in cases where compliance with building code energy requirements is not required or is not enforced.

Page 19

4.1.2 Market Data on New Home Window Performance

In the new construction market, the study showed a greater proportion of products

shipped with U-values lower than USI-1.4. This suggests a number of these manufacturers

are selling to the Vancouver new home market where the maximum U-value is USI -1.4

(Figure 4.2)

Note that several manufacturers are outliers, reporting U-values above USI -1.8. Their

survey responses indicate these companies serve the commercial sector where higher U-

values are permitted.

Figure 4.2 Distribution of U-Values by manufacturer: Houses and Small Buildings – New Construction.

4.1.3 The Current State of Technology

The study also presented a table that showed the impact of glass performance on the

current generation of window frame profiles in wide use in British Columbia. This table

supports the self-reported U-value ranges from manufacturers.

An analysis of well documented thermal performance data for several vinyl and wood

framed product lines representative of the products supplied in BC today is summarized

in Table 4.1. While there are variations between individual product lines, and a minority of

products have significantly better performance than “market average”, this table is

believed to represent performance ranges broadly achievable with today’s available frame

and glass options.

The study confirmed that the majority of residential windows sold in BC today have vinyl

frames and are supplied with glass having at least one low-e coating, argon gas fill, and

warm-edge spacers. These options correspond to performance within a U-value range of

USI -1.80 – 1.61, a level of performance that complies with BCBC 9.36 requirements for

single family homes and small buildings in BCBC climate zones 4 through 7A. Table 4.1

also shows that with the addition of a second, “fourth surface” (room-facing) low-e

coating, many dual pane products can achieve U-values as low as 1.4 W/m2·K, the

maximum allowable U-value under BCBC 9.36 for climate zones 7B and 8, and for single

family homes in the City of Vancouver. U-values below this level require frames that can

accommodate triple pane units.

Page 20

Upgrading to available glass options can improve window performance significantly. To

achieve U-values below USI 1.0. new frame designs will be required. Only a handful of BC

window manufacturers can offer such products today.

TABLE 4.1 RELATIONSHIP BETWEEN GLASS OPTIONS AND WINDOW U-VALUE FOR TYPICAL WINDOW FRAMES

U-value range Product type

Glass options Comments Panes Gap fill Low-e

Coatings

2.00-1.81

Vinyl Sliding Windows 2 Air 1

Dual-pane glass options, conventional frames

Vinyl Casement Windows 2 Air 1

Wood Casement Windows 2 Air 1

1.80-1.61

Vinyl Sliding Windows 2 Argon 1

Vinyl Casement Windows 2 Air 1

Wood Casement Windows 2 Air 1

1.60-1.41

Vinyl Sliding Windows 2 Argon 2 4th surface (room side) coating

Vinyl Casement Windows 2 Argon 2 4th surface (room side) coating

Wood Casement Windows 2 Argon 2 4th surface (room side) coating

1.40-1.21

Vinyl Sliding Windows 3 Argon 1

Triple-pane glass options, conventional frames

Vinyl Casement Windows 2 Argon 2 4th surface (room side) coating

Vinyl Casement Windows 3 Argon 1

Wood Casement Windows 3 Argon 1

1.20-1.01

Vinyl Sliding Windows 3 Argon 2

Vinyl Casement Windows 3 Argon 2

Wood Casement Windows 3 Argon 2

≤ 1.0

Vinyl Sliding Windows — — — New frame designs required from most BC manufacturers

Vinyl Casement Windows — — —

Wood Casement Windows — — —

Page 21

5 Windows and HOT2000

HOT2000 is an energy simulation and design tool for low-rise residential buildings. This

software is widely used across Canada to support program, policy and regulatory

development and implementation. HOT2000 is developed and managed by the Office of

Energy Efficiency at Natural Resources Canada.

There are several ways to define window energy performance characteristics in HOT2000,

but the methods are not equivalent and users need to understand the intended uses of

each of them.

5.1 Defining Windows with the Code Selector

Creating a Window Type using the dropdown lists in the Code Selector screen is only

appropriate for estimating the energy performance of older windows in existing homes.

The Code Selector screen cannot be used to define new Window Types on the basis of

tested U and SHGC values. The Code Selector can however be used to select user defined

window codes created with the Code Editor.

5.2 Defining Windows with the Code Editor

To define the performance of windows for new, unbuilt homes, you need to define new

window types using the Code Editor. Version 11 of the program has a new Window Code

Type, Overall Window Characteristics, that allows users to define windows on the basis of

their actual CSA/NFRC U-values and SHGCs. (Figure 5.1) This is the option to use when it

is necessary to simulate the energy performance of a home when the specific window

energy performance characteristics are known.

Figure 5.1 The new Overall Window Characteristics option in the Code Editor

Code Selector or Code Editor? The Code Selector screen is not appropriate for defining window types whose U-values are regulated by building codes or the ENERGY STAR program. The glass, spacer and frame options available in the dropdown lists do not reflect current window technology, and they cannot be used to define a code compliant or ENERGY STAR window. The correct way to define windows with CSA/NFRC U-values in Version 11 is to use the Overall Window Characteristics option in the Code Editor.

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Example: Window with CSA A440.2 or NFRC U-values Provided by Window Supplier

The “Overall Window Characteristics” option introduced in Version 11allows actual

windows to be defined using overall product USI and SHGC values as well as the product

frame heights. Frame height information is not ordinarily published with energy

performance ratings, and will need to be obtained from the window supplier, or physically

measured from the products.

Step 1 – Obtain window energy performance and frame height data

From the manufacturer, obtain the USI, SHGC, and frame height for each window and

sliding door product type.

Table 5.1 shows energy performance data from one manufacturer’s product lines for vinyl

windows glazed with dual pane, medium-solar gain insulating glass units. Note that USI,

SHGC, and frame heights often differ for each product type.

TABLE 5.1 NFRC-100 U AND SHGC VALUES FOR ONE VINYL WINDOW LINE

Product Type NFRC/CSA Evaluation Size

USI SHGC Frame Height

Fixed (Picture) window 1200 x 1500 mm 1.53 0.24 47 mm

Casement window 600 x 1500 mm 1.53 0.24 62 mm

Awning window 1500 x 500 mm 1.53 0.24 62 mm

Sliding window 1500 x 1200 mm 1.70 0.30 40 mm

Note that the CSA A440.2 and NFRC 100 standards evaluate only individual unit product

types, such as Fixed (picture), Casement, Awning, and Sliding. Each product type has a

defined evaluation size (see Section 7.2). Most homes homes however feature one or

more combination windows, whether composed of individual units mulled together or

supplied within a common frame. Combination windows typically have separate U-value

labels for the fixed lite and operable sash components.

Since HOT2000 does not account for the different U and SHGC values that could exist in a

combination window, the ERS Technical Procedures3 provide this remedy:

For windows that have a window area that is 50% or more operable (i.e. not fixed

and can be opened), assess the entire window with the observed operable

Window Type option (e.g. Hinged, Slider with Sash, Patio Door . . . ). If less

than 50% is operable, assess the Window Type as Picture.”

Once these products are defined in the Code Editor, they can be reused and do not need

to be defined again.

3 EnerGuide Rating System Technical Procedures Version 15.3, p. 42.

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Step 2 – Create new Overall Window Characteristics codes

From the HOT2000 menu bar, select:

File > Editors > Code Editor > Window Codes > New User-defined Code (hard hat icon)

> Overall Window Characteristics > OK:

With the Overall Window Characteristics option selected, the Window Code Editor displays

a new set of options that allow entering the Overall Thermal Resistance as a U-value,

together with Frame Height and SHGC values.

In the Overall Thermal Resistance field, select the U-value option.

Enter the U-value (Resisitivity) and SHGC from the energy performance data provided

by the manufacturer for each product type.

Enter the Frame Height in mm for each product type as measured from the product or

as provided by the manufacturer. (Frame height is measured from the outermost edge

of the frame to the glass daylight opening. Frame heights for operable products are

often higher than for fixed glazing and picture windows.)

Name the window’s Code Label and Description fields.

Save this code for reuse.

This screen allows U-value input instead of RSI!

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Note: It is important to select the correct Window Type, as HOT2000 uses that to

correctly scale from the CSA/NFRC evaluation size to the actual size windows

defined in the walls. The other fields—Glazing Type, Fill Gas and Low-E Coating

are used by the program only for error-checking purposes only.

Step 3 – The new window is now available for use in the Code Selector

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6 Windows and Comfort Considerations

6.1 Windows and Cold Window Surface Discomfort

Cold window surface temperatures produce discomfort in two distinct, but related ways:

The sense of discomfort due to “radiant asymmetry”: when window temperatures are

more than 4–5 °C colder than the walls, we perceive the difference as cold discomfort.

The sense of discomfort due to drafts. Cold window surface temperatures produce

cold air currents that we can feel.

To minimize cold window surface discomfort, choose windows with the lowest available U-

value, but think twice before choosing dual pane glass with “fourth surface” low-e

coatings. These are coatings on the glass surface that faces a room. Because these

coatings reflect heat, they reduce the glass surface temperature, and in winter can

produce the same level of radiant discomfort as a dual pane clear glass window with no

low-e coatings at all. In addition to discomfort, there is a greater risk of condensation

forming than on glass in which the low-e coatings are placed on internal glass surfaces.

6.2 Windows and Overheating Discomfort

Excessive solar heat gain can lead to significant overheating at certain times of the day

and certain times of the year, particularly with east, west and south facing windows. When

a home’s design has significant window areas facing these orientations, measures to

control solar heat gain should be considered, such as:

External shading design features (South-facing windows);

Low solar heat gain glass;

In-window blinds (more effective); and,

Internal blinds (less effective).

For south facing windows without external shading from trees, other buildings, or

external shading devices, and for east and west facing windows, overheating discomfort

can be reduced by choosing windows with the lowest available SHGC ratings. Glass having

“double silver” (good) or “triple silver” coatings (best) is a good choice in these situations.

6.3 Overheating Discomfort in ESNH Homes

Ontario builders have reported significant overheating discomfort with homes built to the

ENERGY STAR for New Homes (ESNH) program requirements, and this has been attributed

to several factors:

High window-wall ratios (> 15%)

Windows facing primarily one or two directions

Use of high solar gain glass

High enclosure insulation levels in ESNH homes

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Builders are increasingly finding that home buyers favour homes with large expanses of

glass, and with windows oriented primarily in one or two directions. This can lead to

overheating discomfort when significant window areas face south, east or west, and when

higher solar gain glass is used. The ability of builders to select very low solar gain

windows to remedy overheating is somewhat limited by the ENERGY STAR qualification

criteria.

The ESNH program requires builders to use ENERGY STAR windows. Figure 6.1 presents

the current ENERGY STAR qualifying criteria for windows. The least expensive ENERGY

STAR products tend to rely on higher solar gain glass to qualify under the Energy Rating

(ER) path. While this may result in less energy required to heat and cool the home, it can

also lead to overheating discomfort at certain times of the year, even in winter.

Builders that are aware of this problem could address overheating by using ENERGY STAR

windows that qualify under the Alternate U-factor Path, but only to a limited extent.

Products that qualify under the U-factor path generally have lower U-values and lower

SHGC values than products qualifying under the ER path. The use of the very lowest solar

gain windows is precluded by the Minimum Energy Rating value on the U-factor qualifying

path.

The ESNH program is aware of the problem, and it is hoped that this issue will be

addressed in the near future.

Figure 6.1 ENERGY STAR® qualifying criteria

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7 Window Energy Standards, Labeling and Certification

7.1 Energy Performance Rating Standards

The energy focus of Canadian building codes is reduction of heating season energy use.

Both our building codes and the ENERGY STAR program recognize two sets of standards

for rating energy performance:

CSA A440.2, “Fenestration Energy Performance”

American standards from the National Fenestration Rating Council:

NFRC 100, “Procedure for Determining Fenestration Product U-factors”

NFRC 200, “Procedure for Determining Solar Heat Gain Coefficient and Visible

Transmittance . . .”

Figure 7.1 Energy performance rating standards

CSA A440.2 follows the NFRC 100 and 200 simulation procedures. The main difference

between CSA A440.2 and NFRC 100 is that CSA allows U-values to be determined by

simulation only, while NFRC also requires physical testing every 5 years.

7.2 U-Values Determined Under Standard Conditions

Both NFRC and CSA standards determine U-values under standard conditions to allow

product comparisons:

Standard environmental conditions

Winter design temperature -18C

Indoor temperature +20C

Wind speed

Standard size for determining energy performance ratings for each product type

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Energy performance ratings for fenestration are defined with respect to

sizes/configurations specified in NFRC 100 and CSA A440.2 standards.

Figure 7.2 NFRC 100 (2014) energy performance reference sizes

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Figure 7.3 CSA A440.2 (2014) energy performance reference sizes

7.3 Energy Performance Labeling

7.3.1 Performance Rating Labels

Both CSA and NFRC standards require two types of labels (or “markings”) to be applied to

window products:

A “permanent” label or marking identifying the manufacturer

A non-permanent performance rating label

Both standards require energy performance rating labels to be applied to each individual

unit product. In the case of combination products, which have both fixed (picture) as well

as operable windows, they require labels for each product type to be present. A single

label can only be applied to a combination product if it is the label of the operable

component, which typically has the higher (worse) U-value (Figure 7.4).

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Figure 7.4 Performance rating label options: each product type must be labeled, or a single label for the operable product may be applied.

Permanent markings are permitted to be:

Affixed to the frame or spacer bar of the product so that the label is visible at all

times

Label affixed to the frame or sash of the product so that the label is visible when the

sash is open

Transparent label affixed to the glass of the product

Label markings etched into the surface of the glass

7.3.2 Certification of Energy Performance

Certification provides a degree of oversight and accountability to energy performance

labeling. Under certification, the manufacturer agrees to label products correctly and

under the rules of the certifying organization, and agrees to be audited by the certifier. In

return the manufacturer is permitted to apply the mark of the certification organization to

these labels.

In general, Canadian building codes do not require energy performance ratings to be

verified, or “certified” by third party organizations. There are exceptions, such as the BC

Energy Efficiency Act and the Ontario Green Energy Act, as well as the Vancouver Building

Bylaw, all of which require energy performance labels to be certified by recognized

organizations.

The ENERGY STAR program also requires energy performance ratings to be certified.

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7.3.3 Certification organizations and label examples

There are four energy performance certification organizations with a long presence in

Canada (Figure 7.5). A fifth, Labtest Certification Inc. now also offers energy performance

certification. (Label and logo information was not available at the time of this publication.)

Figure 7.5 Certification organizations and their brand marks

Example of permanent markings are shown in Figure 7.6:

Figure 7.6 Examples of cerfitied permanent labels

Example removable performance rating labels are shown in Figure 7.7.

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Figure 7.7 Examples of certified removable performance rating labels

7.3.4 ENERGY STAR Labels

The Canadian ENERGY STAR program has specific labeling requirements. First, the upper

portion of the label must have the ENERGY STAR logo and either a map or text

designation of the ENERGY STAR zones the product is qualified for.

Second, the certified energy performance ratings appear below the ENERGY STAR label.

Valid ENERGY STAR® labels will always bear the mark of one of the certification

organizations.

Third, the labels must also report the structural-air-water performance ratings, as

determined according to the North American Fenestration standard (NAFS), below the

energy performance ratings. This label does not need to be certified.

These three label components may appear on a single label or as separate labels with the

NAFS information appearing below the energy performance ratings.

Figure 7.8 displays example labels showing acceptable formats for the ENERGY STAR portion and the energy ratings portions. ENERGY STAR does not regulate the NAFS structural-air-water label that would appear below these labels.

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Figure 7.8 Example ENERGY STAR® labels