slide 1

This Morning’s Agenda:

• Presentation: Envelope Analysis for the PHPP• Tutorial: Thermal Bridge Calculation• Break• Presentation: Introduction to Point Thermal Bridges• Mini-tutorial: Entering Point Thermal Bridges into PHPP• Questions

slide 2

Thermal Bridge Analysis for the PHPPPassive House Conference 2010

PH Consultant SessionPortland, November 4

David White, Right Environmentsdavid@rightenvironments.com

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Overview: Overall Method for Envelope Analysis

1. Analyze the envelope component areas & U-values using outside dimensions. Areas go in the “Areas” tab and U-values go in the “U-Values” tab. This is called “the Simplified Method.”

2. Examine the thermal bridges at the intersections between component areas. Based on best judgment, decide whether the simplified method was accurate enough, or significantly under/overestimated heat loss.

3. If the bridge is not significant (less than 0.01 W/mK), don’t bother to calculate it. If it is significant, you can account for it by taking a value from a catalog of thermal bridges or doing your own 2D calculation. Input the thermal bridges low down in the “areas” tab of the PHPP.

4. PHPP calculates total envelope loss as the sum of component area losses and bridge linear losses. You can compare the performance of different details by swapping them in and out on PHPP and seeing how much difference in makes in heating/cooling demand.

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This workshop includes:

• Key Concepts• Calculation Guidelines• Tutorial with THERM and Excel

It does not include:

• Dynamic analysis methods

slide 5 Key Concepts

slide 6

Component Areas and Intersections

Key Concepts

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Components

roof

wall

slab

wall

Key Concepts

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Intersections

Key Concepts

roof

wall

slab

wall

slide 9 Key Concepts

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Intersections. Note: taking windows from NFRC to PHPP has two problems: 1) physics of ISO vs. NFRC and 2) NFRC has one combined value for glass, frame, and spacer. Check my website for window inputs calculation method.

Key Concepts

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Thermal bridging typically means that the heat gets a short cut across the envelope.

Key Concepts

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In PHPP, whether or not the heat gets a short cut, a “thermal bridge coefficient” can be applied any place where heat flow can’t be accurately calculated using the simplified method, i.e. an intersection.

Thermal Bridges• wall to slab (can be a big one!)• wall to roof• wall to wall• glass to frame (“spacer” in WinType)• frame to wall (“installation” in WinType)• etc

Key Concepts

roof

wall

slab

wall

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= ψ-

(W/K per meter of intersection length)

Key Concepts

Key understanding: the thermal bridge heat loss is the difference between the “true” heat loss, calculated using 2D analysis, and the heat loss calculated using the simplified method.

(Btu/hrF per footof intersection lenth)

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For component heat loss, use simple parallel heat transfer calculation in PHPP.

Key Concepts

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For thermal bridge heat loss, either reference a calculation done by others...

“Details for Passive Houses”

Key Concepts

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...or do it yourself, for instance using THERM.

Key Concepts of Thermal Bridges

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Thermal bridges must be specific to ambient, ground, or “perimeter.”

Key Concepts

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Ambient – no ground interactions

Ground – bridge is in contact with ground, far from grade. Do not include ground or exterior air film in model.

Perimeter – partly above, partly below grade. Special!

Key Concepts

slide 19 Calculation Guidelines

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Schneiders, “Protocol 16: Thermal Bridge Free Construction,” PHI, January 2008

R-0.45 R-0.74

R-0.22

R-1.14

R-0.97

Calculation Guidelines

Surface Film Coefficients

slide 21 Calculation Guidelines

Surface Film Coefficients from ASHRAE Fundamentals – differ slightly from German

IP units METRIC units

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• EN 10211-1 recommends straight sections extend 1m “clear” (consistent construction)• Rule of thumb: 4x wall thickness (although for PH this can be 6 feet!)• Beware: adiabatic boundary will force isotherms to be parallel! Red herring!• Above example is for a simple detail – higher fluxes may need longer straight sections• Too long can cause inaccuracy b/c error limit is based on total heat loss of the model.• When in doubt, test it at various lengths (as above)

Calculation Guidelines

Schneiders

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Useful hint: when applicable, put adiabatic boundary at a line of thermal symmetry. E.g. in a wood frame wall, put boundary at center of stud or at midline of bay.

Calculation Guidelines

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“Perimeter Insulation” is included in ground sheet calculations and in thermal bridge calculations (I think).

Calculation Guidelines

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This is how to model a perimeter thermal bridge (from Schnieders 2008). The indoor/outdoor temps are arbitrary in terms of calculating psi. The specific temps used here are useful because they also tell us something about condensation risk.

Outdoor temp,e.g. 13°F for NYCPHPP peak

Indoor temp, e.g. 68°F

Average of Indoor and Outdoortemp, e.g. 40.5°F

2.5 m 1.0 m

2.5

m

Adiabatic

Calculation Guidelines

slide 26 Tutorial with THERM and Excel

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Thermal bridge analysis: not just for masochists anymore!

Tutorial with THERM and Excel

slide 28

Wal

l

Slab

Tutorial with THERM and Excel

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Draw detail in THERM (to save time, it is pre-drawn for the tutorial).Assign boundary conditions and U-factor tags.

Tutorial with THERM and Excel

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Note: U-factor tags are assigned to red boundary along interior surface. U-factor tags can go inside or out, as long as they mark one “gate” through which all heat flow to/from the interior passes, so that THERM can measure the flow. Note: German and US air film values differ slightly, so there are discrepancies w/ slide #18.

Tutorial with THERM and Excel

adia

batic

13 F with exterior resistance (R-0.17)

68 F, vertical surface film (R-0.68)

68 F, downward flow film (R-0.92)

13 F withrain screen

(R-0.45)

40.5 F (half way between indoor and outdoor temp), no air film

adiabaticadiabatic

68 F, inside corner film(R-1.14) for 8 inches or so

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If you like, you can add a little extra length at the end of each component, and give those lengths their own U-factor tags. This way THERM will calculate the U-value of the “clear” sections at the same time that it calculates flow through the area of the intersection. Ease of use, reliability!

Tutorial with THERM and Excel

U - tag “intersection”

U factor tag “wall”

U - tag “slab”

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Technical credit: Charlie Weedon cweedon@wildblue.net Charlie requests your input!

Tutorial with THERM and Excel

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Set error tolerances and max iterations, then run it. Manual (7.3.2) warns of accumulated roundingerrors below 5% Maximum Error Energy Norm. Software author says 5% is ok, so I use 5%.

Tutorial with THERM and Excel

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Set up calculation sheet on PHPP

Tutorial with THERM and Excel

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Calculate heat loss of straight sections by one of the following methods:

• simulate straight section on THERM (more accurate, but how much more?)• use PHPP calculation (faster)• include clear sections in the same THERM model of the intersection (Weedon method)

Tutorial with THERM and Excel

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Subtract losses calculated using the simplified method from losses calculated on THERM. Be careful to assign the correct temperature difference to each component. Divide the net loss by the deltaT to ambient (not ground) because PHPP asks for perimeter thermal bridges with respect to outdoor temperature. The result is the ψ-value.

Tutorial with THERM and Excel

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Rest