Agricultural Structures:Insulation and Heat Flow

AGME 1613

Fundamentals of Agricultural Systems Technology

                                                                                                                                   

                               

Objectives

• Describe methods of heat transfer• Explain why structures are insulated• Describe common types and forms of insulation• Calculate total thermal resistance of a structural

component• Estimate building heat loss• Determine optimal level of insulation for a

structure

Heat Transfer

• Heat moves from area of high concentration to area of low concentration.

Methods of Heat Transfer

• Conduction – heat transfer where there is direct contact between the hot and cold surfaces.

• Other examples?

Methods of Heat Transfer

• Convection – Fluid (air or water) transfers heat from the hot surface to the cold surface.

• Other examples:

Methods of Heat Transfer

• Radiation – Heat transfer between non-contacting surfaces without change in air temperature.

• Other examples:

Why do we insulate structures?• Reduce building heat

loss in cold weather.– Decrease heating costs

• Reduce building heat gain in hot weather.– Decrease cooling costs

• Reduce / eliminate water condensation during cold weather.– Decrease repair costs

Condensation Process

Outside InsideW

A

L

L

Warm, moist air

Cold, dry air

What Should be Insulated?

What is Insulation?• Insulation – Any

material that reduces the rate at which heat moves by conduction.

• Insulation, including structural materials, may be:– Homogenous

– Non-homogenous

Poured ConcreteConcrete Block Wall

Commercial Insulation Products

                    

• Loose-fill

• Batt-and-Blanket

                                                                      • Rigid

Forms Materials

•Cellulose

•Vermiculite

•Glass fiber

•Polystyrene

•Polyurethane

Insulation R-Values• R-Value is the rating system for insulation.

– Higher R-values = greater thermal resistance.

• Heat flow is measured in BTUs per hour• Heat flow through a component is calculated as:

Q = Δt x A Rtotal

Where, Q = Heat flow (BTU/hr)Δt = Temperature difference (degrees F)A = Area of component (ft2)

Rtotal = Total thermal resistance of component

Determining Total R-values

• Determine the composition of the building component.

• Determine the R-value for each component (Table 14, p. 84 of Engineering Applications)

• Add all the R-values together to determine Total R-value.

ExampleDetermine the R-total for the

wall section shown below

Outside Inside

Air Film

½-in wood siding

½-in plywood

3½-in glass-wool insulation

½-in plaster board

ExampleDetermine the R-total for the

wall section shown below

½-in wood siding

Air Film

½-in plywood

3½-in glass-wool insulation

½-in plaster board

R = .81

R = .62

R = 11.9

R = .45

Inside air filmOutside air film

R = .61

R = .17Rt = 14.56

Heat Loss Example #1

•Assume that a house has a total wall surface area of 2000 ft2.

•Given the R-total just calculated, determine the total heat loss (BTU/hr) through the walls if:

•Inside temperature = 72 deg. F

•Outside temperature = 25 deg. F

Heat Loss Example #2

Un-heated Attic40 deg F

Heated Interior

73 deg F

Ceiling = 60’ x 40’

4-in. glass wool ¾-in plywood

Determine:

•Current R-total

•Total ceiling heat loss (BTU/hr)

•Amount (in.) of loose-fill cellulose insulation required to bring ceiling up to DOE recommendations.

Economic Analysis• Assume that a contractor will “blow

in” the insulation for:– $.25 / ft2 (first 2-in.)– .15 / ft2 (each additional 2-in. increment.)

• You heat with electricity:– $.075 / kW-hr– 3413 BTU/kW-hr

• What is the “optimum” insulation level IF “payback period” must be < 10-yrs?

• Spreadsheet