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Agricultural Structures:Insulation and Heat Flow
AGME 1613
Fundamentals of Agricultural Systems Technology
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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
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Heat Transfer
• Heat moves from area of high concentration to area of low concentration.
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Methods of Heat Transfer
• Conduction – heat transfer where there is direct contact between the hot and cold surfaces.
• Other examples?
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Methods of Heat Transfer
• Convection – Fluid (air or water) transfers heat from the hot surface to the cold surface.
• Other examples:
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Methods of Heat Transfer
• Radiation – Heat transfer between non-contacting surfaces without change in air temperature.
• Other examples:
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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
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Condensation Process
Outside InsideW
A
L
L
Warm, moist air
Cold, dry air
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What Should be Insulated?
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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
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Commercial Insulation Products
• Loose-fill
• Batt-and-Blanket
• Rigid
Forms Materials
•Cellulose
•Vermiculite
•Glass fiber
•Polystyrene
•Polyurethane
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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
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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.
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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
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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
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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
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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.
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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