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Insulation serves to limit the conduction of heat through the building shell. Little of a commercial building’s summer cooling is due to conduction of heat through the skin of the building envelope. However, for buildings with a large amount of roof area relative to floor area, such as a single-story retail facility, reducing heat gain through the roof can be an important consideration.
The type of insulation used in roof applications and the cost effectiveness of adding more depends on the type of structure, the building’s orientation, the amount of insulation already installed, venting of the space below the roof, and the color of the roof. For flat-roofed structures with rubber membrane roofing, it’s common to use a rigid insulation board on the roof deck. For structures with sloped roofs, batts or loose blown-in insulation are more common. Some structures are relatively easy to add additional roof insulation to and others can be more difficult. A building with an attic space can have insulation added at any time, while adding insulation to a building without attic space might not be cost effective unless it can be done at the same time as a roof replacement or other major remodel. The greatest energy savings are typically made when adding insulation to a dark colored, flat, uninsulated roof directly over air-conditioned space. The following table shows the insulating values of common types of roof insulation.
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Characteristics of Common Insulation Types
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Type
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Form
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R-value per inch¹
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Polyurethane
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Expanding foam
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6
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Expanded Polystyrene
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Rigid board
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4 – 5
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Polyisocyanurate, faced
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Rigid board
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7
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Fiberglass
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Batts or Loose
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3.5
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Cellulose
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Loose or wet blown
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3.5
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Mineral Wool
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Loose
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2.5
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Rock Wool
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Loose
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2.5
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R-Value is the thermal resistance of a building material. The higher the R-value, the higher the insulating value of the material.
U-Value is the rate of heat flow through a building element. It is the reciprocal of the R-value and the lower the U-value the higher the insulation value of the material.R-Value is the thermal resistance of a building material. The higher the R-value, the higher the insulating value of the material. |
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Since lighter colors and reflective coatings reflect more of the sun’s heat than darker colors, the color of a roof can affect the demand for cooling in buildings. The savings from applying a light colored or reflective roof treatment vary depending on the orientation of the roof, the ventilation of the space below it, and the roof’s insulation levels. The greatest savings are expected on a flat roof with no ventilation below it and no insulation. Another method available for flat roofs is to apply a layer of white pebbles. In order to remain effective, a light or reflective roof coating or material needs to be kept clean. If it is allowed to become dark over time due to dirt, dust, and pollutants it’s effectiveness will degrade. The effect of different roof insulation levels and color on annual heating and cooling costs is shown in the table below.
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Annual Cooling Costs with Different Insulation Levels and Roof Colors
For a Single-Story 10,000 Square Foot Building in the Phoenix Area
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Insulation Level
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Roof Color
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Annual Heating and Cooling Cost
($/square foot/year)
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No insulation, R-0
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Dark
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2.26
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Light
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2.17
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R-19
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Dark
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1.80
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Light
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1.69
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R-30
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Dark
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1.23
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Light
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1.11
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The following table provides some rules-of-thumb on the cost effectiveness of adding roof insulation to an existing building. For new buildings, APS recommends a minimum R-30 roof insulation. These are general guidelines and it’s always a good idea to consult with your architect or builder about the cost-effectiveness of your application.
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Cost Effectiveness of Adding Roof Insulation to Existing Buildings in the Phoenix Area²
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Existing Condition
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Is it cost effective to add insulation?
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No insulation to R-6
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Yes, always
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R-7 to R-19
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Yes, if attic is accessible or if built-up roof is being replaced
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Greater than R-19
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Not usually cost effective
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¹ The values high are typical, however actual values will vary by manufacturer and installation quality.
² Adapted from the Energy Star Small Business Guide for the Phoenix Climate.
More building envelope answers
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