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Roof Related R-Value


A material’s R-value is the measure of its resistance to heat flow. It is important to know the R-value because many states or regions require that a roof system have a minimum amount of thermal resistance on commercial, industrial, and/or institutional buildings. The way it works is simple: the higher the R-value, the more the material insulates.

Some common roofing materials and their corresponding values for Thermal Conductance (C) and Thermal Resistance (R) are shown in the following table.

 

Material

Thickness In Inches

C-Value

R-Value

Metal

N/A

0.000

0.00

Concrete

1.0

3.333

0.30

Gypsum

1.0

1.667

0.60

Wood

1.0

1.099

0.91

Tectum

1.0

0.500

2.00

Inside Air Film

N/A

1.087

0.92

Outside Air Film - Summer

N/A

4.000

0.25

Outside Air Film - Winter

N/A

5.882

0.17

Vapor Retarders

N/A

0.000

0.00

BUR Gravel

N/A

2.941

0.34

BUR Smooth

N/A

4.167

0.24

Fiberboard

1.0

0.360

2.78

Perlite

1.0

0.360

2.78

Phenolic Foam*

1.0

0.120

8.30

Fiber Glass

1.0

0.256

3.90

Polyisocyanurate

1.0

0.180

5.56

Polyisocyanurate Composite

1.5

0.240

4.17

Polystyrene Bead Board

1.0

0.280

3.57

Polystyrene Composite Board

1.5

0.301

3.32

Polystyrene - Expanded (EPS)**

1.0

0.260

3.85

Polystyrene - Extruded (XEPS)***

1.0

0.200

5.00

Sprayed Polyurethane Foam****

1.0

0.150

6.88

Cork

1.0

0.280

3.57

 

* Problems have been reported with regard to the use of Phenolic Foam roof insulation. Incidents of deck corrosion have been reported in cases where the insulation is in direct contact with steel roof decks and there is moisture present.

** Molded, Expanded Polystyrene Insulation, also referred to as MEPS, can have an R-value that will vary from less than 4.00 to slightly more than 4.00. The amount shown is an average amount used for roof system R-value calculations.

*** Extruded, Expanded Polystyrene Insulation is commonly used in Inverted Roof Membrane Assemblies (IRMA).

****Sprayed Polyurethane Foam (SPF) will have an R-Value of 7.14 when it is newly installed. After it ages a few months, the R-Value will reduce to somewhere around 6.88.

The C-value (C) is a measure of the Thermal Conductance of the material and is the reciprocal of R, or

C is determined only when the Thermal Conductivity (k) of a material is known.

Thermal Conductivity is the measure of the amount heat that will be transmitted through a one inch (1") thick piece of homogenous material, one square foot (1 ft.2) in size, in one (1) hour, when there is a one degree Fahrenheit (1 F) temperature change. The equation for "k" is:

 

Now let’s see how easy it is to figure the R-value. First of all, you need to know what the components of the roof system are. We’re going to figure the value of some common ones. Let’s assume the roof system consists of a smooth-surfaced built-up over " of perlite coverboard over 2" polyisocyanurate insulation on a steel deck in the winter. The season makes a difference with the value of the outside air film. Let’s start from the inside and go out.

Rvalue1.gif (3474 bytes)

 

COMPONENT R-VALUE
Inside Air Film 0.92
Steel Deck 0.00
2" Polyisocyanurate (5.56 * 2) 11.12
" Perlite (2.78 * 0.75) 2.09
Smooth Built-Up Roof 0.24
Outside Air Film in Winter 0.17
TOTAL 14.54

Total R-value for the above roof system is 14.54.

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