| 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 ¾" 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.
| COMPONENT |
R-VALUE |
| Inside Air Film |
0.92 |
| Steel Deck |
0.00 |
| 2" Polyisocyanurate (5.56
* 2) |
11.12 |
| ¾" 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.
BACK TO TOP
All contents © 1998, 1999 RoofHelp.com. All rights
reserved |
