Generally, walls are made up of several layers of different materials. The R-value of a composite wall is calculated by adding the effective R-values of each of the layers of the wall.
For example, the image below shows a wall made up of four layers—½ inch drywall inside for aesthetic purposes, real insulation in between the studs, ¾ inch plywood sheathing outside, and wood siding as the final exterior finish. Together, the layers of the wall are preventing heat loss.
We can calculate this wall’s composite R-value by adding the R-values of each layer.
Plasterboard (1/2 inch)
+ Fiberglass (3.5 inches @ 3.70 per inch)
+ Plywood (3/4 inches)
+ Wood Siding (1/2 inch)
-----------------------------------------
Total R-Value of Composite Wall
We can find the R-values for the walls by using the table from the page about R-Values [1] (the table is repeated below):
Material | R-Value (ft2 o Fh / BTU) |
---|---|
Plain glass, 1/8 inch | 0.03 |
Stone per inch | 0.08 |
Common Brick per inch | 0.20 |
Asphalt Roof Shingles | 0.44 |
1/2 inch Gypsum Board (Drywall or plasterboard) | 0.45 |
Wood Siding, 1/2 inch | 0.81 |
Plywood, 3/4 inch | 0.94 |
Insulating sheathing, 3/4 inch | 2.06 |
Fiberglass, per inch (battens) | 3.50 |
Polystyrene per inch | 5.00 |
Polyurethane Board per inch | 6.25 |
Cinder Block (12 inches) | 1.89 |
Plasterboard (1/2 inch) = 0.4524
+ Fiberglass (3.5 inches @ 3.70 per inch) = 12.95
+ Plywood (3/4 inches) = 0.94
+ Wood Siding (1/2 inch) = 0.81
-----------------------------------------
Total R-Value of Composite Wall = 15.15
A ceiling is insulated with 0.75" plywood, 2" of polystyrene board, and a 3" layer of fiberglass. What is the R-Value for the ceiling?
Solution:
The ceiling consists of three layers, and all three layers together prevent the heat loss. So, we need to add the R-values of all three layers
" of plywood has an R-value of 0.94
2" of polystyrene at 5.0 per inch will have an R-value of 10.00
3" of fiberglass at 3.7 per inch will have an R-value of 11.10
So the R-value of the ceiling is 22.04 ft2 oFh / BTU.
Please watch the following 1:31 video presentation about Example #2. A wall consists of 0.5” wood siding (R = 0.81), 0.75” plywood (R = 0.94), 3.5” of fiberglass (R = 13.0), and 0.5” plasterboard (R = 0.45). What is the composite R-value of the wall?
Please watch the following 1:18 video presentation about Example #3. What is the R-value of a wall that is made up of wood siding (R = 0.81), 5” of fiberglass (R = 3.70 per inch), and a layer of 0.5” drywall (R = 0.45)?
Please watch the following 1:44 video presentation about Example #4. A wall is made up of 8” of stone, 3” of polyurethane board, and 0.75” of plywood. Calculate the composite R-value for the wall.
The United States map below shows insulation needs by region, as indicated by color and numbers.
Instructions: Click on the “zone” buttons below the map to see the U.S. Department of Energy’s Recommended Total R-Values for new construction of houses. Note that insulation R-values are different for the ceilings, walls, floor, etc.
Click here to open a text description of the Insulation Needs activity
The U.S. Department of Energy released recommended total R-values for new construction houses. The info is based on regional zone and covers various parts of the house. The states within each zone are listed below, followed by a data table containing the R-values for each part of the house. The R-values are dependent on the type of heating system being used and may vary for each. Some states may lie in multiple zones.
States:
Zone 1 | Gas Heat Pump Fuel Oil |
Electric Furnace |
---|---|---|
Attic | R-49 | R-49 |
Cathedral Ceiling | R-38 | R-60 |
Wall | R-18 | R-28 |
Floor | R-25 | R-25 |
Crawl Space | R-19 | R-19 |
Slab Edge | R-8 | R-8 |
Basement Interior | R-11 | R-19 |
Basement Exterior | R-10 | R-15 |
States:
Zone 2 | Gas Heat Pump Fuel Oil |
Electric Furnace |
---|---|---|
Attic | R-49 | R-49 |
Cathedral Ceiling | R-38 | R-38 |
Wall | R-18 | R-22 |
Floor | R-25 | R-25 |
Crawl Space | R-19 | R-19 |
Slab Edge | R-8 | R-8 |
Basement Interior | R-11 | R-19 |
Basement Exterior | R-10 | R-15 |
States:
Zone 3 | Gas Heat Pump Fuel Oil Electric Furnace |
---|---|
Attic | R-49 |
Cathedral Ceiling | R-38 |
Wall | R-18 |
Floor | R-25 |
Crawl Space | R-19 |
Slab Edge | R-8 |
Basement Interior | R-11 |
Basement Exterior | R-10 |
States:
Zone 4 | Gas Heat Pump Fuel Oil |
Electric Furnace |
---|---|---|
Attic | R-38 | R-49 |
Cathedral Ceiling | R-38 | R-38 |
Wall | R-13 | R-18 |
Floor | R-13 | R-25 |
Crawl Space | R-19 | R-19 |
Slab Edge | R-4 | R-8 |
Basement Interior | R-11 | R-11 |
Basement Exterior | R-4 | R-10 |
States:
Zone 5 | Gas | Heat Pump Fuel Oil |
Electric Furnace |
---|---|---|---|
Attic | R-38 | R-38 | R-49 |
Cathedral Ceiling | R-30 | R-38 | R-38 |
Wall | R-13 | R-13 | R-18 |
Floor | R-11 | R-13 | R-25 |
Crawl Space | R-13 | R-19 | R-19 |
Slab Edge | R-4 | R-4 | R-8 |
Basement Interior | R-11 | R-11 | R-11 |
Basement Exterior | R-4 | R-4 | R-10 |
States:
Zone 6 | Gas | Heat Pump Fuel Oil |
Electric Furnace |
---|---|---|---|
Attic | R-22 | R-38 | R-49 |
Cathedral Ceiling | R-22 | R-30 | R-38 |
Wall | R-11 | R-13 | R-18 |
Floor | R-11 | R-11 | R-25 |
Crawl Space | R-11 | R-13 | R-19 |
Slab Edge | (c) | R-4 | R-8 |
Basement Interior | R-11 | R-11 | R-11 |
Basement Exterior | R-4 | R-4 | R-10 |