Help on R Values

Note: if you are starting with an un-Insulated space it is very important to read the section below titled "Estimating R Values for Un-Insulated Spaces"


Note: These are all US R values.  To convert to SI R values multiply by 0.176.  For example US R10 = SI R 1.76.

Most Common R Value Upgrades:

Add blown in cellulose to attic:  Add R3.1 for each inch of new cellulose you blow in (1)

Add loose fill Fiberglass to attic: Add R2.2 for each inch of new Fiberglass that you blow in.

Add blown in cellulose to wall: Add R11.2 for walls with 2X4 studs, add R17.6 for 2X6 stud walls.

Add Fiberglass batt insulation:  Add R3.1 for each inch of added batt thickness

Add rigid polystyrene "beadboard" insulation: Add R4 for each inch of board thickness

Add rigid extruded polystyrene (pink or blue) insulation board: Add R5 for each inch of board.

Add Polyisocyanurate (foils faced) insulation board: Add R7 for each inch of board thickness.

(1) adding cellulose over loose fill Fiberglass insulation will also reduce air infiltration and will reduce currents in the fiberglass that can reduce the effectiveness of loose fill Fiberglass insulation :

For other situations, use the R value table just below to look up the R value for your situation.

Note that all R values are in US units.


R Value Lookup Table: has a good table of R Values:

You will find R values for most materials, insulations, and windows in the table at this link.


Estimating R Values for UnInsulated Spaces:

If you are starting with a wall, ceiling or floor that is not insulated, it is important to make a good estimate of the current R value.   Its very important to get the R value for uninsulated surfaces as close as you can, as small changes in these values will make for large changes in your heat loss. 




This applies to conventional wood framed ceilings of either the cathedral or attic type.


Attic air film R 0.61
Sheathing R 0.5
SheetRock R 0.45
Inside air film R 0.61
Total R 2.2

The components for a cathedral ceiling with a 1 inch vent space between the top of the insulation and the bottom of the roof sheathing are a little different, but the total is nearly the same.  The attic calculation assumes a well ventilated attic.



This applies to conventional wood stud walls with interior sheetrock and exterior wood siding:

Outside air film R 0.25
Wood siding R 0.59
Sheathing R 0.5
Inside air cavity R 0.94
SheetRock R 0.45
Inside air film R 0.68
Total R value R 3.4



This applies to conventional wood joist floors with 3/4 plywood subfloor covered with carpet and pad over an unheated  and ventilated crawl space:

Lower air film R 1.23
Subfloor R 0.94
Carpet+Pad R 1.23
Inside air film R 0.92
Total R value R 4.32

I think that it could be argued that some additional R value could be added for crawl space to account for the thermal resistance of the crawl space walls to the outside -- this would be particularly true for people who close up the crawl space vents during the winter.


Concrete and Block Walls:

Above grade concrete wall including inside and outside air films = R 1.2 for 6 inch, R1.4 for 8 inch


Concrete block wall with air in cavities = R 2.0

Concrete block wall with Perlite in cavities = R 2.9



The reason that it is important to make a good estimate for the current R value for uninsulated spaces is that small changes in R values result in large changes in heat loss when the total R value is low.  For example, if you miss the R value by 1 when the total R value is (say) 2.0, this will result in a 50% error in the heat loss.  For an insulated space it does not mater so much -- if you miss the R value of an R30 insulated ceiling by R1, its only a 3% error in heat loss.



Rev. 3/12/06, 3/24/06