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Efficiency per inch for 2-lb. closed-cell spray foam

With most things with layers, each layer provides a different level of efficiency which you can then take this and apply cost to find your best number of layers to go. Does anyone know how many inches of 2# closed cell spray foam is the optimum? I talked to one contractor that said that the first 2" of the foam provide 90% efficiency. Does this sound right? Or should you be looking at going 4" or more? How does one determine how much is enough and the rest is overkill?


Asked by Greg Thornton
Posted Tue, 05/13/2014 - 14:41
Edited Tue, 05/13/2014 - 15:19


7 Answers

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Every inch of closed-cell spray foam provides exactly the same R-value per inch (between R-6 and R-6.5 per inch).

Every time you double the thickness of the insulation (that is, double the R-value), you cut the rate of heat flow in half.

Building codes list the minimum insulation levels required for different types of building assemblies in different climates. It's often a good idea to exceed minimum code requirements if you can afford to do so.

Because spray foam insulation is the most expensive type of insulation on the market, spray foam salespeople find it hard to justify the high cost of their product, and they often argue in favor of thin insulation. Less expensive types of insulation (like cellulose) can be installed at a much higher R-value before these cost-effectiveness considerations enter the picture -- so cellulose is generally preferred in locations where it can be installed.

Answered by Martin Holladay, GBA Advisor
Posted Tue, 05/13/2014 - 15:08
Edited Tue, 05/13/2014 - 15:15.

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I have a brick cape with a sided dormer out the back creating a second floor. My ceiling (which is a 12/1 pitch. pretty flat) follows the 2x6 rafters with little to no insulation from what I can tell. Thus creating a pretty hot second floor that my AC unit can't keep up with. I'm looking at fixing this issue so the second floor is usable during the summer. I was looking at re-pitching my roof creating a cavity to be able to insulate better. Do you believe this should fix my problem? Typically in the summer there is 10-15'F delta between the first and second floor temperatures.

Looking for a resolution


Answered by Greg Thornton
Posted Tue, 05/13/2014 - 15:21

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If your roof assembly has little or no insulation at the moment, then clearly adding insulation will help. The higher the R-value of your installed insulation, and the better your job of air sealing, the better your results will be.

The 2009 IRC requires a minimum of R-38 insulation for roof assemblies in Climates Zones 4 and 5, and requires a minimum of R-49 insulation for roof assemblies in Climate Zones 6, 7, and 8.

For more information on different ways to insulate this type of roof, see How to Build an Insulated Cathedral Ceiling.

Answered by Martin Holladay, GBA Advisor
Posted Tue, 05/13/2014 - 15:38

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Closed cell foam has a high R/inch, but when put between rafters or studs it does nothing for improving the performance of that R1.2/inch framing wood. In a 25% framing fraction wall (typical for 16" o.c. framing) the center-cavity R of a 2 x 6 wall might hit R32-ish with 5" of closed cell foam, but the severe thermal bridging of the framing yield an average or "whole-wall" R of about R16 (and that's including the R-value of the wallboard, sheathing and siding. With rafters you'd have a lower framing fraction - say 15%, yielding about an R20 whole-assembly R, but that's still nowhere near the ~R29-R30 whole-roof performance that you'd get with R38 cellulose or rock wool in 2x12 rafters at the same framing fraction.

The BEST thing you could do with 2x6 rafters would be to apply the closed cell foam budget to installing 5-6" of EPS or polyiso above the roof deck and filling the 2x6 rafter bays with ~R20 ish fiber insulation. With 5" of EPS (R21-ish) the rigid foam layer alone would equal or exceed the performance of 5" of closed cell polyurethane bridged by rafters, and at a 15% framing fraction the R20 between the rafters adds another ~R16 for a whole-roof R of about R37, which BEATS an R38 between joists or rafters solution.

The installed cost of closed cell polyurethane is about 17 cents per R per square foot, compared to about 10 cents R-foot for EPS (with some installation cost and scrap rate.) In the end it comes out comparably per square foot of roof area, but since the EPS thermally breaks the rafters, it's nearly twice the thermal performance.

Clearly this is best done as new construction, or when the roofing needs replacement.

Closed cell foam has a heavy climate footprint, due to the fact that almost all cc foam in the US is blown with HFC245fa, which has a global warming potential of about 1000x CO2. If the shingles are in good shape and this is an interior-side only solution, 1-2" of closed cell foam against the roof deck as a vapor retarder and non-wicking condensing surface with the rest of the cavity filled with blown insulation (or compressed batts, if you must) and NO interior side vapor barrier yields a whole-roof R that is comparable to 5" of ccSPF between rafters and is sufficiently protective of the roof deck sheathing from interior side moisture drives. See:

And with that stackup, when it's time to re-roof it's still safe to add the exterior foam.

Answered by Dana Dorsett
Posted Tue, 05/13/2014 - 17:00

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Dana, can you email me at and I can respond back with images of the roof "re-pitching" that I'm looking at to see what you think? Thanks Greg

Answered by Greg Thornton
Posted Wed, 05/14/2014 - 05:51

Helpful? 0

I have a brick 1960 cape that has a sided dormer out the back that I moved into 2 years ago. Here is a picture of something comparable with a dormer out the back, but I do not have a porch or addition on the first floor.

In the summer time the upstairs just roasts on heat. So last summer I started trying to resolve this issue. Fixing around the dormer wasn’t available at the time so the pitched front for me is just a walk in attic that had little (R-13) insulation in the knee wall and in the floor of the attic. I removed it all and had a company come in and put down 2” of 2# closed cell spray foam on the floor and in the knee wall. I then put plywood down on the floor and had them dense pack the cavity between the plywood and spray foam. This helped some during the winter but still needed some added help.
- The roof on the dormer is a flat roof with a 1/12 pitch.
- The rafters I can see in the attic and in the garage are all 2x6, so I’m assuming that the rafters in the flat roof are also 2x6 with little to no insulation.
- The roof should be replaced within the next 3-5 years but with the second floor not being usable, this is moving the roofing job up.
- The dormer created 3 bedrooms and a bathroom for a second floor
- The ceiling in the dormer I believe is attached directly to the rafter as my ceilings are pitched also.
In pricing roofing, I’ve been told that a flat roof is pretty pricey so I thought of designing something like this, where the front roofline is continued up to create a new peak of the roof and a pitched roof out the back that can be shingled and also give me an area that I can insulate

- Area 1 would be for building up my interior walls so that my ceilings would no longer be pitched but achieve a flat ceiling
- Area 2 would be my new roof design, creating a sufficient area that I can properly insulate
- I would also like to be installing ceiling fan to help with air movement, so I thought creating a tray ceiling to recess it a little higher, not taking up head room as my highest height of my ceilings is 93.5”
- The walls I was looking at ripping out the drywall and insulating from the inside out as part of the ends are covered by brick and would prevent me from doing a full insulation job.

Here is a drawing of what my second floor looks like. I’ve added in to visualize what tray ceilings would look like.

My questions are the following:
- Is this what I should be doing to fix my temperature issues?
- With insulating my knee wall and the floor of my attic with spray foam, should I be looking to stay with spray foam? And how much? Or if something different, what and how much should I be using?

1.jpg 2.jpg 3.jpg
Answered by Greg Thornton
Posted Wed, 05/14/2014 - 11:30

Helpful? 0

On a new roof you're free to use rigid foam above the roof deck, using spray foam only for air-sealing, then filling the rafter bays with fiber.

As long as the ratio of foam/fiber meets or exceeds the IRC prescriptive levels it'll work just fine as-is.

eg: In climate zone 5 you need about R20 out of R50 (R49 is the prescriptive code minimum) to be above the roof deck, so that's 40% / 60% ratio of foam/fiber. (To beat R20 with EPS takes 5".) Further south in climate zone 3 you're looking at R5 out of R38 (the prescriptive code minimum) for a ratio of 13% / 77%. If you went with 2x10 rafters and used R38HD batts tight to the roof deck that means you'd have to bump up the exterior side insulation to R6 or more. A higher foam/fiber ratio is always better in terms of moisture control at the roof deck AND thermally breaking the higher heat conductivity of the rafters themselves. From a dew point control point of view it's the ratio and not the total R that matters.

Answered by Dana Dorsett
Posted Thu, 05/15/2014 - 18:34

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