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Ineffectiveness of increasing R-value with more than 4 in. of closed-cell spray foam?

I have been told a number of times that using more than 4" of cc spray foam (in a wall or roof assembly) has little value at increasing r-value. In other words, does using more than 4" not have a linear increase in r-value? The reason I ask is b/c I need the recommended r-35 against the bottom of my roof sheathing (with blown-in cellulose in the remainder of my cathedral truss) to prevent the roof sheathing to dip below the dew point and create condensation. Any thoughts? Thanks.

Asked by Matthew Michaud
Posted Feb 2, 2013 8:27 PM ET
Edited Feb 4, 2013 8:55 AM ET


15 Answers

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Every inch of insulation adds as much R value as the inch that came before it. Foam salesmen have a rationale for using less of their product, but it doesn't really bear explaining IMO.

I'm not clear on your proposed installation. Are you going to use both foam and cellulose to insulate the same surface, i.e. and flash-and-fill hot roof? Or, is the cellulose not in contact with the foam?

Answered by David Meiland
Posted Feb 3, 2013 11:57 AM ET


Quoting Martin Holladay:"doubling the thickness of insulation from 6 inches to 12 inches will cut the heat flow through the assembly in half. "

Answered by John Brooks
Posted Feb 3, 2013 12:19 PM ET


Thanks David/John.

David-My unvented cathedral roofing assembly is as follows: metal roof, air space, synthetic felt, advantec, cc spray foam against under side of roof sheathing (R-35 for Zone 8, ~6 inches), and the remainder of the space (~14") filled completely with blown-in cellulose, osb (to support weight of cellulose above), taped sheetrock.

Answered by Matthew Michaud
Posted Feb 3, 2013 2:12 PM ET


I'm curious, why not 30# ASTM D226 felt?

There is another thread recently with an assembly similar to yours.... less foam, regular OSB, synthetic underlayment, asphalt shingles... possible but unconfirmed issues with trapped moisture.

Answered by David Meiland
Posted Feb 3, 2013 3:57 PM ET
Edited Feb 3, 2013 3:58 PM ET.


Yes, I'm using that felt.

I created my assembly based on the following article: "How to Build an Insulated Cathedral Ceiling" article by Martin Holladay, GBA Advisor:

Answered by Matthew Michaud
Posted Feb 3, 2013 7:57 PM ET


Matthew; Yes R-value does increase incremental with the amount of inches. I think what the sales personnel you are consulting with are trying to say is that once you achieve more than 3" of closed cell foam your cost per return ratio (amount of foam inches vs. cost) reaches a point of diminishing returns very quickly. Which is true.

With that being said you still have to abide by local code and achieve your R-35, which is approx. 5 to 5.5 inches cc foam depending on product used.

As for the condensation that will be a non issue when using foam on a closed roof assembly due to lack of dew point being formed with foam.

Hopefully I was of some help. More info. can be found here http://ctsprayfoaminsulation.com/

Mike L.

Answered by Michael Lieto
Posted Feb 3, 2013 11:35 PM ET


Every inch of closed-cell spray foam that you install has an R-value of about R-6.5 per inch.

The first inch you install has an R-value of R-6.5.

The 8th inch you install also has an R-value of R-6.5.

If you are aiming for R-35, you need about 5 1/2 inches of closed-cell spray foam.

If you stop at 4 inches, all you will get is R-26.

Answered by Martin Holladay
Posted Feb 4, 2013 9:38 AM ET


What foam installers are really saying between the lines is that any thickness beyond 4" may cost more than 25+ years the energy savings of that "extra" thickness buys you.

At 17-18 cents/R-foot it's about the most expensive way to insulate a roof to code levels. Cellulose is usually less than 5 cents/R-foot, installed price, which means you can go much higher-R before the crossover in the cost-effectiveness curve. Codes are mindful of long term cost effectiveness using standard methodologies for meeting code minimums. But meeting code at triple or quadruple the cost the time periods over which a present-value financial analysis breaks even are far longer, or even never.

At 3-4" the foam itself is a strong class-II vapor retarder, which is more than enough to protect the roof deck from interior moisture drives. (At 8" it's nearly a class-I vapor retarder.) The issue then becomes wintertime moisture accumulation in the cavity below the foam. With cellulose as cavity fill and air-tight methods on the interior gypsum the cellulose itself is capable of buffering the amounts of moisture getting in via diffusion through standard latex paint.

Using one of the "smart" vapor retarders (eg. Certainteed MemBrain) on the interior is a lot cheaper than a few more inches of oam, and would both limit the amount of moisture the cellulose has to buffer, and would let the assembly dry toward the interior far faster than if using vapor-retardent latex as the interior vapor retarder.

More here: http://www.buildingscience.com/documents/reports/rr-1001-moisture-safe-u...

Answered by Dana Dorsett
Posted Feb 4, 2013 12:18 PM ET


Thank you all for your input.

Dana-How would you suggest to control the moisture build-up at the roof sheathing level if no foam was used? From my understanding, I'd have to resort to a vented cathedral roof if I were to just fill the space with cellulose. Does the "smart" vapor retarder just allow for prevention of winter time moisture from rising into the roof and allow summer moisture to escape from the interior portion? How does it play into the entire picture from roof sheathing-down. Thanks.

Answered by Matthew Michaud
Posted Feb 4, 2013 7:40 PM ET


One alternative to spray foam -- an alternative that is usually cheaper -- is to install one or two layers of rigid foam on top of the roof sheathing.

More information here: How to Build an Insulated Cathedral Ceiling.

Answered by Martin Holladay
Posted Feb 5, 2013 8:06 AM ET


I wasn't suggesting "no foam", but I am suggesting LESS foam than specified in the most recent IRC. At 3" most closed cell foam is running about 0.3-0.4 perms, which is as-tight or tighter than asphalted facers on batts, but not as tight as poly. At 3" the roof deck can still dry during the warmer periods, but it's pretty slow. At 6" the closed cell foam is half-that, and you may not have enough warm months to purge any substantial moisture. If any moisture gets into the roof deck (say, from a minor roof leak) it never gets out, since the permeance of the roofing materials is even lower, and drying MUST be toward the interior.

Martin's recommendation for rigid foam above the roof deck is preferable to putting spray foam on the interior, because it keeps the roof deck warmer, and the "drying season" thus begins sooner, ends later, than when the insulation is inside the roof deck. In colder snowy areas using 2x furring above the foam through-screwed to the rafters with timber-screws 24" on center, with a cheap non-structural 1/2" OSB nailer deck for attaching shingles, etc allows the nailer deck and shingles to vent into the furring cavity. The cavity allows the nailer deck some drying capacity even under a snow load, and lowers the temp of the shingles to mitigate against ice damming.

While many high-R builders are putting the nailbase directly against the foam (though screwing the nailbase to the structural roof deck) and seeming to get away with it, shingle-leaks can go un-detected for years in that stackup which will rot the nailbase over time, and since the nailbase is what's holding the foam sheets in place in those roofs it's not the most resilient assembly ever invented (though cheaper than a furring vented nailbase approach.)

Smart vapor retarders are class-II vapor retarders when the air is dry (which it usually is in a cold winter), which limits the rate at which moisture gets into the roof deck. When the roof warms up and starts releasing whatever moisture it picked up, the air in the cavity becomes more humid, which makes the vapor retarder much more vapor open- the assembly dries at about 3-4x the rate at which it accumulated the wintertime moisture. The drying rate of the roof deck is limited primarily by the drying rate of the interior paint during the peak-humidity in the cavity, but 3-5 perm standard latex is still pretty vapor-open, sufficiently open that it dries quickly toward the interior.

Answered by Dana Dorsett
Posted Feb 5, 2013 12:52 PM ET


Thanks for the responses.

Dana-if using foam sheets on top of the roof sheathing, should I still be aiming for some spray foam underneath (the ~3" you mentioned) to reach my recommended R-35 for Zone 8, or can I bring the blown-in cellulose directly up to the roof sheathing without the spray foam? Thanks for your time.

Answered by Matthew Michaud
Posted Feb 6, 2013 9:05 PM ET


If you are able to install R-35 foam above your sheathing, you are all set to install cellulose up to the sheathing. Six inches of polyiso would do the trick.

Answered by Martin Holladay
Posted Feb 7, 2013 6:16 AM ET


Thanks. So as long as I have R-35 on the above, below or in combination, I'm all set?

Answered by Matthew Michaud
Posted Feb 7, 2013 6:40 PM ET


I would not put foam on both sides of the sheathing.

Answered by David Meiland
Posted Feb 7, 2013 9:05 PM ET

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