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How much closed cell spray foam do I need?

I'm building a home in northern Wisconsin on the southern edge of zone 7. I'm planning on R40-45 in the walls (double stud) and R60-65 in the ceiling (parallel chord trusses). I had planned on using dense pack cellulose but there are lots of valleys and no good way to ventilate them. So I'd like to have "the appropriate amount" of closed cell spray foam applied to the inside surface of the exterior sheathing (1/2" walls/5/8" roof OSB) and fill the rest of the wall/ceiling cavities with dense packed cellulose. I assume the R value of the spray foam needs to be enough to prevent condensation in the cellulose - no interior vapor retarder. Also using this approach would it be wise to not use synthetic roofing underlayment, i. e. DiamondDeck?

Asked by Roy Brohaugh
Posted Wed, 11/14/2012 - 13:52


9 Answers

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You can find the answer in this article: How to Build an Insulated Cathedral Ceiling.

You're in Climate Zone 7, so you need at least R-30 of spray foam against your roof sheathing -- which means between 4.5 and 5 inches of closed-cell spray foam. Note that this isn't enough R-value to meet minimum code requirements -- you'll also need some cellulose to reach the minimum code goal.

Make sure that you have a plan to net the roof trusses in a way that allows for a smooth cellulose installation.

Answered by Martin Holladay, GBA Advisor
Posted Wed, 11/14/2012 - 14:08

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With typical asphalt shingle roofing or dark metal your actually pretty safe with just 2" of closed cell foam, which is (barely) permeable enough to let the roof deck dry, but sufficiently vapor retardent to limit the wintertime moisture accumulation. As a non-wicking condensing surface the cc foam won't wick moisture to the roof deck, and the cellulose can buffer quite a bit of moisture without losing performance. (More dry-lbs of cellulose= greater the buffering capacity at any density, but dense packing is important to minimizing the moisture accumulation rate.)

See table 3 of this document:

(note the International Falls case.)

But putting even three inches of rigid iso above the roof deck would allow you to get by with a 1" shot or cc on the interior, since it moves the roof deck into a warmer layer of the stackup, with much fewer seasonal hours below the dew point of the interior air.

Air sealing between the conditioned space and the cellulose has to be taken pretty seriously though, as well as dense packing, since convective currents can transport a lot more moisture than vapor diffusion through latex paint.

Answered by Dana Dorsett
Posted Wed, 11/14/2012 - 15:09

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Your answer differs from my answer, obviously.

Here's one difference: my suggestion complies with section R806.4 of the 2009 IRC, while yours does not. Here's a link to the code:

Answered by Martin Holladay, GBA Advisor
Posted Wed, 11/14/2012 - 15:28

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I totally get it that the IRC disagrees with Lsiburek, Straube t & co. on this, but WUFI is a pretty good simulation tool, with an excellent record of predicting the real-world performance on moisture levels.

It's really a quibble about the placement of the vapor retarder in clause 4. At 2" all HFC blown closed cell foams are class-II vapor retarders, and would be at least somewhat protective of the roof deck, but not necessarily the fiber insulation. Since it limits the adsorption rate of the roof deck as effectively as if it had been placed on the interior side, it then becomes only a question of whether the fiber becomes too wet in that configuration. The fiber they used in the foam/fiber hybrid stackups (1.8lb Spider fiberglass) has almost zero buffering capacity, unlike cellulose, yet the simulations look pretty good for 2" foam + Spider. It's not a stretch to assume that it would look even better with 2" foam + cellulose.

I personally prefer putting half or more of the R above the roof deck as EPS or iso per Table R806.4, which doesn't involve ANY interior SPF. But there's no science that says interior foam needs to hit the prescriptive R values in Table R806.4, which only specifies insulation on the exterior (not the interior) of the roof deck. And the difference in placement matters.

The whole point of RR-1001 is that by putting a modest semi-impermeable layer of foam on the interior it's vapor retardency (rather than it's R value) is is protective of the roof deck, since it 's vapor retardency slows the adsorbtion rate in the wood layer to something reasonable. But at the 4-5" required to hit the IRC prescription it is SO impermeable that the roof deck is then in a "dries never, or maybe when baking under the August sun provided you don't have anything less permeable than #30 felt" situation. It doesn't take on winter moisture, but any moisture that gets in (even the moisture in green wood, or truss elements that took a rain-soaking on site during construction) could take years to get out when sandwiched between low-perm roofing and lower-perm foam.

If foaming the interior to the IRC prescriptive levels, use 6" (R5/inch) of water-blown Icynene MD-R-200, which would still be over 0.6 perms (about the permeance of 2" of HFC blown cc foam) which still gives the roof deck a reasonable drying rate toward the interior. This is 3x the drying capacity that you'd get with 4-5" of an HFC-blown closed cell polyurethane, and 1.5-2x the drying rate you'd get through kraft facers used on batts.

Answered by Dana Dorsett
Posted Wed, 11/14/2012 - 16:38

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Thanks, Martin and Dana. I haven't read the Building Science report yet, but I will soon. Then I guess I need to chose between the two approaches although I don't see myself using open cell foam or adding exterior insulation to the roof deck at this point. If I had to forge ahead with any plan without pro input at this point I would have the ceiling sprayed with 6" closed cell foam and the walls with 4" closed cell foam and add dense packed cellulose. And have the roofer replace the DiamondDeck underlayment with regular 15# felt.

Answered by Roy Brohaugh
Posted Thu, 11/15/2012 - 12:35

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If you're bound by IRC code Martin's approach does in fact meet the letter of all recent versions of the IRC, whereas the BSC approach does not, but WUFI sims are generally pretty good.

Going with the full IRC-compliant approach I can't stress strongly enough how important it is to have a drying path > 0.5perms in at least ONE direction, which would limit you to only the MD-R-200 product for the cc foam in the roof.

Heavier 30# felt is typically under 2 perms, it might hit 3 when fully saturated (like after spending months under a snow load), the 15# goods might hit 7 when wet, but with lapped layers of sub-1 perm asphalt shingles above there's very little ventilation for moving that moisture out on the exterior side, and the perm rating of the stackup is under 0.2 perms. Reference:

And basically any time the roof is wet from snow rain or dew, there's nearly zero (or even negative) drying going on, since it brings the temp of the wood down to the dew point (or lower) of the outdoor air.

There are many many existence proofs where the miniscule drying capacity is sufficient- nothing bad happens. But double-vapor barriers always carry a risk, which is why the higher-perm 2lb Icycene would be the product of choice if you go that route. The ASTM E96 perm rating can be found on p.6 of the spec:

With homogenous materials like foam, the permeance drops in inverse proportion to thickness, so if it's 1.3 perms @ 3", it'll be around 0.65 perms @ (2x3=) 6", which is the thickness it takes to hit the prescriptive R30.

Most closed cell foam on the market run ~1.2 perms @ 1" and it takes 5" to hit the prescribed R30, which puts it under 0.25 perms- tight enough to be a moisture trap at the roof deck layer when you have low-perm roofing above it.

The walls are much less of an issue, since A: They're never buried in snow for 2-3 months at a time and B: They won't have a super-low-perm siding or paint, and can dry to the exterior at reasonable rates. But even in double-stud stackups a rainscreen gap between siding and sheathing buys a whole lot o' resiliance to the assembly.

Answered by Dana Dorsett
Posted Thu, 11/15/2012 - 17:06
Edited Thu, 11/15/2012 - 17:07.

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Most experts accept the fact that it is unnecessary for roofs to dry upward. The vast majority of roofing materials and roof assemblies provide very little upward drying; yet they work.

More important than upward drying is choosing an insulation type that limits water vapor transmission from the interior to the roof sheathing.

Asphalt felt is a smart vapor retarder. #15 asphalt felt has a permeance of only 5 perms when dry, but a much higher rating of 60 perms when wet. I think that the permeance of wet #30 asphalt felt is higher than you state, but I don't have the number handy.

Answered by Martin Holladay, GBA Advisor
Posted Thu, 11/15/2012 - 17:27
Edited Thu, 11/15/2012 - 17:28.

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We agree that upward drying isn't happening and thus not an important consideration, but indpendent of the permeance of the felt,with the overburden of lapped asphalt shingles, the permeance of the stackup drops to well under 0.2 perms, according to Lstiburek's modeling and testing:

Limiting the vapor transmission of the underside foam too much carries some risk, and 5" of typical closed- cell goods drops the interior permeance to nearly the vapor tightness range as the felt & shingles, which is a "dries never" situation- any moisture that gets in (or is built-in) could take years to get out.

It's a classic double vapor-barrier stackup, which can work, but only if nearly perfect, which is a lot to ask.

But with 6" of the water-blown 2lb Icynene you'd be a bit over the half-perm range, slow enough to limit the wintertime peak moisture content in the roof deck, but fast enough that it can dry seasonally. The wintertime upload is gone before May, whereas with 0.2-0.25 perm foam it takes more than the summer, and incidental water leakage never gets out. A WUFI comparison between 0.6 perm and 0.2 perm foam layers starting with a high initial moisture content would probably make the difference pretty clear.

Answered by Dana Dorsett
Posted Fri, 11/16/2012 - 13:16

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Looking again at Straube et al's WUFI simulations in Tables 3 & 4:

This was 1.2perm/inch foam, and modeled on the north-facing pitch of the roof. In the zone 7 International Falls case even a FULL CAVITY DEPTH of cc foam proved to be fully protective under a reflective metal roof(!), yet with dark shingles even 1" (1.2 perms) was enough (as was 2" /0.6 perms, as was 4"/0.3 perms and whatever the full depth fill ended up being. )

Apparently a darker roof aids drying speed (in both directions) by increasing the daytime temperature peaks of the roof decking, even through very low permeance roofing & foam. But if 0.6 perms is enough (the 2" case)

In Table 4 they compare what happens when the conditioned space is maintained at the high-humidity profile vs. normal humidity proflle, when air is allowed to flow through the fiber layer. Under those worst-case conditions it takes 4" (0.3perm) foam to get there even under the normal humidity profile, and it's still in the danger zone with a high-humidity profile. This highlights the primary importance of air-tightness between the fiber layer and the conditioned space.

But the fact that 4" of foam gets you there even in the semi-worst-case condition and that it requires a darker roofing material means there's some work to be done on tuning up the next rev if the IRC. Foam-R under the roof deck really IS a different case than R value above the roof deck, for which the case for the prescriptive values pretty much done.

Answered by Dana Dorsett
Posted Fri, 11/16/2012 - 15:24

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