I was initially spec'ing my home with open cell foam, and I received a quote for it.  Then last week, I was informed that the open cell foam needs an ignition barrier, and the ignition barrier is ~40% of the cost of the foam.

I was then told that closed cell foam doesn't need the ignition barrier.  The cost of the closed cell foam is 60% more than the open cell, so with the ignition barrier, the costs are closer.

I have decided to spec closed cell foam, because the cost differential wasn't very high, and then I read some articles about damp roof decks with open cell.

Based on Dana's pricing, I guess my pricing is decent.  Based on the roofing squares, I have about 4400 sq ft of roof deck, and I have some walls separating the garage.  I have been quoted $17k for R38.  That roofing number might include waste.

Is that a good number, and a good decision?

4400 sq ft for $17,000 would only get you ~4" of closed cell spray foam around here (@ $1/inch per sq ft). 4" of closed cell spray foam is good for ~R24 when aged to a typical R value of R6/inch. I would ask them to confirm how many inches of foam they intend to spray for $17K. Often times, spray foam installers make wild claims about how little foam is needed. 4" of closed cell spray foam does not equal R38. To get R38 would require over 6" of closed cell spray foam. This would require multiple spray lifts to get the desired depth because they can't spray that much foam in one pass because of the risk of fire from the heat generated in the foam due to the exothermic reaction.

The installer is saying that it would require 5.75", but they will have 6-7" to make sure that it will be at least 5.75" everywhere.  He said that they will do it 2" at a time.

Is that a good number, and a good decision?

I used a couple inches of closed cell which protects the sheathing and helps seal things up real nice. Then we filled with blown-in. I'm pretty pleased with it.

First, SFAIK there are no fire-rated closed cell spray foams out there- to meet code they ALL spray foams need either an ignition barrier or intumescent paint to be legal in crawlspaces/attics, and a timed thermal barrier in areas that are actually used (even for storage.)

Anyone telling you that it doesn't need an ignition barrier needs to show you both the relevant section of code, and the testing that proves it meets spec without ignition/thermal barriers. My gut feel is that they are either mis-informed or outright lying on that part of it.  But if they can show you in the spec the test data meets R316.6 requirements of the IRC section 316, it might be true.  (That's not to say it's the right way to go though.)

To install closed cell foam without risk of shrinkage cracking/separation or setting the place on fire while curing requires installing it in lifts of no more than 2" at a time.

A single layer of 2" of closed cell foam is sufficient protection of the roof deck to allow you to fatten out the center cavity R to R50 with much cheaper sprayed fiber (damp cellulose or Spider.) It doesn't meet the letter of code for unvented assemblies, but it works even in cold US zone 7 climates. The reason it works is that at 2" closed cell foam is a Class-II vapor retarder, and forms a non-wicking condensing surface. Even 1" of closed cell foam is at the Class-III/Class-II boundary, and sufficiently protective in most climate zones & assemblies. See Table 3 in this document:

See: http://www.buildingscience.com/documents/bareports/ba-1001-moisture-safe-unvented-wood-roof-systems

Note that this is at odds with the IRC prescriptive R, that uses the same values whether the foam is above the roof deck or on the underside, apparently ignoring the vapor retardency aspects of the foam:

http://publicecodes.cyberregs.com/icod/irc/2012/icod_irc_2012_8_sec006.htm

At 3" or more of depth damp sprayed cellulose or JM Spider is a sufficient thermal barrier to meet fire code issues for the foam.

With only ~6" of foam between rafters or joists you have severe thermal bridging of about R7 at the rafter or joist since the rest of the joist is exposed to the full conditioned space temperature. At at a typical 10-12% framing fraction for simple roofs that makes an R38 achieved via 6" of closed cell foam delivers only about R26-R27 performance as a whole-assembly average. A full cavity fill on a 2x12 rafter with 2" of foam and 9.25" of cellulose (or open cell foam) delivers about R37-R38 whole-assembly, due to the rafter fraction now being R13.5 instead of R7, with only the interior edge of the rafter at the conditioned space temp.

Installing the full IRC prescriptive R as rigid foam above the roof deckfor you climate zone and a cheap cavity fill of batts is likely to outperform 6-7" of closed cell spray foam, and will be a more resilient assembly to boot, since there would be at least some drying rates toward the interior. At 6" most 2lb foam is only ~0.2 perms, which means the moisture that was present in the roof deck on the day you install the foam will take years to leave, so it had better be done with material that never got rained on, and had a season of warmer weather to dry.

Lastly, 6" of closed cell foam has a very substantial greenhouse gas hit due to the HFC245fa blowing agent (about 1000x CO2), which means it may not break even on greenhouse emissions over it's lifecycle. Open cell foam is blown with water, and doesn't have this issue. Rigid XPS is blown with HFC134a, at about 1400x CO2, whereas EPS and polyiso are blown with pentane at ~7x CO2. A 2" shot of closed cell polyurethane to adjust the vapor retardency is one thing, but 6" is probably always going to be a net-negative for the planet, with 3x the lifecycle global warming potential of a 2" ccSPF + fiber solution.

The installer has included some icc-es documentation, but I am not sure of everything it says.  I am not sure what my options for different insulation packages might be.  The builder is only offering, bats, open cell, or closed cell.

I think they are installing the insulation next week some time.

Anything less than a full rafter fill is going to severely rob performance of high R/inch insulation, so it it's an either/or, take the open cell foam.

To protect the roof deck from wintertime moisture accumulation, in addition to the intumescent paint, finish it off with spray-applied "vapor barrier latex", which when applied directly to foam (as opposed to the paper facer of wallboard) will be good for about 5 perms, and it shouldn't affect the fire retardency of the intumescent paint. (On wall board vapor barrier latex takes the vapor permeance all the way down to about 0.5 perms, but not when applied to foam.)

With 5-perm paint it will control wintertime moisture accumulation sufficiently to be protetive of the roof deck in climate zone 5 or lower, even zone 6 if you limit the interior humidity to 35% RH @ 70F or lower during the winter months. See Figure 10 of this document showing the moisture content of the sheathing over time on the north facing roof pitch in Minneapolis with both closed and open cell solutions at the IRC prescriptive minimums for foam, and the paragraphs both before and after figure 10.

http://www.buildingscience.com/documents/bareports/ba-1312-application-of-spray-foam-insulation-under-plywood-and-osb-roof-sheathing

The IRC prescriptive minimum for the foam fraction in zone 6 is R25, so you're looking at about 7" open cell foam there to your 10" for R38. At 7"/R25 half-pound foam has a vapor retardency of about 9-10 perms on it's own, but even at 10" the vapor retardency of open cell foam on it's own is still going to be over 5 perms, and it's the vapor latex paint that would be limiting the moisture transfer rates between the conditioned space and roof deck.

Hi Dana,

I think I am in zone 3.  I live in Santa Clara county, california.  I don't know if the requirement is R38, but that is what is being installed.

Posted By ba_icf on 27 Feb 2014 02:46 PM
Hi Dana,

I think I am in zone 3.  I live in Santa Clara county, california.  I don't know if the requirement is R38, but that is what is being installed.

In Santa Clara you don't need to sweat the vapor retardency NEARLY as much, unless you go overboard and are using the highest SRI cool-roof shingles.  CA Title 24 mandates minimum SRI for different CA-defined climate zones, which can be ignored if you provide sufficiently higher-than-code R values.

But even if you DID go with a high SRI roof, you'll be just fine with 5-perm paint on the interior of the open cell foam. Your average wintertime humidity is higher in Zone 3C than in Zone 3A, but your wintertime roof deck temps are warmer too. To meet the full IRC prescriptive using insulation above the roof deck is a measly R5, and that would allow you to use much more air permeable insulation under the roof deck.  You'd probably be just fine even without the spray-applied vapor retarder, but it's cheap insurance.

California is now at IRC 2012 for R-values, so in US zone 3C Santa Clara County, R38 is now code min.

R38 air-tight open cell foam will outperform the typical R38 batts between joists with no top-side air barrier by quite a bit.

Posted By Dana1 on 27 Feb 2014 03:26 PM

Posted By ba_icf on 27 Feb 2014 02:46 PM
Hi Dana,

I think I am in zone 3.  I live in Santa Clara county, california.  I don't know if the requirement is R38, but that is what is being installed.

In Santa Clara you don't need to sweat the vapor retardency NEARLY as much, unless you go overboard and are using the highest SRI cool-roof shingles.  CA Title 24 mandates minimum SRI for different CA-defined climate zones, which can be ignored if you provide sufficiently higher-than-code R values.

But even if you DID go with a high SRI roof, you'll be just fine with 5-perm paint on the interior of the open cell foam. Your average wintertime humidity is higher in Zone 3C than in Zone 3A, but your wintertime roof deck temps are warmer too. To meet the full IRC prescriptive using insulation above the roof deck is a measly R5, and that would allow you to use much more air permeable insulation under the roof deck.  You'd probably be just fine even without the spray-applied vapor retarder, but it's cheap insurance.

California is now at IRC 2012 for R-values, so in US zone 3C Santa Clara County, R38 is now code min.

R38 air-tight open cell foam will outperform the typical R38 batts between joists with no top-side air barrier by quite a bit.

I am not sure what kind of roofing paper was installed.  I have certainteed presidential tl shingles installed because that was in the builder's quote, and to change it something else was not cost effective.   i didn't know about cool roofing.  they frames installed a reflective OSB for the sheathing.  I am not sure of the exact product.

The roofing paper is of no consequence, the solar reflectance index (SRI) of the shingle is.  For roof pitches steeper than 2:12 the statewide mandate is an SRI minimum of 16, which can be met by almost any shingle except dark black. Lighter colors will have a higher SRI, and some moderately dark colors can still have surprisingly high SRI when the materials used for the coloring are highly reflective in the non-visible part of the solar sperctrum.

A short form of the cool roof requirements can be found here. You are probably in CA climate zone 4, which has no special requirements beyone the state minimums. I couldn't find an online SRI spec for any of the Certainteed Presidential TL colors, but none look light enough to have a very high SRI, and if it WAS a high SRI they'd probably be labeled as such, so don't sweat it.

OSB with a reflective foil or paint on the interior side won't improve the thermal performance once foam sprayed on it, but if it's a foil-cladding the foil is a powerful vapor barrier that keeps wintertime moisture loads from accumulating in the OSB.  The thermal benefits of low-E paints and foils are pretty limited, but require an air gap between the foil and the next layer to have any thermal effect at all.  It may still be worth applying vapor-barrier latex over the intumescent paint to limit moisture accumulation in the outermost inch of open cell foam, but with a foil clad OSB it won't make any difference in the moisture content of the OSB.

Posted By Dana1 on 28 Feb 2014 02:36 PM
The roofing paper is of no consequence, the solar reflectance index (SRI) of the shingle is.  For roof pitches steeper than 2:12 the statewide mandate is an SRI minimum of 16, which can be met by almost any shingle except dark black. Lighter colors will have a higher SRI, and some moderately dark colors can still have surprisingly high SRI when the materials used for the coloring are highly reflective in the non-visible part of the solar sperctrum.

A short form of the cool roof requirements can be found here. You are probably in CA climate zone 4, which has no special requirements beyone the state minimums. I couldn't find an online SRI spec for any of the Certainteed Presidential TL colors, but none look light enough to have a very high SRI, and if it WAS a high SRI they'd probably be labeled as such, so don't sweat it.

OSB with a reflective foil or paint on the interior side won't improve the thermal performance once foam sprayed on it, but if it's a foil-cladding the foil is a powerful vapor barrier that keeps wintertime moisture loads from accumulating in the OSB.  The thermal benefits of low-E paints and foils are pretty limited, but require an air gap between the foil and the next layer to have any thermal effect at all.  It may still be worth applying vapor-barrier latex over the intumescent paint to limit moisture accumulation in the outermost inch of open cell foam, but with a foil clad OSB it won't make any difference in the moisture content of the OSB.

It rained quite hard, and some of the sheathing was exposed.  Will the OSB be able to dry?  Very soon after it rained, paper and shingles were installed, and if the foil doesn't let the OSB dry downwards, get the water get out through paper and shingles

With rain-soaked 1-side aluminum-faced OSB covered over with #30 felt and composite shingles it can still dry, but it will take years, not months. The permeance of a shingle & #30 felt layup runs around 0.1 perms, and it can only dry when it's not actively wet, so even the number dew-hours, not just rain wetted hours becomes a factor in how long it takes. The good news is it's not a cool-roof shingle, which would lower the average roof temp extending the drying time dramatically.

IMHO foil-clad OSB is generally a bad idea, but can work under inherently-ventilated roofing like terra cotta tile or metal roofing on purlins. The amount of thermal benefit it buys is pretty tiny even in vented attics, and for shingle or membrane roofs it's a setup for creating a moisture trap.

Posted By Dana1 on 03 Mar 2014 12:00 PM
With rain-soaked 1-side aluminum-faced OSB covered over with #30 felt and composite shingles it can still dry, but it will take years, not months. The permeance of a shingle & #30 felt layup runs around 0.1 perms, and it can only dry when it's not actively wet, so even the number dew-hours, not just rain wetted hours becomes a factor in how long it takes. The good news is it's not a cool-roof shingle, which would lower the average roof temp extending the drying time dramatically.

IMHO foil-clad OSB is generally a bad idea, but can work under inherently-ventilated roofing like terra cotta tile or metal roofing on purlins. The amount of thermal benefit it buys is pretty tiny even in vented attics, and for shingle or membrane roofs it's a setup for creating a moisture trap.

I will  have to ask my GC what he thinks happened versus  the rain because I don't the exact sequence of things, and how they mitigated things.  It is too late now to do much.  The rain happened and the shingles have been installed.