Any limit on the thickness of roof-applied open-cell SPF?
user-4435615
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In northern Climate Zone -4 ( Louisville) it is my impression, from this and other sources, that:
1. There is no upper limit on the thickness of open-cell spray foam that can be applied to the under side of the roof sheathing; and that application will not result in roof rot.
2. There is a potential problem with open cell foam and roof rot in CZ-5 and up but not in CZ-4.
It is not my intention here to address the cost of ocSPF versus other forms of insulation, just the safety of the application from a vapor/rot point of view.
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From a vapor/rot point of view- could the open cell spray foam be sprayed on the top f the ceiling from above? There would also be less board feet of foam and a smaller envelope, and the gables would also be uninsulated. But from that issue of rotting the sheathing, I think this would be a safer enclosure should you have concerns, AS LONG AS YOU INCLUDED: vented ridge and soffits( or gable vents) AND a vapor retarder on the ceiling like OSB with taped seams directly above the sheetrock, or a smart membrane like Intello. Dana would probably have some input on this that makes sense.
I would prefer to keep my insulation at the ceiling level for lots of reasons but it may be necessary to have an Unvented attic. Hence the question about the potential thickness of the insulation on the underside of the roof sheathing.
There's a limit to how much can be installed in a single lift (about 5.5-6"), but you need at least 10" for it to be even as vapor tight as standard latex paint, and it needs to be if it's going to fully protect the roof deck in zone 4. With 15" it's even better, but probably still not enough to be fully safe without a class-II vapor retarder on the interior side, or a smart vapor retarder.
Download a copy of BA-1001, and take a peek at Table 3, the "Full-depth ocSPF" column, and all of the zone for rows:
http://buildingscience.com/documents/bareports/ba-1001-moisture-safe-unvented-wood-roof-systems/view
R38 (about 10-11") is clearly not vapor retardent enough to be fully protective for a zone 4 climate, not even close.
But with a smart vapor retarder on the interior it's pretty safe even at R20.
It's pretty much OK in zone 3 & lower, as long as you don't use a "cool roof" shingle.
Ted, you seem a little holding back on info. Why might it be necessary to have a conditioned, unvented attic? Do you plan on having the air handler or furnace up there? Ducts? More info might help people solve your dilemma. Because if you want you furnace up there you might need thermal resistance paint on the foam and the price of that stuff will crush you. That is why more info is never a bad thing. Dana, if no ducts are needed up there, would a oc foamed CEILING work at R38 numbers (10-11" oc foam) with OSB below as a vapor retarder, along with vented soffits and ridge in Ted's zone? Assuming it can be sprayed from above via a hatch or open gable end or something.
Ref. Paragraph #3
Thank You for the reference, which I have since read, and perhaps I'd read before. I, however, came to slightly different conclusions. Perhaps you can help clarify. The article was written in 2010 and precedes the update in recommended levels of roof/ceiling insulation from 38 to 49 for CZ-4. So presumably, at this time, "Full-depth ocSPF" would be somewhat thicker. But, ignoring that for the next part of the conversation:
My roof will be asphalt shingle with high exposure and NO shading in CZ-4A.
I have included attachments (excerpts from that article, BA-1001) for ready reference.
Attachment number One is Table 3 which you referenced and contains the pertinent information. It shows that the risk is Class 3 for High moisture interior conditions and Class 2 for Normal interior moisture conditions. I believe that I will maintain comfortable, Normal, moisture conditions with my intended continuous balanced ventilation system. Thus, at these insulation levels, placing me in Class 2.
Attachment number Two is an excerpt from the same article describing Class 2 in detail and presents slightly differing opinions (ASHRAE vs. Straube). That leaves me with a cautionary feeling about Class 2.
Attachment number Three is at the end of his Conclusions and presents his FIELD EXPERIENCE regarding moisture content in roof sheathing. His final sentence is: "However, Zone 4A should be a safe location for such a system except for High interior humidity or low solar heating of the roofing". This gives me a distinctly better feeling about being in Class 2.
In previous readings at GBA and Building Science Corp websites about the use of ocSPF foam and roof rot, I believe that similar conclusions were expressed, i.e. that though there were definite problems in northern CZs, and some instances of homes in northern CZ-5 having problems, us Zone 4 folks are safe using ocSPF on the underside of roof sheathing. Though I am now unable, readily, to find or name the references.
Would any of the above affect your opinion ?
Would changing the level of ocSPF to R-49, instead of R-38, as referenced in BA-1001, change your opinion ?
I'd like to rephrase and expand my original poorly phrased questions:
1. Can some level of full thickness ocSPF be safely used, avoiding roof rot, in my area - northern
CZ-4A ?
2. What would that thickness be ?
3. Is there any limit, done correctly in lifts, to the amount of ocSPF than can be applied to the underside
of roof sheathing ?
Ted, why dont you look into this open cell product which is a medium density product. Dana is actually the one who shared this link with me a few weeks ago. http://dmsecureweb5.com/idthermal/PDF/MDR200BroID.pdf
You may be able to have a lesser thickness with this product to achieve the desired R value and vapor permeance. ???
Joe,
Re: paragraph 6.
Thanks for the reference.
I have spoken to our local Icynene contractor at length some time back. The number of inches of MD-R-200 that I would need to reach a satisfactory R-value at the roof sheathing would obviously be less at R-5.2/inch for that medium density ccSPF than for R-3.75/inch ocSPF. Though I don't currently recall the cost differences, It seems to me that the $/R was CONSIDERABLY higher for the MD-R-200. I believe that he recommended it for those circumstances where there was a higher amount of insulation needed in a smaller cavity.
I believe that in my problem above, I have lots of space in which to apply insulation to the underside of the roof sheathing and would be looking for the most economical way to do it. It also seems to me that ocSPF would likely be the way to do that as long as the vapor permeability allows me to do so without risk of damage to the roof. My discussion with Dana relates to that.
Also, if I were to go the vapor impermeable closed cell route, I'm not sure why I would use MD-R-200 at R-5.2/in. as opposed to their usual higher density R-6.5/in ccSPF ? Unless it was a cost factor, but then again both of them are more costly in $/R than the ocSPF.
Joe,
Ref. paragraph 4:
I was trying to be brief in my question and address only few questions.
I'm in the home design stage. I've had to go to a slab-on-grade. I'm trying to preserve my goal of reaching Net Zero by achieving very low infiltration levels; high levels of insulation; wide overhangs; high quality windows and doors; controlled humidity, PV, etc. in this Mixed-Humid northern CZ-4 climate. I will be using balanced ventilation. I'm considering the use of various Conditioning combinations that may require that I move from a Vented attic design to an Unvented one. My Conditioning choices boil down to a 45 EER, 5.1COP 7-speed GSHP; multiple single ductless mini-splits to achieve a zoned effect; a ductless multi-zone mini-split; or a Chiller with up to 7 zones. No furnace. Each has variables with respect to cost; ability to produce hot water; efficiencies for heating, cooling, and hot water; need for defrosting; load capacities; requirements to have ducting an area above the ceiling; etc.
In any event, though I appreciate your willingness to delve into the complete picture, I think that it's too involved for this format. I will have professional help in solving some of these problems but I like to have some idea of what I'm talking about with that professional when the time comes.
My initial question, though inartfully phrased, was to answer a specific question of how much ocSPF I can apply to the underside of roof sheathing and what risk of roof rot might result.
Thanks!
Closed cell foam is not vapor impermeable, not even close. But at R49 it's tighter than optimal.
The R6.5 /inch Icycnene goods is 0.9 perms @ 1.5" (R12.35). It's vapor permeance at R49 is then about 0.9 x (12.35/49)= 0.2 perms, which is approaching Class-I vapor retardency (= dries never). The R6.5/inch goods are all blown with HFC245fa, which among other joys, has a 100 year global warming impact about 1000x that of CO2. It's overkill from a vapor retardency point of view, and it's planet-kill from an environmental impact point of view.
http://www.icynene.com/sites/default/files/fb/MD-C-200%20-%20USA%20Technical%20Product%20Data%20Sheet.pdf
There are a couple of reasons for going with MD-R-200 rather than a higher R/inch foam:
*MD-R-200 is about 1.3 perms 3" (R15.3) so at R49 it would be about 1.3 x (15.3/49)= 0.4 perms about twice the drying rate of the other stuff, and somewhere in the middle of the class-II vapor retardency. That's sufficiently vapor tight to be protective, but sufficiently vapor open that it can dry at a reasonable rate.
**MD-R-200 is blown with water, which has a very low environmental impact.
http://foamworksinsulators.com/pdf/ICYNENE%20MD-R-200T%20Technical%20Data%20Sheet.pdf
Half pound open cell foam like LD-R-50 has a vapor permeance of 17 perms @ 2" (R7.4), so at R49 it's vapor permeance would be 17 x (7.4/49)= 2.5 perms, which isn't terrible in a zone 4 climate, but it's barely more vapor retardent than standard latex paint. It's a Class-III vapor retarder, and would not meet code on vapor retardency on it's own.
http://www.icynene.com/fbdocs/LD-R-50-USAProductInformationMar2013.pdf
To get it down to true Class-II vapor retardency of 1.0 perms would take about R125, which would be a bit ridiculous (and expensive.)
There are 0.7lb foams out there that can come in at more optimal vapor retardency at R49. Icynene's "Classic Plus", isn't one of them. At 20.7 perms @ 2" (R8), and would be about 20.7 x (8/49)=3.3 perms
http://www.icynene.com/sites/default/files/US%20content%20uploads/Classic%20Plus%20-%20USA%20Technical%20Product%20Data%20Jan%202015.pdf
If the MD-R-200 is too expensive, a compromise all-foam solution would be to put 1-2" of the nasty crud on the roof deck (R6.5- R13) to protect the roof deck, with 10-11" of half-pound foam under that. There would be some wintertime moisture accumulation at the colder layers of the open cell foam, but it would dry quickly. But the wintertime accumulation in the moisture-susceptible roof deck itself would be negligible through the 0.45-0.9 perm closed cell foam.
Dana,
Thank You. That was VERY helpful.
Did you have a chance to review paragraph 5.
Thanks Again!
I don't think they will spray the half lb. foam more than R55 or so, either. The mid density foams seem to be up and coming as the formulations are an attempt to mitigate the exact concern which you have, Ted. No one will guarantee you safety in regards to your roof deck should you put an open cell product up there. I am in Northern NY zone 5 and am going through similar evaluations. There are so many ways to skin a cat and a lot of it depends on the competency of the trades you have to work with in your area. In my own area there is a dirty of people doing dense pack cellulose from what I have found, but many spray foamers. They certainly have varied opinions as well.