Closed-cell foam on underside of roof with no AC: Do I need to vent?
I am working with a local energy efficiency contractor. They are replacing my old roof (which badly needs replacing) with a new roof. They will be spraying five or so inches of closed cell foam directly against the decking. This will fully seal my attic. My question is, how does this work if you do not have air conditioning in the house, other than window units in the bedrooms and one in the living room.
This is a two-story house with a quasi-finished basement, a first floor with living room, dining room, and kitchen: second floor with three bedrooms and two baths, and a walk up attic, half of which is unfinished, and half of which is a finished room that is boxed inside the 12-12 pitched roof.
My concern is that, because heat rises, the attic will gather heat. How do I get that heat out of the attic if it has no vents? Also, how would I integrated a whole house fan into this design?
Any help would be greatly appreciated.
Thanks so much!
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Replies
Rob,
If you install closed-cell spray foam on the underside of your roof deck, creating an unvented attic, there is no reason to believe that your attic will be warmer than a vented attic. If anything, it will probably be cooler.
Your attic will be within 5 or 10 degrees of your living room temperature. If your living room is hot, your attic will be, too. If your living room is cool, your attic will be, too.
Martin
Will the use of foam have an effect on the life of the roof shingles? Wondering if the shingles will age faster due to heat retention...
John,
Q. "Will the use of foam have an effect on the life of the roof shingles? Wondering if the shingles will age faster due to heat retention."
A. The shingles will be somewhat hotter, and the increase in temperature may have a small effect on shingle longevity. However, the presence of insulation under the sheathing is just one of many factors affecting shingle temperature.
The most important factor affecting shingle temperatue is shingle color. If you want cooler shingles, out of the belief that cooler shingles last longer -- and they probably do -- then install white shingles, not dark shingles.
Martin -
Thanks so much. How would a whole house fan work with this configuration?
Rob
Rob,
Whole-house fans are designed to be installed in homes with vented unconditioned attic. If your attic is unvented, you can't use a whole-house fan. Instead, you could install a NightBreeze unit.
More information on the NightBreeze:
NIGHTBREEZE PRODUCT AND TEST INFORMATION
NightBreeze: Integrated Heating, Ventilation and Cooling System
Thanks, Martin. So how does warm air leave the attic? Currently, I have a fan in an attic window to pull cool air through the downstairs windows at night. Would you recommend spray foam against the roof deck for a house without central air? It sounds as if the air in the attic will never get changed, and I will no longer be able to bring in cool air at night.
The Night Breeze system looks interesting, but it is an add-on to a central air conditioning system, not a stand alone solution.
Rob
Rob,
You're the one who has decided to install spray foam on the underside of your roof sheathing. If you have all kinds of misgivings, maybe you shouldn't do it -- if it's not too late.
When you convert a vented unconditioned attic into an unvented conditioned attic, you are incorporating the attic into your conditioned space. It's just like another room in your house. It shouldn't be significantly warmer or colder than any other room. It's just like a second-floor bedroom or a third-floor bedroom.
But If you want nighttime ventilation cooling using a whole-house fan (usually installed in the ceiling of the upstairs hallway), it’s much easier to do with a vented, unconditioned attic.
On a 12:12 roof with a vented attic almost ALL of the shingle cooling is re-radiation and convection on the exterior. The venting primarily serves to purge moisture (not heat) from the interior side of the roof deck.
Putting in 5" of closed cell foam would trap moisture at the roof deck, and it mostly wasted from a thermal point of view by the thermal bridging of the rafters. What's more, the blowing agents of most 2lb foam have a HUGE lifecycle global warming hit, making anything beyond the first inch or two net-negative for the climate.
But putting 1-2" of closed cell foam against the roof deck and bringing the rest of it up to code-R with damp-sprayed cellulose or high-density fiberglass can be the best of both worlds. At 1" you have about a 1 perm vapor retarder limiting wintertime moisture uptake into the decking, while having ample drying capacity toward the interior. At 2" it's about 0.5 perms (similar to a kraft facer on a batt), which is also good enough drying capacity.
While this would be in contravention to IRC-prescribed minimums in colder climate zones, it still works at 2" even in climate zone 7. There is little science behind using the same prescriptive R values for air-impermeable foam under the roof deck as is used for foam above the roof deck (where there is PLENTY of science.) It actually matters which side of the roof deck the vapor-retardent foam is on, and you can get away with a LOT less foam-R if the foam is on the interior than you can if the deck is still vapor-open to the interior. That was the point of this very good set of WUFI simulations done by the folks at Building Science Corp:
http://www.buildingscience.com/documents/reports/rr-1001-moisture-safe-unvented-wood-roof-systems
Scroll down to Table 3 on p.12 (.pdf pagination) for the summary results. Only if you're using a light colored "cool roof" on the north facing pitch would you run into any issues in climate zones 6 & 7, even with just an inch of closed cell foam.
"Currently, I have a fan in an attic window to pull cool air through the downstairs windows at night."
I see no reason why this arrangement will not continue to work in the new attic regime. To get the proper benefits of course there has to be an open air path to the attic space where the window is located and all window a/c units need to be shut off. A whole house fan is simply a bypass arrangement to enable night-time cooling in homes without such a convenient layout as you currently have. It's not so much that it won't work, but that you don't need it.
Thanks to all. It seems as if this is generally a good idea. If I understand the answer provided correctly, Dana Dorsett suggests that putting side on the underside of the roof deck would trap moisture in the roof deck, which others do not see as a particular issue. Also, Dana notes that the proposed 5.5" of foam/R-38 is overkill, and that more than one or two inches is wasted due to thermal bridging of the rafters. I don't understand the full post, but It sounds as if Dana is suggesting an air gap between the decking and a second decking on the interior of which the foam is sprayed. Is that more or less correct?
Regarding the GHG issue, I believe the contractor is using a water based foam to avoid that issue.
Thanks for all the info. Keep it coming!
Rob
Rob,
All insulation systems are compromises. The more R-value, the slower the heat flow -- but eventually the cost of the insulation reaches a point where we say, "that's enough."
If you choose to install insulation between your rafters, you will have thermal bridging through your rafters. This problem can be solved by adding a continuous layer of rigid foam above the roof sheathing or under your rafters. Only you can decide whether the improved performance is worth the added cost and other disadvantages of the continuous layer of rigid foam.
Unlike Dana Dorsett, I don't think that installing 5 or 6 inches of closed-cell spray foam on the underside of your roof sheathing will cause any moisture problems. It's true that the roof sheathing won't dry to interior to any degree -- but as long as the roof sheathing is dry on the day that it is insulated, the system should perform as designed.
Thanks so much, Martin. Just so I am sure what you are suggesting: You are suggesting rigid foam board either above the rafters, between the sheathing and the plywood decking, or directly on the underside of the rafters (I could then put drywall on top of the foam). Do I understand that correctly? If so, what thickness rigid foam board? Would I want the foil backed board, or is than not necessary?
Thanks again so much for your help.
Rob
Rob Gaff,
There are lots of ways to build an insulated sloped roof assembly. You can read about them here: How to Build an Insulated Cathedral Ceiling.
These roofs can be vented or unvented. They can have rigid foam above the sheathing, or no rigid foam above the sheathing. They can have lots of thermal bridging through the rafters (I don't recommend that type of roof) or very little. It's up to you how to build the roof assembly, based on your budget and the condition of the existing roofing.
The most important rule to follow, as far as I am concerned, is to install at least the minimum R-value of insulation required by your local building code.
To be clear, 5" of closed cell foam is a potential moisture trap, but at 2 " it isn't.
The exterior roofing is extremely vapor retardent, and cannot be counted on for ANY drying capacity, (whereas you CAN count on it to eventually leak.) With 5"+ of foam any moisture that's in the roof deck will remain there for years. With 2" of foam it has sufficient capacity to dry over a single summer season. OTOH 5" of 2lb foam is structural- almost as structural as the roof deck itself, but it's also going to be a net global-warming negative due to the blowing agents used. If that's the route you take, use only Icynene's water blown MD-R-200 (not MD-C-200) or any of Aloha Energy's 1.8lb+ density water blown foams, to avoid the very heavy greenhouse gas hit from the usual HFC blowing agents. At 2" you'll eventually break even with the avoided carbon from lower energy use even with the worst HFCs, but never at 5-6".
And I'm NOT advocating "...an air gap between the decking and a second decking on the interior of which the foam is sprayed...". If you hold the line at 2" of closed cell foam on the underside of the deck, you do not need to provide a ventilation gap- the roof deck can still dry (albeit slowly) through 2" of foam. Then a full rafter-fill with cellulose reduces the thermal bridging at the rafters, though not NEARLY to the degree rigid foam on the exterior does.
If going with rigid foam on the interior, how much youne ed is climate dependent. The IRC has prescriptive minimums at the code-min total R, which should be adjusted up if you're going higher than code-min:
http://publicecodes.cyberregs.com/icod/irc/2009/icod_irc_2009_8_par093.htm
But you can "cheat" on that substantially, with as little as 1" of spray foam on the underside of the deck, as outlined in my previous post.
short answer, don't go for an unvented roof. it's a design with no safeguard, which isn't a smart thing to do. would you install a toilet fill valve with no overflow protection?
unvented roofs are a fantasy supported by a guy named "Joe" who presumably is compensated to promote spray foam. trapped moisture is the keyword.....
in a utopian idealized world where roofers build a perfect roof and insulators seal 100%, like in a computer simulation, it works perfectly
Essentially, sealing 100% is a bad idea...; lets not confuse water infiltration with vapor permeability.
Aside from energy efficiency and climate, properly constructed non-vented attic space is perfectly fine even in the dystopia world.