Converting Vented Attic to Nonvented Attic
I’ve been researching several methods to insulate my attic. First, some background.
I live in Maryland and own a house that has the HVAC system in the attic. Currently the attic is vented and has zero insulation anywhere. Not ideal!
From my research, it looks like the best option is to make the attic a conditioned (nonvented) space. I’m unsure of what to do, and I’m unsure of the most economical way to do it. The attic is fairly small at about 1,000 sqf total.
Option 1:
* Use closed cell foam insulation. Total cost to do this would be around 3,500 dollars. Lowe’s sells a closed cell foam for 688 (619 with military discount). I’ve calculated that I would need 5 kits to get approximately 2 inches total. For 5 inches it would be a total cost of around 5,000 dollars. The downside is that closed cell needs to be applied at surface temperature of 60 F degrees or higher. Can this be confirmed?
Also, I chose closed over open since I am worried about vapor. I’m not comfortable with open as it will only act as a air barrier. So air would not be allowed in, but vapor would. Where would the vapor go? Is it going to cause ice damming, leaky roof, etc?
Option 2:
* Attach rigid foam to the rafters, but have the roof vented. Total cost to do this myself would be around 2,000 dollars. I would use Rmax thermasheath (2″x4’x8′). I would get do 4 inches total at around 31 dollars per sheet.
What am I overlooking? What other options should I consider?
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Replies
Samuel,
Start by reading this article: Creating a Conditioned Attic.
Once you've finished reading the article, you can post follow-up questions here.
By the way, for a job this size, it's almost always cheaper to hire a spray foam contractor than it is to buy 5 or more spray foam kits at the lumberyard.
The closed cell option is the least-green and most expensive option, due to the HFC245fa blowing agent used. Doing it with kits has a learning curve- pay close attention to the chemical temperatures as well as the roof deck temp as you go. The tanks drop in temperature as the chemical is drawn- sometimes you need to let them bob in a tub of warm water to keep the temps right.
The vapor permeance of 2lb foam is about 0.6 perms @ 2", which is sufficiently low permeance that the roof deck will not accumulate much wintertime moisture from interior vapor drives (though some), but still has some capacity to dry toward the interior. Roof decks have effectively zero ability to dry toward the exterior- a layup of #30 felt and asphalt shingles runs about 0.1 perms, and half the time the shingles are covered with snow/rain/dew, preventing even that miniscule amount of drying capacity from contributing. At 2" you're still fine from a moisture trap point of view, but at 4"+ it's getting into "fuggedaboudit" range.
A couple inches of foam is also only R12-R13, woefully shy of code-min performance, and undercut by the thermal bridging of the rafters, and way to low to be able to prevent ice damming issues. It won't cause ice damming, but it won't prevent it either.
If you have 2x6 rafters or larger you'll get more bang/buck out of soffit-to-ridge venting, using high-density batts thin enough to leave an inch or two of space between the fiber & roof deck, followed by interior side rigid foam board taped & sealed on the underside of the rafters.
If you have 2x6 rafters you have enough room to use R5 rock wool or R15 "cathedral ceiling" fiberglass. If you then added 4" of polyiso (R24) on the interior you'd be looking at R38 at center cavity (which used to be code min up to US climate zone 5, and would still be WAY better than what you have.) If the rafters are 2x8 you can use R23 rock wool or R21 high density batts. Added to the R24-ish rigid polyiso you'd be pretty close to the R49 code min, but it would outperform R49 batts between joists. If the rafters are thicker still, you can go even higher on the fiber batts, but if it's not a high-density type you need to install an exterior-side air barrier between the fiber & vent channel (which is a good idea even with HD batts) to not lose some performance to convection between the batts & vent channel. The air barrier material needs to be at least modestly vapor permeable (not foil faced foam), but it doesn't need to be anything special. Even housewrap would be good enough, as long as you ensure that there is no pillowing that would restrict the vent space to less than an inch with the batt installed.
You don't say where you are located, but a thermally broken R40+ on a soffit-to-ridge vented assembly goes a long way toward reducing ice damming potential in US climate zones 4 & 5.
So, how deep are the rafters?
Are you planning to re-roof any time soon (say, in the next 5-10 years)?
Yes, I do have 2x6 rafters. I would say a re-roofing job will be occurring within 5 - 10 years or so.
The insulation job that Dana states seems so complex. Additionally, my roof is slightly odd. It's a low sloping roof rancher house. But, it has a split level roof since part of the second sloping roof is at an even lower angle. Trying to do everything Dana suggests in tight quarters may be almost impossible.
I do know that I want to convert the attic to a conditioned space so my hvac system is more efficient. If the batte insulation plus rigid foam board is cheaper than spray foam and it will not cause any ice damming or related issues, maybe that's the way to go.
Would it make sense to do 2 inches of close cell of SPF and then use open cell SPF for increased R value? Or maybe do a combination of closed cel SPF and then rigid foam?
http://www.buildingscience.com/documents/digests/bsd-149-unvented-roof-assemblies-for-all-climates
Your answer to Dana's re-roofing question points to the best solution: insulate ABOVE the sheathing as part of the re-roof. All issues related to cost and efficiency aside, it will be profoundly difficult to successfully insulate such a low-pitch roof from below. I'm guessing you can't even reach the last 6' by the eaves.
Andy,
Although Dana's advice may appear complex, he's trying to keep you out of trouble. Insulating a cathedral ceiling quickly and cheaply is easy -- but doing it the right way takes a little bit of thought and quite a bit of money.
Most of the "quick and easy" jobs eventually develop sheathing rot and ice dams.
If you read the article I linked to, you'll find several possible approaches. Thick rigid foam installed above the roof sheathing is by far the best way to proceed.
I agree that the best solution will be to insulate when the roof gets re-done. Unfortunately, I am getting killed with electric bills right now.
I'm looking for an alternative solution right now besides a new roofing job (will get this done in the summer or late fall of this year).
It may be best to air seal the attic floor and then add about a foot or so of blow insulation as a compromise. My HVAC equipment may run a little less efficient, but at least the heat won't be escaping too much.
Does anyone have any alternatives that are a fairly cost effective?
With a plank sheathed roof you can insulate the interior side with cellulose and have more than a decade before roof problems would appear. With plywood you probably have more than 5, as long as it's in good shape. If it's OSB, it's riskier.
With any fiber you can buy a lot of time (maybe forever) if you install a smart vapor retarder like MemBrain on the interior side of the assembly, but it has to be air-tight. If you're desperate to get going on it, cellulose + MemBrain (or Intello Plus) would be the safest way to insulate between the rafters ahead of re-roofing. It'll only be ~R20, but that's not terrible, since you'll be dramatically reducing the air-handler driven infiltration losses.
In MD (US climate zone 4) as long as at least 1/3 of the total-R is exterior to the roof deck, the roof deck is pretty safe, from a wintertime dew-point perspective. With R20 cavity fill that means you'd only need R10 above the roof deck to keep it under control. With R23/R24 high-density fiberglass or rock wool it would take only R12. While the final assembly would still be shy of IRC 2012 performance, it would perform pretty well, and reduce the ice dams considerably.
You can always go higher on the exterior foam, if you have the budget for it. Rigid foam can be pretty cheap- cheaper than batts if you buy reclaimed roofing foam from commercial demolition rather than virgin stock.