Spray Foam in a Vented Attic
I have a few questions about spray foaming my attic. If I decide to move forward, the work will be done by a professional. I’m hoping the folks on this forum who are a lot more knowledgeable than I can help me with my decision.
First, some details:
– House is in Northern NJ (climate zone 6A)
– Built in the mid-1940s
– Stick construction
– Relatively simple gable roof currently covered with asphalt shingles that will probably need to be replaced in the next five years
– The roof stops at the edge of the house – there are no eaves/soffits, and thus no eave/soffit vents
– The attic is vented by two gable vents and three roof deck vents (there is no ridge vent)
– Attic floor area is about 700 square feet, and it’s currently covered with really old (paper crumbles when you touch it) fiberglass batt
– Home energy auditor estimated the current ceiling R-value to be about R-4, and there is no air sealing
– Attic contains central air equipment and ducts and is also used for storage
– Ceilings are plaster over what appears to be a precursor to drywall (no lath)
– The ceilings curve down along the front/rear walls of the house where the pitch of the roof encroaches on the rooms
I have a spray foam contractor coming out on Sunday to provide a quote. Over the phone, we discussed lightly spray foaming the floor of the attic for air sealing, and then spray foaming the rafters and gables for insulation.
Essentially, this would create an unvented attic.
Here’s how I’m thinking about this:
Pros:
– R-value is R-value, so spray foam will improve comfort just as much as the equivalent R-value of cellulose
– Better air sealing than cellulose
– Not as messy as cellulose both in general, and especially if the roof were to ever leak (fortunately, haven’t had this problem)
– Creates an unvented/semi-conditioned space, which will improve the performance of the HVAC equipment
– Potential to stiffen up the structure, especially with closed-cell (not that it needs it, just a side benefit)
Cons:
– Price, based on what I’ve read
– Permancy (won’t be able to get to wires/electrical boxes without some serious foam removal)
– Flammability
– Potential to hide problems (like a leaky roof deck)
Overall, money aside, my feeling is that an equivalent R-value of spray foam will improve comfort and efficiency more than cellulose. And I like the idea of an unvented/semi-conditioned space for the HVAC equipment and the stuff I store up there.
Am I missing any major downsides? Are there horror stories out there about people converting vented attics to unvented in old houses and causing tons of domino-effect problems? Any reasons not to do this?
Any info you can provide will be appreciated. Thanks!
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Replies
Matt,
The most important reason to transform your vented, unconditioned attic into an unvented, conditioned attic is to bring your air handler and ductwork inside your home's conditioned space.
There is no need to install any spray foam on your attic floor, since you will be moving your air barrier and your insulation layer to the sloped roof assembly.
For more information on this work, see Creating a Conditioned Attic.
Q. "Are there horror stories out there?"
A. Yes, there are horror stories -- but the horror stories occur in a very small percentage of jobs like the one you are describing. For more information on the horror stories, see Spray Foam Jobs With Lingering Odor Problems.
Thanks, Martin.
After reading the Creating a Conditioned Attic article, it seems there are a few key points:
- If I'm planning to leave the HVAC equipment in the attic, turning the attic into conditioned space is really the only smart way to go
- That said, the BEST option, if money, etc. were not an object, would be to move the HVAC equipment and ductwork someplace else, insulate the attic floor, and leave the attic vented
- Spray foaming the rafters will cause the roof sheathing to remain damp longer, and prevent me from noticing leaks and rot
It's that last point that has me concerned, given the house's age. We've been fortunate so far, and haven't had any leaks, but I still worry about the possibility.
With all of this in mind, it seems there are really 3 smart options:
1. Create a conditioned attic and not worry about the potential roof sheathing problems
2. Create a conditioned attic and, a few years from now when I put a new roof on, have it re-sheathed with two layers of sheathing and a ventilation channel in between
3. Replace the HVAC system with ceiling-mounted ductless mini-splits, blow insulation in the attic, and leave it unvented
The only reason option 3 is even a consideration is because I've heard that spray foam is quite expensive, so ductless mini-splits might not be THAT much more expensive for my small, 3-bedroom house. I have 2 in the house already, and love them. Plus, the current HVAC equipment and ductwork looks like they were installed by a 3-year-old, so the mini-splits would provide a significant increase in efficiency in several ways. The major drawback, though, is the "logistics" of fitting a 2'x2' mini-split unit in a 16" OC ceiling.
What do you think of the 3 options?
I'm guessing most will agree that #2 is the best overall. Does anyone think #3 would be worth the expense and headache?
Thanks.
Spray foaming the floor THEN insulating the roof deck would technically be a code violation, not that it's particularly hazardous.
Spray-foaming the roof deck with 2-3" of closed cell foam would NOT increase the average moisture content of the roof deck- it can still dry toward the interior (somewhat slowly through 0.4-1 perm foam), and would limit the amount of wintertime adsorption of moisture in the roof. If you have 2x6 rafters, at the warm edge of zone 6A climate(where you probably are) you can safely install 2" of closed cell foam (R13) on the underside of the roof deck, and R13 batts or damp-sprayed cellulose below that. At the cold edge of 6A you'd be better off with 3" of closed cell foam and 2.5" of fiber, or 2" of foam and R11 batts. A ~50% foam-R/total-R is good enough for dew point control at the foam/fiber boundary in any part of zone 6. At that point you must NOT use an interior side vapor barrier. Kraft facers would be OK, but not foil, and no polyethylene sheeting.
That would put the center-cavity R at about R25, but an air-tight R25- it's a pretty good performer. When it's time to re-roof you could then add 5" of EPS (or 2" polyiso on the roof deck plus 2" of EPS above that) held down to the roof deck with half-inch OSB through screwed to the rafters with timber screws 18-20" o.c. to bring it up to something more like code. There is no need to add a vent channel- the nailer deck is somewhat sacrificial (you may have to replace a few sheets if you let the shingles go to hell an it leaks), but even an ~R40 center-cavity roof built up this way will outperform an R49 code min, since the rafters are thermally broken by R20 foam.
At 2" the closed cell foam will come in at about $2-2.25 per square foot, installed. (about 17-18 cents per R per square foot You'd have to get quotes on damp-sprayed cellulose vs. batts, but it should be in the 3-5 cents/R-foot range, installed. Sheet polyiso & EPS will run about 10 cents/R-foot, plus installation. For simple gabled roofs the installation is pretty easy and the scrap rates much lower compared to roof lines with lots of dormers hips & valleys.
Mini-duct cassettes will fit between 16" o.c. joists, if you opt for that route, and may be worth doing even if you insulate at the roof deck.
Thanks for the very detailed info, Dana.
You're suggesting a combo of spray foam and other insulation.
Forgive my ignorance, but other than the cost, what is the disadvantage of getting 100% of my R-value from spray foam?
Asked another way, what is the purpose/advantage of using other insulation (batts, cellulose, etc.) over spray foam?
Thanks.
For starters, spray foam that doesn't fully fill the cavities has more severe thermal bridging, since the distance from the cold side to the warm side of the wood is much shorter. When thermally bridged by wood you are severely undercutting the potential thermal performance of high R/inch foam.
Second, closed cell spray foam must be sprayed in 2" lifts to avoid shrinkage/cracking/air-leak issues.
At higher thicknesses, say 10-11" (R65-ish) filling a 2x12 cavity the vapor permeance of most 2lb foam is under 0.1 perms, a class-I vapor retarder, essentially a vapor BARRIER, which in a roof assembly forms a moisture trap at the roof deck unless you vent the roof deck from above.
But more importantly, closed cell spray foam (with very few exceptions) is blown with HFC245fa, which has a global warming potential (GWP) about 1000x that of CO2. No matter what you heating/cooling energy sources are, at some R-value there will be a crossover between being a net-benefit from a climate perspective to becoming a net-problem over a 50-100 year lifecycle. While closed cell polyurethane has many wonderful & useful properties and can be a huge boon to the resilence & thermal performance of a building, using any more than the minimum necessary for moisture control isn't necessariliy the greenest way to build.
That said, there are a few closed cell & semi-open cell ~2lb density foams out there that are blown with water, and don't have the GWP problem. A smaller player in upstate NY named Aloha Energy (http://aloha-energy.com/ ) has a range of 1.8lb density water blown polyurethane products. The much bigger player Icycne has two similar products: MD-R-200 and MD-R-210. Both run about 2lbs/cubic foot density and are water blown, and are about R5/inch. The MD-R210 product has lower vapor permeance, and is a class-II vapor retarder at 2.4"
http://www.icynene.com/sites/default/files/downloads/ProSeal%20Eco%20MD-R-210%20Technical%20Data%20Sheet_August%202013.pdf
The MD-R-200 has ever so slightly higher thermal performance, but is more vapor open, only hitting Class-II performance at about 4".
http://www.icynene.com/sites/default/files/downloads/ICYNENE-MD-R-200-Specification-Sheets-US_0.pdf
Either of the 2lb water blown Icynene products would be appropriate for a rafter-fill, as long as the thickness is sufficient to bring it into the 0.5-1 perm range. (Don't confuse these with their MD-C-200 R7/inch foam- that product is both much lower permeance, and blown with HFC245fa.)
Hi could you expound on this statement?
"At higher thicknesses, say 10-11" (R65-ish) filling a 2x12 cavity the vapor permeance of most 2lb foam is under 0.1 perms, a class-I vapor retarder, essentially a vapor BARRIER, which in a roof assembly forms a moisture trap at the roof deck unless you vent the roof deck from above."
I've been asking about pros/cons of roof deck spray foam vs venting in a related question (here: https://www.greenbuildingadvisor.com/question/will-this-work-retrofit-vented-unvented-hybrid-attic ) and still not clear on the water/moisture risks with foam directly applied to the underside of roof decking.
I think what you're saying here is that thick enough closed cell is impermeable and doesn't allow the roofing to dry to the inside? This assumes that the underside of the roofing can get wet (vapor drive? nail holes?). Is it a concern?
KT,
This article may answer your questions: "Sandwiching Roof Sheathing Between Two Impermeable Layers."
Thank you that was helpful though still a bit confusing. The 2nd diagram, here https://s3.amazonaws.com/greenbuildingadvisor.s3.tauntoncloud.com/app/uploads/2019/05/18153239/Vented-roof-assembly.jpg is what I am aiming for but with board foam rather than batt, with the foam panel forming the vent cavity.
In the 3rd diagram, it looks like the sheathing dries to the interior, however there's a top layer of sheathing directly under the asphalt shingles that's got nowhere to dry to. https://s3.amazonaws.com/greenbuildingadvisor.s3.tauntoncloud.com/app/uploads/2019/05/18153908/Rigid-foam-above-roof-sheathing-4.jpg
I read some pieces on JFC too and it seems most think vented is lower risk than unvented. What's not clear is too low perm foam directly below the roofdeck is bad (i.e. thick closed cell) on the one hand, because it prevents drying to the inside but then open cell (higher perm) foam below the roofdeck is a no-no even though that would dry to the inside.
Ultimately my concern with the spray foam directly on the roofdeck is any moisture migration from above, that gets trapped between the closed cell foam and the underside of the sheathing. When a roof is replaced there's bound to be a lot nail pulling and replacement that disturbs the foam and provides water or water vapor micro-entry paths.
Thanks again, Dana.
I didn't realize the chemicals in some types of foam are be so potent.
I'm waiting for a call back from an Aloha Energy installer, and will be calling an Icynene installer today about their water-blown options. The installer that was out this weekend only uses regular (Quadrant) foam.
In terms of the overall approach, my plan is this:
- 4" of closed-cell in the rafter bays/gable ends, plus a light coat over everything to provide a thermal break for the rafters
- intumescent ignition barrier
This should meet the R-20 insulation requirement, class-II vapor retarder requirement, and ignition barrier requirement. Additionally, this will be a considerable R-value improvement over what's in the attic now, and will also be a great air barrier. The only requirement not being met is R-49 for ceilings/roofs.
I bought a small temperature/humidity data logger and am hoping to gather some before/after data to evaluate the performance. Will post results here.
Appreciate the help!