Spray foam on top of upstairs ceiling?
Getting down to the details now on my design (Upstate NY Zone 5 near Zone 6). I plan a cold attic with vented soffits, baffles protecting the upwards airflow, and no ceiling intrusions like wiring, lights, outlets ducts or holes/hatches for access from inside. My plan is to build a fake ceiling and soffited sheet rock below the original ceiling WITHIN the envelope, in which to run my ducts and put a few ceiling lights or fans up there. I plan on building a door on the gable peak large enough to get into the attic by removing some screws that will hold it in place. This door will be similar to the old barn hayloft doors in style and will be waterproofed and weatherstripped underneath the rakes which should be 12″ away and I expect where it is to stay pretty dry as a rule and out of sight against an East facing woodline. This is a two story house and the ceiling area is about 1100 sf. So what I am asking is: Would it be acceptable to closed cell spray foam an inch or (whatever needed amount) on the top of the sheetrock ceiling, BEFORE the cellulose is blown in to R-50 standards, in order to thoroughly AIR SEAL the ceiling in case the sheet rock men are not capable of eliminating every bit of air into that attic from the ceiling? Of course the workers doing insulation would be able to get in there through my “hayloft” door to install insulation when the time is right. In my ever vigorous approach to making sure the attic has no moist air rising up through it, I am wondering if this is ‘just’ overkill or a flat-out BAD idea?
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An inch of closed cell foam is usually overkill for air sealing the ceiling, unless that ceiling is full of holes & cracks. Since you're building a full service chase from below air sealing the seams of whatever you're supporting the cellulose with is pretty easy, or alternatively using broad-sheet goods like housewrap.
R50 is barely code-min. In your climate with a dead-simple open blown cellulose going to R65-R75 is almost always cost-effective (barely an up-charge from R50.) See Table 2:
http://www.buildingscience.com/documents/bareports/ba-1005-building-america-high-r-value-high-performance-residential-buildings-all-climate-zones
For cellulose under trusses 24" o.c. using half-inch OSB ring-shank nailed to the under side of truss chords or joists supports substantial weight without sagging WAY more than either 5/8" or 1/2" gypsum and is a "smart" vapor retarder, limiting the water vapor diffusion into the insulation layer during winter. It also gives you a convenient fastening surface for components in your service chase, with much better fastener retention then gypsum board. Material cost for 7/6" OSB a wash against half-inch gypsum (often cheaper), and is usually quite a bit cheaper than 5/8" gypsum. It's WAY cheaper than an inch of closed cell polyurethane (at about a buck a square foot)! You can either tape the seams with the appropriate tapes, or seal over them with duct mastic.
Oh, I like the idea. So 1/2" OSB, ring shank nails, flashing taped seams, 8" service chase for lights and ducts, and R75 blown in through my service "hayloft" door on top. Sounds like the answer I was looking for! What is the exact tape I should be dealing with and do you need a primer for it to stay stuck? Gobs of duct mastic could work too as you say, but it depends on what the carpenter prefers working with.
Joe,
To seal OSB seams, you might want to use one of the following four tapes: Siga Wigluv, 3M All Weather tape, Zip System, or Pro Clima Tescon No. 1.
For more information on tapes, see Return to the Backyard Tape Test.
Thank you guys as always! I like that scheme.
Joe,
Sounds like you've gotten some good advice and have a plan for the ceiling barrier. I'll pass along what we did for our house (zone 6, NH) in case it's helpful, particularly for the venting...
We have an unconditioned attic with 20"+ of cellulose on attic floor. The ceiling air barrier is continuous 5/8" gypsum on strapping, installed before interior partitions and before interior stud wall (double stud wall). Only two intrusions (one plumbing vent pipe and one solar conduit), well sealed. We also used 3M tape on the gypsum to exterior stud wall top plate transition. Overall, I'm confident this barrier is keeping conditioned house air out of the attic.
Because of this, we are only using gable vents (fixed on west side and larger hinged on east side for access) and the soffits are totally blocked off. Some inspiration and advice for this came from Up Hill House.
Hinged gable vent: http://www.customgablevents.com/Hinged-Gable-Vents.html
This worked out well (much easier than dealing with soffit vents) and we are happy with the gable vents from that company.
Wow, Brian, I am super inspired by this. I assume you have a ridge vent. What do the energy gods think of no soffit vents? Would large gable vents provide the needed air flow to cool the roof? Blocked off soffits? Hmmmm... Sounds ideal if it would work. Nice color by the way. I am doing similar with light gray trim and black Marvin windas.
Just a first time homeowner here with no experience before this project, so keep that in mind...
No ridge vent. The larger hinged vent is 24x30" and the smaller fixed is 24x24", both have nailing fins. The space between the trusses, above the top plate, and below the roof sheathing is blocked by wood by the framing crew....we added the foamed XPS to further seal this area and the cellulose from wind.
I don't think the concern is so much the actual roof temperature (our roof is 5/8" Advantech sheathing, felt, strapping, metal panels), but the attic space. I believe the consensus is that with a very well sealed/insulated building envelope, there isn't the need for much attic ventilation.
I've been in our attic a lot in winter and summer, it's dry and things seem fine. 24' ladder is just enough to access.
Joe,
Here is a link to an article with more information on attic venting: All About Attic Venting.
After reading that article I bet that Brian's design works fine but I am still unclear whether the code would allow it without the soffit vents, so it is something I need ask my inspector, I guess.
You can have either soffit-to-ridge venting (with 25-50% more soffit opening than ridge vent opening to limit stack effect at your ceiling barrier), or gable venting, but generally not both. Gable vents short-circuit the draw through the soffit vents.
Oh, one last Q for Brian, why the sleepers on the ceiling?
Joe,
In the Northeast, ceiling drywall is almost always installed on 1x3 or 1x4 strapping, installed 16 inches on center, 90 degrees to the joists. It's done because drywallers like it that way. Ceiling strapping is optional.
If you are blowing cellulose above the ceiling, some of the cellulose should find its way into the 3/4" gap between the strapping and the bottom of the joists, slightly reducing thermal bridging. But if you are installing R-60 cellulose, the cellulose should be deeper than the joists, addressing thermal bridging on the top side of the joists.
I was also mentionng the concept of theMooney Wall, Martin, in a different thread which does the same thing as to the walls (reducing the bridge) yet drywall people don't seem to specify or like to see that often. Ultimately I think it is a good idea in both instances and I would think it also reduces acoustic bridging and noise transmission to some extent as well. Part of living in comfortable and HEALTHY home in not having to listen to someone's rap music at 100 decibels at 2am. Particularly when you live next to a bank drive through ATM, like I currenty do. I learned a lot on this thread in regards to both ceiling construction for second floor and attic venting. Thank you, all!