Chicago Bungalow Insulation
nzamorski
| Posted in Green Building Techniques on
I have a story and a half bungalow in Chicago (zone 5a) and have a partially finished attic space on the second floor. The 2×6 roof rafters are lacking insulation and the knee walls have fiberglass batts that are falling out. I have some good ideas from this site on how to insulate the knee walls, but am unsure how to insulate the roof rafters and avoid condensation.
I’ve heard different recommendations on spray foam, cellulose, etc, but am unsure what the best solution is.
Any suggestions on what the best assembly might be to insulate the 2×6 roof rafters?
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Replies
nzamorski,
You may find this article useful. It shows five options for the roof insulation: https://www.greenbuildingadvisor.com/article/five-cathedral-ceilings-that-work
If you need more depth for insulation in the rafter bays, you can fur them down.
Thanks, I'll take a look at that article. Unfortunately, head room is already at a premium, so furring down the joists isn't an option.
I realize I might not achieve required R values, but looking for the best option to improve the comfort and performance of the space without creating a concern of condensation at the roof deck, etc.
About the best you can do in terms of R value is cut'n'cobble polyiso, which would give you about R36 in the 2x6 rafter bays, and a layer of 1/2" polyiso under the rafters behind the drywall both brings the total up to about R38 and also provides an interior side vapor barrier which will help limit how much moisture can sneak up into your attic space above. I assume the attic is vented, so you need to leave vent channels of at least 1" in any rafter bays that connect the upper and lower attic spaces, which will cost you about R6 from that total I mentioned in those areas where vent channels are needed. This is what I did in my own attic kneewall areas to maximize the R value in the available space (I had 2x8s to work with though).
For the kneewalls themselves, I would be sure to use high density batts (I like mineral wool), and put a layer of polyiso over the attic side of the studs to provide a back side air barrier and additional R value. I find that my attic kneewalls ran hot in the summer and cold in the winter with batts alone, and since they're only 2x4 walls, R15 was the most that would fit in the studbays. I put 3" polyiso over the attic side of the studs, but any amount is better than none. The addition of continuous insulation on the attic side of the studs greatly improves the overall thermal performance of the kneewalls. In my own home, the extra insulation made the "hot in summer / cold in winter" problem completely go away.
Bill
Thanks Bill. Very helpful info.
What's the best way to seal the pieces of polyiso to the roof joists as I put them in place?
I assume the seams in the final 1/2" later of polyiso behind the drywall also have to be taped or something similar to seal?
The way I do it is to tack furring strips to the inside edges of the rafters up tight to the underside of the sheathing. I usually use 1x2 furring strips to create 1.5" vent channels, but in a really tight space like yours, I'd recommend you rip 1" strips of plywood to save space. Those furring strips act as spacers to create a vent channel under the roof sheathing. Next step is to cut pieces of polyiso to fit between the rafters with about a 1/4-1/2" gap. Fill that gap with canned foam. I like to use Loctite's TiteFoam product for this since it's a bit more durable than most canned foams, but Great Stuff works too. Build up layers until you fill the rafter bay with polyiso sealed in place like this. I recommend leaving some space around the first piece of polyiso, then install the second piece, then fill the gap the rest of the way with canned foam so that the last layer of canned foam seals the polyiso you just installed plus the last bit of the piece installed previously. This way the canned foam doesn't expand out of the gap and block the next piece of polyiso from going in.
Once you have the rafter bay filled like this, install some 1/2" (or whatever thickness you want) polyiso under the rafters and tap the seams (I like Nashua's 324A foil tape for this, which the orange store stocks). This inner layer of polyiso provides a vapor barrier, but also provides an air barrier and a bit more R value plus reduced thermal bridging from the rafters.
Bill
Thanks Bill. Any benefits or concerns with polyiso board that has a facing to it?
Or am I better off finding unfaced polyiso?
Not Bill, but since he and I are already on another thread together I figured I'd jump in. I think the normal faced board is the better choice - it forms a tighter air seal, which I understand is valuable in this case because you are/should-be maintaining the venting of the roof with the gap/spacers he describes, meaning the roof sheathing can still dry through the vent channel. The air sealing will help reduce moisture getting into the attic in the first place, which is the first step to preventing it from condensing where you don't want it.
As for the rest of his suggestions, I like them and they get you to the best R-value for the assembly, though it can be a bit fiddly to cut and cobble that much foam. I'm going through a very similar plan right now, Zone 5, 2x6 vault bays that connect a lower vented attic to an upper vented attic, currently empty. I've now insulated my knee walls (spray foam over batts, given the odd space constraints), but left the kneewall unsealed at the entrance to the vault bay until I figure out this detail.
The main option I'm considering right now, that I think would all work for you, are to do as Bill is doing - use vertical furring strips to create a 1" vent channel against the sheathing, then build up polyiso foam into the space. Cap the whole assembly with 1" polyiso glued to the joists and taped for maximum air sealing. Drywall on top. Drywall can go straight on the foam, or onto a 1x3 furring strip, which loses a bit more height, but gains a little R-value, and gives you a place to run wiring if you need to.
Another to throw out there would be to use a plastic baffle, like DCI Smart Baffle (24" OC and 16" OC options), secure that to the vault bays with a small nailer, then have my spray foam contractor spray over that, to about 1" shy of the top of the joists. Foil-faced 1" polyiso still goes over the top of the joists for air sealing, with foil facing outward and the small gap there will be a little radiant benefit. That results in less total R value, but some of my spaces are nice and easy shapes, and some of them are not, and this has the benefit of being able to be sprayed into those tricky spaces where I don't think I could cobble and seal foam board effectively.
Yet another would be Smart Baffle -> mineral wool -> polyiso interior board or rockwool board is also an option, and has the benefit of using no foam, but it also has the lowest total R value of these options.
I think in this case the key elements are making sure you air seal the final assembly as best you can, and use a baffle of some kind against the roof sheathing to allow air to move from the lower attic up to the upper attic (or go through the process of deciding if you can switch to an unvented attic, which has considerations that go a lot farther). If you do those things then the rest of the detail comes more to your preferences, local costs, and the specific challenges of your installation.
-Sam