Rafter Size and Ice Dams
hi GBA, There are several articles on GBA that address ice dams. What I couldn’t find an answer to is what size of rafter do I start worrying about thermal bridging making ice dams? Climate zone 5b, half of the roof faces north. The engineer drew up a roof plan with 12inch rafters that are to be insulated with closed and open cell spray foam to about R50, with a 2x vent channel above deck. My concern, other than the cost of the spray foam, is how much risk is a ~R14 2×12 rafter for ice dams? How much thermal bridging is acceptable? Putting rigid foam on the interior is not an option due to beams. The builder thought rigid on the exterior would be too labor intensive. any advise much appreciated!
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With a top vented roof, I doubt you'll get any ice dams.
None the less, I would look at swapping the 2x12 for I-joits. These have much narrower web and significantly less thermal bridging. This also means that you can pass code on U factor basis, which generally works out to R38 of SPF. Way cheaper install and more energy efficient than your 2x12+R49.
Depending on your local lumber costs, especially for long spans, I-joists tend to be cheaper plus you can usually increase the spacing (ie go from 16OC to 19.2OC or 24OC) and save even more cost. A bit more labor to install for a roof and you tend to use more joist hangers, so overall it should still be a wash in terms of cost. Just be careful with any double joist, make sure the framers insulate between them.
Thanks once again Akos,
That’s reassuring, the neighbors above me installed heating cables due to ice dams on their older house so just wanted to check with experts. I agree ijoists sound like they make a lot more sense and I pitched that and the idea you gave earlier about lifting the rafters off the deck and dense packing, but my builder was against it. Ultimately, the builder is already trying a lot of new things on my behalf so I can settle on him wanting to take a safer and more familiar path on the roof and stick with the one the engineer recommended.
Carson,
I've seen multiple homes built recently here in the mountains of NH that included spray foam between 2x12 rafters without an air gap/vent. So far, there is no sign of ice dams and each roof has healthy 2' of snow on top.
I think you'll be fine with 2x vent channel especially if you can keep your ceiling air tight. Ideally, this would be achieved without relying
upon the cc spray foam as the dedicated air barrier.
One of the homes that insulated with spray foam that I referenced earlier still had an air leakage rate of 900 cfm @50 pascals. It was a smallish home and this eqated to around 4 ACH. The cc spray foam install 'looked' nearly perfect but was still not a reliable air barrier. The crappy-ish windows were definitely playing a role too though.
A tight ceiling will prevent heat from getting to the roof deck. My own anecdotal observations tell me that heat loss through material (rafters/insulation/etc) is far less important than air leakage for the formation of ice dams. A vent gap washes away any warm air that finds it way through. The vents serve as the suspenders you wear with your belt. I think you'll be fine!
Thanks Rick. Debating the extra air sealing layer given that intello/etc are a bit pricey and drywall not an option, unless it was between beams which i hear is hard to get sealed well. Are you sure the bad ach score was due to the roof and not just from other parts of the home?
Hey Carson, I should have put poly over the windows during the blower door test but didn't have the time. I think most of the gaps in the spray foam were within the insulated floor over a crawl space.
If I had to guess, I would attribute the air leakage as follows:
40% crappy windows/installation
40% poorly executed crawl space (gaps in spray foam around penetrations)
15% gaps around spray foam in remainder of home
5% air movement through materials
For what it is worth, these houses have no separate air barrier- just spray foam between rafters. One of them has spray foam over trusses with top chord buried. No Ice dams and lots of snow on top.
I have a 2x10 unvented cathedral ceiling with spray foam, about R38 worth of closed cell. Ice damming is very minimal, and only happens when conditions are just right (usually periods of sun and then clouds). I actually have more issues with ice damming on my vented "regular" roof over attic spaces than I do over the unvented roof insulated with spray foam, and I think that's due to more air leaks in the attic spaces (something I've been working on).
I have noticed that the areas of the unvented roof where the rafters are sistered I can see there is some leakage BETWEEN the rafters. I did not think to put a bead of sealant between the sistered rafters when this roof was built (rebuilt actually, due to some roof leaks before I was here). I recommend you make sure you air seal any gaps in the framing that the spray foam doesn't seal. I'd be especially careful of any sistered framing members, and where the ends tie into other parts of the structure.
Bill
Bill, what’s your take on membrane at the bottom as a belt and suspenders approach? Worth it? Would something cheaper like tyvek be fine instead of intello I often see used here?
Tyvek isn't any kind of vapor barrier/retarder. Tyvek is specifically designed to be vapor open. Tyvek is not a substitute for something like Intello.
If you do a good job of air sealing, a vapor retarder won't hurt and might help. With a non-complex roof, a good job of airsealing and a painted drywall ceiling is probably enough though.
Bill