Unvented Insulated Roof and Ice Dams
I just finished watching Joe Lstiburek talk about ice dams (https://youtu.be/Ld8pzIu45F8). He says you need to be concerned about ice dams in a “high snow load area”, which he defines as one where the ground snow load is 50psf or above.
I live in NYC. According to 1608.2 of the the 2022 building code, the ground snow load is only 25psf. Does this mean I can build an insulated, unvented roof without having to worry about ice dams? I would like to avoid the expense of building an “over roof” (https://www.buildingscience.com/documents/insights/bsi083-mea-culpa-roofs) if I can avoid it. However, if doing so is unsafe, I want to know about it now so I can design appropriately.
Building an uninsulated, vented attic is unfortunately not in the cards, due to space constraints. I really need the extra attic space for mechanicals.
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I would look at spray foaming your roof deck then and make it a conditioned space. We see pretty high snow loads and very low temps here. Ice dams are pretty obvious on a lot of older homes, and some newer ones too. Pitch of roof and orientation has a lot to do with ice dams as well. A low pitch north facing roof will see ice dams before a higher pitch south facing.
If you have persistent snow in a season, high heat loss at the eaves, and temps that stay below the -10C or so range, you will see ice dams.
I did an unvented spray foam approach for my shop, and it has worked quite well (no issues with dams). The downside though is a that a leaky roof has no where to leak. I've not reviewed the latest thoughts on an unvented roof spray foamed roof here on GBA. I have FLIR images of ours, and it works, minus some thermal bridging via the roof rafters. That said, the shop is only heated when in use.
I do intend to turn my attic into a conditioned space, but I think we're in very different climate situations (assuming your profile is still correct). I looked up the ground snow load for Thunder Bay, and it's 60psf. That's quite a bit more than Lstiburek's 50psf rule of thumb, and waaaay more than NYC's 25. I really want to know how concerned I should be in a climate well below the 50psf guideline.
I'm a bit further south with a 1.1kPa (23 psf) snow load and there are no issues with unvented shingled roofs. I'm in the land of 2.5 story houses, you can tell which ones are recently insulated with SPF as they don't have any ice. If you have a metal roof, it will always create a bit of ice melt when the sun hits it, I don't think there is a way around this.
Thank you! I think 23psf is close enough to 25 that I can treat the two as identical for these purposes. I'm glad I won't have to build a vented over-roof.
I assume a higher pitched roof, and obviously proper ice and water shield underlayment at the eaves will buy you some insurance in any case.