Ice dams and soffit insulation
I am planning a heated utility building in a cold climate with significant snow (western Montana, zone 6B, 53 psf snow load). I am using the “Classic Control Approach” depicted in Figure 2 of Joseph Lstiburek’s recent posting on ice dams:
https://www.buildingscience.com/documents/building-science-insights/bsi-097-de-icing-ice-dams
In my case there is 11″ between the top plate top and the roof deck bottom, so I’m thinking it is best to use closed cell spray foam insulation above the exterior insulation and above the top plate up to 2″ below the roof deck bottom. My understanding is that building code requires 2x blocking between the roof trusses from the top plate to 2″ below the roof deck bottom, which seems like a significant thermal bridge, but I don’t see how that can be avoided and still pass the framing inspection. The continuous soffit ventilation planned will satisfy the recommendations of Tobiasson, et al.,
https://www.erdc.usace.army.mil/Portals/55/docs/CEERD-RV/CEERD-RR-H/BuildingTechnology/ResearchPapers/MP-02-5778,%20Guidelines%20for%20Ventilating%20Attics%20and%20Cathedral%20Ceilings%20to%20Avoid%20Icings%20at%20Their%20Eaves.pdf
I understand that an effective air control barrier generally, and specifically at the ceiling, as well as adequate ceiling insulation are critically important to minimizing ice dams.
What is not clear to me is the significance of the solar heating mechanism of ice dam formation Lstiburek writes about (see Figure 9 of the above reference). In my case the south wall is a dark colored eve wall with a 18″ deep soffit, so that is certainly going to happen to my building. Tobiasson, et al., Figure 1 suggests that maybe this doesn’t cause much of a problem.
Is it likely to be a problem?
If it were a significant problem I imagine the fix would involve 2 steps: 1. installing a baffle over the soffit vent and extending beyond the fascia so the vents draw in air mostly unaffected by the solar heading from the wall, and 2. fill the soffit completely with insulation except for the volume needed to allow ventilation. The baffle installation can easily wait until a problem materializes. However, waiting to install soffit insulation will involve a bit of re-work, I expect.
Suggestions?
Thanks for your help.
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Replies
> Is it likely to be a problem?
I don't know, but injecting heat into the vent channel does seem like a bad idea.
Maybe "Shingle Over Intake Vents" placed a foot up the roof would address this issue.
One way to reduce how much warm air gets trapped by overhangs would be to close off the eaves with a soffit.
It does not matter if that warm air moves up the soffit vents and into the roof venting; what matters is that warm air getting trapped and heating the overhang, which creates the ice dam.
Peter
Lstiburek explains in his second paragraph that it's cold (not warm) overhangs that cause ice dams. But I suppose one could differentiate between different parts of the overhang (higher portion above the wall gets warmed, further out portions don't). This suggests that it would be better to angle soffits downward (left to right in the drawings). But there is no way that warm air doesn't warm the path that it travels. So either avoid pulling in warm air or do it as outboard as possible (so it heats the entire overhang). Fig 2 and Fig 3 seem to support the latter.
Warm roofs melt the snow, the meltwater runs down along the roof under the snow and then refreezes when it gets to the cold soffit. That’s the usual formation process for ice dams.
The non-green bandaid way to deal with the problem is heating cables on the roof over the soffit area to keep things clear. On a house of any size, this can use up a LOT of electricity even with snow and freeze sensors controlling the cables. The better solution is to air seal the attic floor, and add enough insulation so that you’re not loosing enough heat through the attic and roof to melt the snow in the first place. If you’ve already done a thorough job of air sealing, and you have a lot of insulation, then it’s time to look into improving attic ventilation (assuming a vented attic).
Rigid foam on top of the roof sheathing also helps a lot with ice dams, when you have a conditioned attic. A If you can cut down on thermal loss in the top part of your house, you cut down on how much melting you have going on on the roof. Less melting from heat loss = less water to form the ice dams.
Bill