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:
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.,
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.
Thanks for your help.
GBA Detail Library
A collection of one thousand construction details organized by climate and house part