Comments on: Ice Dam Problems

By: Todd

Todd — Sat, 17 Dec 2011 00:19:01 +0000

I agree with their assessment.

By: Todd

Todd — Sat, 17 Dec 2011 00:17:29 +0000

There are calculations you can do for air flow. However, you’ll be fine if you choose a proper vent that fills up as much of your rafter space as possible. It doesn’t have to fit perfectly, but I would get all the air you can.

By: Dave

Dave — Fri, 16 Dec 2011 15:46:03 +0000

Hi Todd
I poked around on your site a bit more, excellent by the way. It sounds like my “proper” or rafter vents are not installed correctly. They are not attached to the underside of the roof decking, but stapled to a cross member about a foot up from the soffit so that they are off the underside of the roof decking by a few inches. They are not providing a very good channel for the air coming in the soffit up the rafter toward the ridge vent. I think for now I will skip the 1 inch piece of cut to fit foam board that the auditor recommended, thats a lot of work in a tight space, and just remove the improperly installed proper vents and replace with new ones stapled to the bottom of the roof decking as they should be. How concerned should I be with the sizing the proper vents? Do they need to span the whole rafter, or is the idea just to get a good channel for each rafter even if it is not the whole width of the rafter?
Thanks
Dave

By: Dave

Dave — Fri, 16 Dec 2011 15:13:19 +0000

Hi Todd
I have an issue with frost forming on the interior of the drywall, in very cold weather. The frost forms where the wall and ceiling meet on the second floor on the backside (north side) of the house. I recently had a home energy audit and discussed this issue with the inspector. The roof is a 7/12 pitch and has no ice dam or snow accumulation problems, not because of heat loss, the snow just does not stay on the roof due to the prevailing winds. The attic has two layers of 12 inch fiberglass that extends all the way to the soffits. There are also rafter vents (channels of foam) from the soffits about 2 feet up the rafter toward the roof vent. The rafter vents are not attached to the underside of the roof decking, they are stapled to a cross member about a foot in from the soffit. The energy auditor suggested adding a piece of 1 inch blue board between the rafters at the soffit extending vertically up to the underside of the roof decking leaving enough space to reinstall the rafter vent and attaching the rafter vent to the underside of the decking. This should direct the air venting the attic coming in the soffits up to the rafter vent and up the underside of the decking so that the flow of cold air away from the wall/ceiling joint wont cool that joint enough for frost to form. Any help with those ideas?
Thanks
Dave

By: Carrier Carpentry

Carrier Carpentry — Tue, 22 Nov 2011 22:22:50 +0000

Very glad we don’t have to deal with these issues in (my part of) Texas. Was interesting though. (Randomly came to this article from another Carpentry blog.) Thanks.

By: Todd

Todd — Wed, 02 Feb 2011 14:04:51 +0000

Jeff – Thanks for sharing. The whole process can be quite complicated. The best defense is cleaning the roof! But for many people that’s not a great option.