I have a 1913 four-square. It has 20 plus year old shingles and needs insulation.
There is enough space in the attic to be usable. I was wondering about sprayed insulation between the rafter and/or SIP on the roof.
We are in the mid South. The house is about 32' x32' and the roof has a 9.5/12 pitch. There is knob & tube wiring below the attic floor that has some batt insulation.
Asked by Jim Brooks
Posted Wed, 03/03/2010 - 17:02
Edited Thu, 03/04/2010 - 04:42
Posted Wed, 03/03/2010 - 17:02
Edited Thu, 03/04/2010 - 04:42
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Jim,
I take it this house is a 1 1/2 story, that means some knee walls at mid span of the 2x4? rafters. The problem with a retrofit of this space in it's current form is heating and cooling, tough to get hvac routed effectively. The structural condition of the roof would be my main concern, undersized rafters for the span and removal of the knee walls making the situation worse. The ceiling joists are actually part of the roof support structure if my guess is correct on the house type.
Posted Wed, 03/03/2010 - 21:52
As i read the post closer I see the house is a 2 story with a hip roof more than likely, the rafter sizing is still a concern and is the roof supported to some extent by the ceiling joists? I don't see where the SIPs come in, adding rigid over the existing roof sheating would be more cost effective. Getting a continuous air barrier in an older house is a challenge, identify the thermal boundary and make the connection between the walls and the ceiling.
Posted Wed, 03/03/2010 - 22:03
Jim,
1. I'm guessing you are in Climate Zone 3. The IRC minimum prescriptive ceiling insulation requirement for that zone is R-30. Don't accept any insulation scheme that provides less than the code minimum requirement.
2. As long as your structural requirements are addressed, you can insulate your roof from the inside, the outside, or a combination of both. If you are planning to install new roofing, it will probably be cheaper to insulate on the exterior of your roof. You can achieve R-30 with 6 inches of XPS (3 layers of 2 inch) or 4.5 inches of polyiso.
The problem with spray foam — you would need at least 4.5 inches of closed-cell foam — is that it won't fit between 2x4 rafters. If you have to sister new rafters in place for structural reasons, however, this may not be a concern. Get some bids to find out which option is cheaper -- exterior rigid foam or interior spray foam.
3. No matter what you do, now is the time to remove all of the accessible knob & tube wiring (and even the inaccessible knob and tube wiring).
Posted Thu, 03/04/2010 - 03:53