Conditioning Attic Space when Job is Small
Zone 2A. I’ve done ventilation baffle/batt insulation of most of the roof decking. What remains is a windowed fascia and associated gable. See attached. The exterior side of the windowed facia is a decorative cedar shingle that I would guess has tar paper beneath. The roof is asphalt shingle. I’m estimating a total of 200-300 sq ft of this remaining area that I’d like to hit with closed cell spray foam. Hard to find a contractor who will do a job that small here in Houston. I could order one of those kits from Tiger Foam and do it myself. Or I could do another insulating method that this community might recommend. I am choosing NOT to baffle/batt this section because of blocking that prevents venting on decking underside and no interior facing tar paper or similar. Thoughts?
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Replies
Mark,
A few questions:
1. Will this attic become finished space (a bedroom, for example) or will it only be used for storage and HVAC equipment?
2. Do you have any insulation on the attic floor?
3. How thick are the fiberglass batts you've been using to insulate your sloping roof?
4. If you don't want to buy any spray-foam kits, you could install pieces of rigid foam insulation between the rafters. These should be sealed to the rafters with spray foam or caulk. In your climate zone, you need R-30 ceiling insulation, which means at least 6 inches of XPS or 4.5 inches of polyiso.
Hello Martin. Thanks for the response. To answer your questions:
1) It is only being used for storage, HVAC equipment, and 40-50% of my ductwork.
2) Yes, the attic floor is insulated and partially decked. This is a side attic space on the 2nd floor.
3) Just R-19.
I am not trying to achieve a certain green standard or rating. Just making best efforts to follow your suggestions of bringing the HVAC inside the conditioned envelope. I can already tell I am making progress. Last night the temp outside was 33 degrees, and my attic temp is now registering 51 degrees. So in addition to getting hotter air from the heater (and presumably cooler air from the air conditioner in summer), I have less of a differential between the attic and the adjacent living spaces. The Tiger Foam (or Handi Foam) that is E-84 rated appears to be the right polyiso product to allow me to avoid a thermal break being applied on top.
I know there will still be thermal bridging, some air seepage, and other imperfections in my final solution. The basis of my question was just making sure I treat that fascia and gable with something comparable to the R-19 batts. Based on your calculations, are you suggesting the best 'comparable' solution would be 2.5"-3" of polyiso applied to those areas for the R-19 equivalent (XPS is not an object due to installation interference by some of the attic roof cross-bracing)?
Mark,
I think you may be misusing the term "fascia." A fascia is a vertical finish board located at your eave, under the drip-edge of your roof. On most homes, the fascia is aligned 90 degrees with respect to your soffit.
Does your question concern insulating your sloped roof or insulating your gable wall?
Sorry. See this latest attachment to better understand what I am looking at and what I am trying to do.
Mark,
1. Your gable wall can be insulated with whatever insulation type you want -- it certainly doesn't require spray foam or polyiso.
2. Concerning your remaining section of uninsulated sloped roof, you can insulate it many ways. If there won't be any ventilation baffles, then you need to use either spray polyurethane foam or rigid foam between the rafters with sealed seams.
3. In Zone 2, the code calls for a minimum of R-30 insulation at your roof or ceiling, so your R-19 is pretty thin. However, you may know that. If you can afford to beef up the R-19 by installing rigid polyiso board on the underside of your rafters, you'll get much better performance from your roof insulation.