Cathedralizing attic spaces in a Cape
I have modified cape style house in zone 5. Originally built in the early 70’s, extensively renovated and added onto in the early 2000’s by the prior owners. About 1/2 or 2/3’s of the roof / attic spaces are accessible without demolition, though these spaces (e.g. the “top attic”) are mostly pretty tight. The remaining 1/3-1/2 of the roof is above sloped ceilings which appear to have baffles and fiberglass insulation. All of the roof pitches have a direct path from soffit to ridge vent. HVAC serving second story is in the “top attic”. Shingles are around 15 years old and seem in good condition, so don’t want to tear them up for exterior roof insulation.
Main goal is simply to increase energy efficiency, as well as reduce variability (“cold areas” vs. “hot areas”).
In the attic over the addition, there is currently about 6 or 8 inches of fiberglass insulation, and judging from the cold air coming down from the can lights below, its pretty leaky (its also one of the coldest areas of the house). There are 2×10 rafters 16″OC and a vent at the top of the ridge. To improve the insulation and air sealing for this area, I was planning to “cathedralize” the attic, but because I can’t access 100% of the roof, I was planning to maintain a pathway from the soffit to ridge vent. Specifically I was planning to make a 1.5″ baffle using 1.5″ rigid foam with some thin strips of the same foam as spacers beneath. I was planning to also add 2″ rigid foam strips under the rafters. I would spray in ~2.5″ of ccSPF, leaving ~5.5″ in the cavity for R-23 rockwool batts. I calculate the cavity R at about 45. For the gable end walls (2x4s 16 OC), I was planning to repeat the 2″ foam on the studs, spray on ~2″ of ccSPF, and finish with R-15 rockwool batts. That should be R-27 in the cavities so again above code.
1) Overall does this seem like a reasonable plan? Any major issues I’m missing or opportunities to make it cheaper / less labor / better performance?
2) For the homemade foam baffles, since there is ccSFP on top, can I skip the air sealing and just kind of “rough fit” these and then let the ccSPF do the rest?
3) Would I really need to pull up the existing insulation from the floor / ceiling below once complete? And some rigid foam previously installed on a vertical wall leading to conditioned space from this attic?
4) Any tricks to connecting the new baffles and insulation to where the existing baffles and sloped ceiling fiberglass insulation emerge from below? I think the basic idea is to keep an open connection baffle to baffle, and then seal out any other spaces (e.g. cover the fiberglass from below with a piece of foam board to force any air that is coming up from the soffit through the fiberglass into the new baffle)?
This has gotten pretty long as it is — I’ll post separately with a similar set of questions but some different specifics for the “top attic” and a large knee wall area in the original 1970’s part of the house.
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