1880 Renovation – Cathedral Ceiling Insulation
Hello all, I have a multi part question that I believe people will have strong opinions on. Thanks in advance for the help on this one.
I bought an 1880 house in Southern NH (Climate zone 6A) and am currently renovating the upstairs unit. I am converting the bedroom (which is half the second floor) to a cathedral ceiling and leaving the other half as a low ceiling (bathroom, kitchen/living, stairs). The rafters are rough cut 2x6s (most are 2 1/2 to 3″) spaced 27″ to 30″ apart. I am planning on adding 2″ of closed cell spray foam on the cathedral ceiling side of the house and filling in w/ R30 fiberglass batts. The spray foam contractor I talked to claims that I will end up being slightly more than 2″ all around and I will end up with R15 from the spray foam. For the walls I was planning on just doing batts (my budget isn’t unlimited) but I am open to other suggestions. My current plan is to cover the walls/ceiling with blueboard & plaster.
Other info:
– Currently steam radiators with an oil furnace but I am switching to mini-splits
– There is an obsolete chimney on one side of the room that I am leaving in place as the roof is currently dry and I don’t want to mess with it by adding another opening.
– The current heating system is steam radiators but I am removing those and converting to mini splits.
– The roof is half shingle/half corrugated metal. It is unvented.
My questions are:
1. I have to add depth to the rafters and the wall framing to make room for batts. Is there anything I can place on the existing rafters/posts to decrease thermal bridging?
2. If I insulate the wall between the bedroom half of the house and other half of the house can I not spray foam the entire ceiling and just leaving the loosefill in the attic of that half?
3. Is there any special way to insulate the chimney?
4. With this amount of insulation, bathroom fan, kitchen fan, and the mini-splits will I have enough air exchange?
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Replies
You might want to read this article: https://www.greenbuildingadvisor.com/article/flash-and-batt-insulation.
Martin indicates you need a minimum of R-25 for a safe flash and batt assembly. So that's more like four inches of foam plus an R-23 thickness of air permeable insulation. I'd be suspicious if a contractor claims that two inches of closed cell is good for R-15.
If the walls are open, you should focus on air sealing. See https://www.greenbuildingadvisor.com/article/getting-the-biggest-bang-for-your-air-sealing-buck and https://www.greenbuildingadvisor.com/article/how-to-hang-airtight-drywall.
If you are installing mini-splits, you should pay for an independent Manual J. See https://www.greenbuildingadvisor.com/article/who-can-perform-my-load-calculations.
On decreasing thermal bridging, you could use the Bonfiglioli method. See https://www.finehomebuilding.com/2014/11/26/breaking-the-thermal-bridge. But it might be simpler to add a new layer of parallel studs to mitigate things. If you are on a budget, this step is probably not worth the effort or cost. (Let's see if the professionals weigh in.)
If you tighten up the home significantly, you will need a ventilation strategy. This article provides guidance that should be helpful: https://www.greenbuildingadvisor.com/article/how-much-fresh-air-does-your-home-need
Steve, that is about as thorough of an answer as I could have hoped for. Thank you.
Hypothetically speaking, lets say the 2" spray foam gets me R-14. If I need a minimum of R-25 for a safe flash & batt, wouldn't I then only need to make up R-11 in batts (plus whatever inefficiency there is in layering the batt on top of the spray foam).
I am trying to balance my limited space for insulation with my limited budget for insulation.
(I assume your installer is recommending Icynene Proseal, which advertising R-7 per inch. Be aware that proper installation is critical with any open- or closed-cell foam.)
It's my understanding that some jurisdictions follow older or local versions of current code recommendations. In Zone 6, a minimum of R-49 attic insulation is usually recommended. But your foam contractor may be suggesting a U-factor approach, which would be R-38.4 in Zone 6.
Martin notes, "At first glance, the U-factor alternative path appears more lenient than the prescriptive path. In fact it is, although it may not be quite as lenient as some people assume, for the following reason: While the prescriptive path is based on the nominal R-value of the insulation rating listed on the insulation packaging, the U-factor path is based on actual U-factors calculated for the entire insulated assembly. The so-called “nominal” R-values shown on insulation labels don’t count."
See this article for more info: https://www.greenbuildingadvisor.com/article/three-code-approved-tricks-for-reducing-insulation-thickness
If cost is an issue, installing a flat ceiling and blowing in cellulose might be worth considering. (It would have to be ventilated, and you might not be able to get the full-depth where the roof meets the top plate.)
If you needed a new roof, installing reclaimed rigid foam above the sheathing would be an option. But this strategy is usually more expensive than working from the inside.