Insulation Issue: Renovating EPDM low slope cathedral ceiling with flash and batt and venting problems
After researching GBA to death, I have come to the conclusion I need to ask the experts. Forgive the novel but I’d rather explain in as much detail as I can.
I am in the midst of an addition/renovation project and have hit a major snag with the roof/ceiling. I’m never going to achieve GBA nirvana, but ‘good enough’ is better than what I had. My grandfather built the house in the 1960’s and it was recently willed to me, but it’s time for changes. Kudos to my grandfather for building a freakishly strong house, but not a well insulated one.
-Zone 5 (Southern NH), on a lake, lots of snowfall and cold. Higher than I’d like heating and cooling bills and colder and warmer than I’d like. Baseboard heat (from tankless LP water heater) and Mitsubishi mini-split (1 year old)
-EPDM roof (approx 6 years old), low slope (approx 8 degrees), unknown thickness of nailbase/insulation (if any)
-Cathedral ceiling 2×10 rafters with tongue and groove inside (24” OC in existing house, 16” to 8”ish in addition)
-Multiple skylights/ construction obstructions with a number of ‘dead’ rafter bays and areas – no venting possible. Soffit vents along whole length of house, both sides. No ridge vent.
-Major ice dams but thankfully no leaks
Much of my existing ceiling in the kitchen/living room is stripped to the rafters. It consisted of, in order: EPDM, 2 sheets of plywood, 4” or so air gap, 2x 2” ISO cut and cobbled in bays. Below rafters was ½” thick fiber board of some kind and 2” ISO sheets ‘sealed’ with clear tape. Then strapping and tongue and groove pine.
My 9’x20’ addition is basically extending the length of this area of the house following the same roof line and walls.
The rafters in the existing house are true 2×8 (8” deep bays) so I’m going to fur them 1.5” to bring them down to the same level as the addition with 2x10s (9.25” deep bays). When I’m done, it won’t look like an addition inside or out.
My budget is far from infinite, but know I need to spend to save.
My current plan was to do the following:
-Site built 1” air gap using 1” ISO I have on hand. Make 1” spacer and cut and cobble in the ISO. Read an article on making your own soffit vents.
-2” closed cell spray foam
-R21 unfaced batt (no double vapor barrier)
-3/8” (maybe 1/2”) plywood (structural stability and ignition barrier)
-3/4” tongue and groove
I’m looking at the following for R-Values and thickness:
1” ISO = R6
2” Closed Cell = R14
5.5” R21 Batt = R21
Total = R41, exceeding the required R38 and with a total thickness of 8.5” leaving me about 3/4” of wiggle room in the 9.25” bay. This will make my building inspector happy and hopefully leaving me warm and cool with a nice tight ceiling.
My concerns are this. I have maybe 1/3 of the ceiling/roof with no air flow and the perfect storm of bad – Cathedral ceiling, low slope, EPDM roof, pathetic baffles, and closed cell spray foam. If I do my plan over the entire area I would have a combination of hot and cold roof. Should I attempt to cross drill some small holes through the rafters from the vented areas to the un-vented? One baffle manufacturer said that is the only option and to go for it and hope for the best.
I have limited depth to work with so closed cell is my only option to reach R38. My roof is basically new so adding insulation on top is not an option. After reading dozens if GBA articles, I know I’m in a pickle.
Help!
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Replies
Robert,
Important point #1: "Multiple skylights/ construction obstructions with a number of ‘dead’ rafter bays and areas – no venting possible." Implication: You are creating an unvented roof assembly.
Your ratio of foam insulation to fluffy insulation (20/21) will work for your climate zone (Zone 5). For more information on the ratio issue, see Combining Exterior Rigid Foam With Fluffy Insulation.
Q. "Should I attempt to cross drill some small holes through the rafters from the vented areas to the un-vented?"
A. No. It sounds like you should assume you have an unvented assembly.
Martin,
Thank you very much for your reassurance but is it worth keeping the air gap anywhere? This part of the renovation is relatively easy; the rest of the house is frustrating and I don’t know for sure what the ceiling baffle situation is up there. I can only speak to the portions of the ceiling I have personally examined. Those baffles are going to be a pain, but I am willing to do it provided the risk/reward is acceptable to fine folks like yourself. Yes, I’ve read about the ratio hence how I came up with that recipe.
While I realized that my initial plan would be making portions of the roof unvented, I always understood one should never sandwich sheathing between two impermeable layers (EPDM and closed cell), hence my initial conundrum.
However with the extra inch gained by not having the baffle, I can get readily available and less expensive 6.5” thick R19 in there as opposed to more expensive and less available 5.5” R21. My very rough calculations are 215 sq/ft of 15” wide and 475 sq/ft of 22” wide. Granted not a huge space but keep in mind the 2” of closed cell up there to provide a nice seal.
I’d be losing R2 and a questionable air gap, but putting more money in my pocket I will be spending on additional insulation elsewhere in the house. I have a LOT more insulating to do. It’s like a screen door in a submarine. Plus with the way the house was built and added on to, I just might have more nooks and crannies than an English muffin.
Thoughts?
Robert,
I think the air gap is beneficial. Your plan is fine.