Insulating a shed roof addition from the 60s
I have some questions about insulating an addition built in the 1960s. I am located in Downeast Maine, in zone 6. The main home was built in 1952, with a traditional gable roof. Over the years, 4 additions were constructed, 3 of the shed roof variety. The room in question is a shed roof addition, resting on a poured foundation expanding the footprint of the original foundation, and topped by rafters resting on the original gable roof of the main portion of the house. The town has no records of the addition (not unexpected due to this being a rural area), but I’m guessing a build date of 1960s due to asbestos fiber-cement siding, and actual, not nominal, rough-sawn 2×4 wall framing and 2×6 rafters. The addition was missing a soffit board when we purchased the property, and that seemed to allow rodents in, who set up shop and destroyed the 1″ of old mineral wool that was stapled to the wall sheathing and set on the ceiling joists. The room leaks air terribly, and sits 4 – 5 degrees warmer or cooler then the main house. All the interior demolition has been done, and both exterior walls have studs exposed and rafters are accessible. The total enclosed footprint of the room is about 12×16, with the rafters resting about 4 feet beyond the original exterior wall of the main house. The walls are sheathed with some kind of 5/4 rough-sawn board, covered with felt, and clad with the asbestos shingles. The roof is sheathed with similar boards, but I can see a layer of plywood has been added over that board sheathing at some point. The roof is a standing-seam metal roof that was installed last year. No exterior foam insulation exists. I don’t have any plans to remove the roof if it can be avoided, and I have no plans to disturb the asbestos-bearing siding either. There is no evidence of water infiltration or damage to any of the sheathing in the room, and I’d assume after 5 decades it would be visible had it occurred. The house is reasonably dry (I’ve install 6 mil poly over the dirt floor of the basement, and I purchased a heat-pump hot water tank [Maine has a rebate program and the tank had a net cost of $0.01 after the rebate] and the resultant dehumidification is a happy byproduct of the tank), maintaining around 40% RH during heating season. Bathrooms and range are properly exhausted to the exterior. Heating is accomplished via an air-source heat pump with a single head, and a direct-vent LP stove. Cooling consists of a single window-mounted air conditioner which is only needed a few weeks a year.
Regarding the walls. I’m intending to cut and cobble 4″ polyiso into the stud bays (taped and caulked), and install 1″ of polyiso (taped and caulked as needed) on the studs to address thermal bridging. Walls will be finished with taped drywall. Electrical outlets on exterior walls will be removed and relocated to the floor. That should give me approximately R-32 and reasonable air sealing. However, I remember reading somewhere on GBA that a small gap between the rigid board and sheathing of this variety is beneficial. I can switch to 3″ polyiso and a 0.5″ gap and still exceed code for the assembly. I’d appreciate any comments and advice on that.
Regarding the ceiling. We’d like to eliminate the ceiling joists and convert to a cathedral ceiling. However, the shed roof creates a relatively low slope. The roof, as designed, is unvented. I’d rather not build the doghouse in the center to create a vented assembly, but I have a question about venting. Could I install 4″ polyiso on sticks to create a 1″ gap between the sheathing and the foam, and cut a 1″ opening into the roof of the main house where the rafters meet, and connect the rafter bay airspace from the shed roof to the airspace of the vented gable roof, and vent each bay at the soffit? I’d install 2″ polyiso (taped and caulked) onto the rafters and finish with taped drywall. That would bring the assembly to around R-39. While that is not code-compliant, the reality is, no code official will ever be investigating here. Alternatively, I could install 4″ polyiso directly to the sheathing, finish with a 1″ skim of closed-cell spray foam, and top with the 2″ polyiso. That would bring me closer to R-45. I also read on GBA of failures associated with cut and cobble and cathedral ceilings and I’d also appreciate advice and comments on this.
I can’t afford to fill the rafter bays to depth with closed-cell spray (they are around 5.75″ deep), nor can I afford to pull the roof and install 4+ inches of rigid on the exterior of the sheathing. As much as I’d prefer to handle it the correct way, I don’t have the funds for that. I have no problem spending the extra time required for cut and cobble. We are comfortable losing the small amount of interior space that the rigid foam installed on the studs and rafters requires. The room had a wretched drop ceiling installed and my plan of 2″ polyiso on the rafters will only cost an extra inch of headspace, which will only be noticeable where the rafters have been trimmed to turn horizontal on the top plate of the wall assembly. And just in case there is any concern, I will not be installing any recessed lighting, and will probably just surface mount the single romex run for the ceiling fan and light to maintain my air barrier.
I’ve been a long time lurker on GBA and have found it to be a tremendous resource, and I very much appreciate anyone who takes the time to respond to me. Thank you in advance.
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Maybe instead of focusing on home repairs, you should become a novelist. You like to write.
1. If your walls have asphalt felt, you don't necessarily need an air space between the sheathing boards and the cut-and-cobble foam. But an air space can't hurt (as long as the rigid foam is installed with attention to airtightness).
2. Whether or not your plan for 1-inch ventilation channels in your roof makes sense depends on the roof slope. The closer your roof is to flat, the less viable your plan becomes. What's the slope?
Ask my wife... she'll regale you for hours on my lack of brevity.
Roof Slope = somewhere between 4/12 and 5/12. I'll climb up and get an exact figure if it is helpful.
The usual definition of a low-slope roof is a roof with a shallower pitch than 4-in-12. Some sources say 3-in-12.
So you probably don't have a low-slope roof, and you can probably move ahead with your plan for a vented roof assembly using the cut-and-cobble approach.
If you can increase the depth of the ventilation space to 1 1/2 inch instead of 1 inch, I think you'll be able to sleep better at night.