Adding roof vents to slate roof before insulating
Hello folks! I’ve got a 1940 townhouse with a slate roof in Zone 5A (southern CT). We moved 2 years ago and I discovered that there was no insulation in the roof at all. It’s an unvented cape style roof on the third floor with a couple of rooms and a small triangular attic space over top. After reading about insulating this type of roof on here ( https://www.greenbuildingadvisor.com/article/the-bs-beer-show-unvented-roofs-and-fluffy-insulation) and talking to contractors, I think we’re likely going to go with dense pack cellulose in the sloped closets and loose fill over the top of the rooms under the roof. (We’re avoiding sprayfoam for a few reasons, partially because we have a party wall and apparently it would mean evacuating the neighbors and partially because if anything goes wrong it’s a real pain to remove etc.).
Especially with a slate roof any roof damage would be very bad, basic construction of the roof is wooden shiplap slats over old (real) 2×6, then there’s some tar paper and the slates. No ridge, soffit or gable vents, but the roof definitely isn’t completely airtight. So I worry a bit about taking all the breathing surface area away.
I have a couple of questions:
1) would there be any benefit / tradeoff to going with mineral wool batts in the sloped section of the roof? I worry a little bit about a roof leak going undetected because it gets soaked up by the cellulose: https://www.finehomebuilding.com/2009/06/29/why-i-dont-use-cellulose-or-blue-jean-insulation
2) The video above talked about how it’s much better to have ventilation in the attic triangle over the living space (where the ceiling of the living space / floor of attic is insulated). We’re getting our valleys redone (the previous owners tarred them, big no-no for slate) before the insulation and can have the roofer install a couple of roof vents at the same time, it’s a bit expensive as is everything with slate roofs, but it feels to me like it could be good insurance to avoid moisture buildup at the ridge and bigger problems down the line from insulation, but I wanted to see what folks here think.
3) I’ve seen some questions / answers where people recommend installing baffles between the roof and insulation to allow airflow along the roof line even though it’s an unvented roof (ie https://www.greenbuildingadvisor.com/question/attic-insulation-slate-roof-double-brick-walls-no-soffit-vents), is there any reason to do this in this case? Is it “safer?
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