Seeking tricks or techniques to air seal blocking (cathedral 1950’s ski lodge)
I’m looking for the right strategy to seal up the blocking at the ends of rafters, where the ceiling is open beam.
The building was built in the 1950’s and they stuffed felt into the cracks. At least for most of the rafters wind, snow, and certainly embers from a fire get in now when the wind blows. It literally can be snowing inside, kind of cool, but also cold.
Can anyone suggest at good set of methods to achieve all the goals:
* Air seal
* Insulate
* Armor any caulk or sealants from future sandblasting of the outside. The T&G outside siding will be sandblasted this summer.
* Avoid introducing any combustible foam where sparks from outside may find them.
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Replies
Is the diagonal TnG both the sheathing and the siding? If not, and you plan on re-roofing and re-siding, maybe a good approach would be doing a sort of "chainsaw retrofit" (looks like there's rafter tails) ... see fine homebuilding article here - https://www.finehomebuilding.com/project-guides/roofing/chainsaw-retrofit-and-energy-performance?oly_enc_id=9785J1277156D3D
You saw the rafter tails off the outside, go over the TnG sheathing on the outside w/ a membrane like Henry Blueskin VP100 (depending on climate zone). If youre concerned about embers perhaps choose a closed-gap or TnG siding if you plan on doing a rainscreen.
Airsealing from the inside - especially over the felt and stuff - will be lots of work and probably expensive in terms of caulk and sealants. ccSPF is not a reliable air barrier and it looks like you/those in the cabin will be close to it (loft is inhabited).
Youll prob gain best energy performance from eliminating the windwashing of the conditioned space by air/weather sealing from the outside (given the self adhered WRB like Henry Blueskin) and then can figure out best way to insulate from there -
maybe some exterior mineral wool (comfortboard) and batts (fiberglass or sim.) on inside stud/rafter bays in terms of availability/performance/fire tolderance? Vapor open insulations and WRB will also keep your old cabin drying where ccSPF or rigid will not.
To @jhwehrli . No. No residing. There's 3/4" skip sheathing, then 3/4" T&G cedar siding nailed to the skip. The skip is 6" nominal, the cedar is 1" nominal.
It's that existing siding that will be sandblasted.
And obviously the building will shift and groan from time to time.
Then your best bet may be just using a bunch of caulk and backer rod (stuffing the felt in and putting backer rod over as good substrate?) like SashCo big stretch where the rafter blocking ends and then doing inorganic (rockwool probably) batt insulation w/ a carefully taped and detailed smart vapor retarder as your interior air barrier on the inside. Adding a 2x2 interior service cavity would make electrical etc. easier if you dont mind losing the couple inches.
I am sure the usual experts here will have a well rounded opinion on this but doing something like flash/batt with ccSPF on the inside seems like a poor idea given that your insulation has so much exposure to the elements - it will probably crack or separate from the interior of the sheathing and since it's vapor closed become bad news fast. So my recommendation is from out to in:
Sandblasted siding
Skip sheathing
Caulk/backer rod at all felt / open joints and/or low expansion canned foam
Mineral wool batt insulation
Smart vapor retarder (taped, continuous)
2x2 service cavity (1.5" mineral wool optional in bays) (service cavity optional)
finish (gwb / tng / etc.)