Flash-and-batt retrofit for kneewall attic with hipped roof
A comment by Walta got me thinking about our attic space and its leaky kneewall glory.
I would like to explore this a little, in context of conditioning the attic and sidestepping some air sealing issues. Zone 3a.
We have a kneewall attic; the kneewall is below a ledger for the attic rafters (shed roof type of ledger attachment, etc.). Rafters are 2×10. The upper part of that 10′ wall is sheathed and sided, per normal for exterior walls.
Attic floor is directly above two bedrooms, which are currently an office and a storage room unless we have guests. There are comfort issues.
There are all of the normal kneewall attic issues:
Attic floor is fiberglass batts and a subfloor that almost extends to the soffit area.
Kneewall is just batts and 2×6 studs, no sheet goods on the attic side because of ducts and things. It’s complicated to sheet and seal.
There is a poorly sealed access door.
There are electrical outlets.
There are minor ducts (no equipment though), but they will be removed at some point. I might put some equipment there if the attic is conditioned.
And, the roof is hipped one one side!
Cheap option: Cut-in a hip vent when the roof is replaced, and do a vented shed roof and all the associated air sealing details.
Better option(?): Flash-and-batt, takes care of the hip venting and the air sealing. Can be done at any time.
Now for the minor details, which I’m a bit hazy on:
Is there any reason to remove the existing fiberglass batts under the subfloor?
What to do with the ledger-to-wall connection? Install horizontal blocking (similar to fireblocking) between the kneewall stud cavities and the ledger, and seal with spray foam?
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Replies
When insulating between rafters, there really aren't any good ways to properly vent a hip (from the interior) where the jack rafters die into the hip rafter, correct? I've seen people drill holes for crossflow, etc., but that's pretty sketchy for airflow and everything else.
I think spray foam against the roof deck is the only likely candidate because of the hip.
Also, how critical is air sealing the ceiling below, if the attic will now be conditioned space? I've seen mention of air sealing the ceiling penetrations, but I'm struggling with the actual importance of that if the attic space can remove air leakage-based humidity through the ceiling and wall plates below.
I'm not sure if this is just best-practice, or if this is a mandatory step to prevent issues. The existing attic subfloor would complicate that step, etc., as I'm sure the existing sealing is pretty poor.
Does this just depend on supplying the conditioned attic with adequate HVAC supply/return flow?