Question About Air Gap in Conditioned Attic
I am completely redoing a 100+ year old house in upstate NY and I am trying to make a decision on the insulation details in the attic. It has 1×2 purlins that go from gable to gable.
My initial thought was to spray foam the underside of the roof deck, but I saw a few other strategies here on GBA for making an airtight ceiling, such as placing baffles to create an air gap to the soffits and then putting rockwool in the cavities, followed by a continuous layer of rigid foam on the underside of the rafters, etc…
My question is, if I were to go this route, could I use the existing air gap that the purlins provide at the gable ends, instead of going to the soffits with styrofoam baffles? The attic does not have a ridge vent either as it was previously finished. Anything else I should consider with going this route?
To keep the insulation out of the air gap, would radiant foil/bubble insulation be useful in this position or would something like tar paper be just as good? I’m in a cold climate so I’m assuming the foil insulation wouldn’t do as much above the insulation?
Thanks!
Josh
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Replies
Josh,
Unfortunately not. You need some mechanism to move air in and out of the vent channels, and that relies on there being a difference in height from the intake to the outlet. That's why flat and low-sloped roof are so difficult to vent successfully.
Thanks a bunch Malcom. That makes perfect sense!
Josh,
One option to consider is to add baffles to the bays from the eaves to the mini attic, fur down the rafters to get as much permeable insulation as you want, and add gable vents to each end instead of a ridge vent.