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Air sealing top plate

ddiluciano's picture

I have an 80 y.o. bungalow in Chicago.  Attic had no insulation, so went with an un-vented attic and had closed cell foam sprayed into the rafters.  Sadly, I failed to have the top-plate foamed at the time.  If I pull up the attic floorboards and foam the top plate between the joists from the inside, does this still leave the house vulnerable to stack effect from the exterior?  Can I open the soffit and apply closed cell foam to the exterior (home is brick construction) at the location of the top plate, essentially "sandwiching" the plate in foam, and then restore the soffit?  The sill plate is foamed in the basement. 

d (post #207474, reply #1 of 2)

Any opening that allows exterior air into that space would be a leak.

The joist bays at the exterior walls are large areas that only have the sheeting at the exterior of the frame as "insulation".  So they should be insulated.  Further, there are maybe/probably penetrations or gaps that go down through the top plates and into the wall cavities-perhaps all the way to the basement-barring insulation and / or firestops within the walls.

As far as the soffit area, good question.  Without taking a look, it would only be a guess.  I think once you take a look from the inside, you might be able to figure if the seal is appropriate and the best action is taken on the inside.

I'm assuming that they did take the foam down to the flooring at the ends of the rafters?

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Remodeling Contractor just outside the Glass City.


Thanks for the input Calvin.  (post #207474, reply #2 of 2)

Thanks for the input Calvin.  Yes, the foam contractor did take foam in the rafter bays to the attic floor, but didn't offer to pull the floorboards to do the top plate and I didn't think to ask (first time homeowner, hadn't had the benefit of the Fine Homebuilding air-sealing articles at the time, and construction knowledge base was a lot smaller than it is now!). 

That said, the bungalow has an octaganol front room with a low roof and a crawl space with no floorboards above (google images of  "chicago octagon bungalow" for examples).  That crawl space top plate is well sealed, and the room is quite comfortable, so maybe the interior sealing will be enough.  I too anticipate penetrations through the top plate, and figured on firebreak foam for them.  Looks like I'm pulling floor boards!  I may pull a piece of soffit this summer just to have a look, as  I finish the exterior paint.  It is the original beadboard soffit.  Kinda cool.