Replies

1. 
   Roger Berry|Nov 16, 2021 05:58pm|#1

   Tom,

   A simple drawing of the section over the top plate would be helpful. Is the soffit height placed below or right at the top plate? What type of roof framing are you using?

   If you are creating an attic space using traditional framing with dimensional lumber then you need to detail the insulation very carefully up past the top plate. Your top plate and sheathing without the 2 1/2" of exterior insulation will come in at roughly R10 for 2x6 wall framing (less for 2x4). Your through stud, mid wall height R value would fall closer to R22 by comparison. Your mid bay, through wall with R19 batts, should be closer to R31.

   You should also consider how much insulation you can get above the top plate to further head off cold corners. It may help to imagine swinging a line going from the ceiling corner outward to where insulation or sheathing stops. At minimum you should aim for the same R value as you have for your wall (going through a stud location). Depending on details, you should aim for same or better R levels as you swing upward into the attic.

   As an analogy to consider, think about putting on a warm hat and coat, but wearing your scarf an inch above your collar. This is somewhat like stopping your exterior foam at the top plate.

   不幸的是,可用的填充高度for insulation at the roof/wall intersection in traditionally framed roofs can be low, even for 2x12's. After subtracting for vent chutes (if that is your plan) you might only have 6-8 inches of depth depending on pitch and how you cut the rafters. With cellulose insulation this leaves your R value over the top plate some where between R19 and R30 or roughly half of what should be in the ceiling for Wisconsin though close to your wall value.

   Hopefully, you are planning on energy heel trusses if using loose fill across the attic floor. In that case I would recommend going up past the top plate about half the height of the energy heel with the sheathing up to the height needed for your vent gap to prevent the loose fill spilling out. Depending on your truss details it should be possible to apply a 2x4 over the 2 1/2" foam so you have a place for fastening soffit support profiles.

   Futz-ing foam between rafters or trusses is always a PITA, but well worth the effort. I knew of houses in Illinois that had frost in room corners thanks to bad attic insulation details. I also lived in a house that had developed rafter ghosts over time due to the previous owners smoking. The rafters cooled the ceiling during cold months enough to selectively condense the moisture and smoke onto the ceiling. A stud finder confirmed my suspicions of the correlation.

2. 
   Wiscoguy|Nov 16, 2021 06:12pm|#2

   I am using trusses with a 14” energy heel. And the soffit does come in directly inline with the first plate of a double plate on the top of the wall. I have a small cathedral but I can still get r50 in it and have it vented the main concern is that connection. I do agree that it would be best to run the insulation past. I like the thought of just screwing a 2x4 over it that seem like the easiest fix not sure of why I never though of that.

3. Expert Member
   Zephyr7|Nov 16, 2021 07:59pm|#3

   As roger mentioned, a drawing would be really helpful here...

   The basic thing you want to do is make sure your top plate is insulated. If you have exterior rigid foam, you want that rigid foam to cover the exterior of the top plate too. You want whatever insulation is in the roof/attic floor to cover the top of the top plate. The end result you want is to have the top plate "wrapped" in insulation, just like the rest of the home's envelope, so that you don't have any thermal bridges hiding anywhere.

   Note that if you have vents, you want to make sure the rigid foam doesn't block the vent channels. What I have done in the past is to tie the air baffles into the top of the rigid foam so that the rigid foam doubles as the "end" of the baffle. This is needed if you want to prevent wind washing of insulation above the ceiling. Without a drawing it's hard to give any specific advice here.

   Bill

   Bill