Replies

1. 
   unclemike42 | May 27, 2021 04:22pm | #1

   As long as the duct is well-sealed (to avoid air leaks), and insulated, it should not make a huge difference.

   You were not specific as to where this house is, or how you will insulate the knee wall and ceiling below from the knee wall to the outside wall.

   Is there more going on, or are there registers you need to work in and connections to the main duct?

   Overall vapor barriers and insulation R value should be considered with this detail.

2. 
   florida | May 27, 2021 05:08pm | #2

   I wouldn't worry about it either way as long as the duct is sealed.

3. 
   Ben_ten | May 27, 2021 06:17pm | #3

   Thanks for the replies. This is climate zone 6 (southern Ontario), so very cold winters and some hot summer months too. It's an existing house, so the kneewall and the attic floor of the kneewall is already well insulated with fiberglass (walls) and blown in cellulose (floor) to R-value 60.

   There will be one take-off a small room register before it ends at a register.

   There is an existing vapor barrier in the wall assembly. For the duct, if I mastic seal and then spray foam the duct directly, I assume that would be good enough as a vapor barrier?

4. 
   unclemike42 | May 28, 2021 07:09am | #4

   Since you have to cut through the vapor barrier to put the registers in, make sure to seal around them well.

   Mastic should be fine to seal up the holes.

   I would pull the existing insulation back, install/seal the pipe, and replace/supplement the insulation over, focusing on the attic-facing side.

   Spray foam could work ok, but more fiberglass would work fine too. You would want enough spray foam to provide similar R value.

5. 
   Ben_ten | May 28, 2021 10:42am | #5

   Great, thanks for your advice! I will probably end up with a spray foam coat followed by cellulose/fiberglass on top of the duct.