Attic half conditioned half not – GreenBuildingAdvisor

Replies

1. Expert Member

   Michael Maines | Mar 27, 2026 06:32pm | #1

   Nope, I do that often, including on a few current projects. The trick is that where the attics change, you need to create what is essentially an exterior wall, though cladding and a WRB aren’t necessary. (Airtightness is though.)

2. Expert Member

   DCcontrarian | Mar 27, 2026 06:55pm | #2

   There’s no theoretical reason it wouldn’t work.

   The practical difficulty is going to be that your building envelope line is going to be a complicated shape that goes up one end wall of the house, across the roof of the conditioned part, down the dividing wall and across the floor of the unconditioned part. My experience is a lot of sub-contractors just don’t understand the difference between conditioned and unconditioned space or the significance of the barrier between them and when you have a complicated boundary it’s a constant struggle to put things on the right side of the boundary.

   I don’t know how far along you are with the build, but I think this is a better way:

   https://www.greenbuildingadvisor.com/article/plenum-truss-keepin-the-ducts-inside

3. 

   walta100 | Mar 28, 2026 09:07am | #3

   If by “unconditioned attic” you really meant to say “vented attic” that would be workable. If by unconditioned you mean to violate the code with something unconditioned and unvented that would be a poor idea.

   To my ear HVAC in the attic of new construction screams poor planning and someone unwilling to pry open their mind and do the work to make room in the conditioned space for the equipment and duct
   
   work.

   Walta