GBA Logo horizontal Facebook LinkedIn Email Pinterest Twitter Instagram YouTube Icon Navigation Search Icon Main Search Icon Video Play Icon Plus Icon Minus Icon Picture icon Hamburger Icon Close Icon Sorted

Community and Q&A

Vapor Barrier in Knee Wall

Victor_V | Posted in GBA Pro Help on

My question relates to the need for an air barrier only vs an air+vapor barrier in a knee wall application.

I have a 1.5 story cape-style house in Zone 5, Massachusetts (mid-50s).  My goal is to make the space behind the knee wall a conditioned space. 

The roof assembly from outside in is Asphalt shingle -> Vapor/water/air barrier -> Wood tongue and groove roof (not OSB or plywood) sheathing -> Rafter vent in a 2 x 6 rafter -> R-21 fiberglass batt insulation.  (In one newer area the roof sheathing is plywood.)  The rafter bay is air sealed at the soffit end from the bottom of the rafter vent to the wall plate using a 1” foam block with the perimeter of the foam block sealed.

What’s Next

Some posts on this site have stated that a Class I vapor retarder is needed where some have stated that a Class III is better for this application.   The 2020 DOE Ueno/Lstiburek report (p. 98) suggested the need for an air+vapor barrier, but it wasn’t clear on rating, or what sort of barrier was beneath the shingles.  My obvious concern is condensation. 

Summary questions:  Given that a Class I vapor retarder is under the roofing shingles, should I use a Class I or Class III vapor retarder under the rafters?  

I appreciate your guidance.

GBA Prime

Join the leading community of building science experts

Become a GBA Prime member and get instant access to the latest developments in green building, research, and reports from the field.


  1. user-2310254 | | #1


    I’ll give you a bump. If you are concerned about condensation, then you need to address leaks by air sealing the structure. Of course, that’s a challenge with 1.5 story capes. (See for a related discussion.)

Log in or create an account to post an answer.


Recent Questions and Replies

  • |
  • |
  • |
  • |