Expert: Ted Barnhill (Principal, Design45 LLC) - 11/23/2009

Question
Hello,
  
I have been searching the internet for properly insulating
a knee wall in an ventilated attic. Half the places tell me
to use fiberglass insulation on back side of knee wall in the attic and air seal with installing rigid foam board over top
the fiberglass insulation. The other half say I can't use
rigid foam board over fiberglass insulation in attic side of knee wall. What is the right way to insulate knee wall from the attic side? Which rigid board is recommended by code?

  Thank You  

Answer
Michael,
Many foam boards are flammable, and thus are required by code to be covered with drywall or another approved fire barrier.  Dow Thermax or other polyisocyanurate foam board is one exception to this rule, and may be left exposed.  If you intend to finish the space as a room, it is normally simplest and most economical to use 1/2" drywall over a less expensive insulation solution, such as EPS board, XPS, or batt insulation.  For an exceptional air seal, spray foam insulation is the preferred choice.  In an attic, it is normally applied to the rafters (with venting if feasible, though sometimes as an unvented "hot roof"), providing you with additional insulated storage space behind your kneewall.

I hope this is helpful.  Good luck with your project.

Ted Barnhill
Design 45, LLC