GBA Logo horizontal Facebook LinkedIn Email Pinterest Twitter X 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


sunrisehomestead | Posted in General Questions on

This is a topic I have been trying to get to the bottom of for some time. 

What is the best approach to insulating the shimming gap around doors and windows?

For ages we spray foamed and called it good. Now we have building science experts like building science fight club (christine williamson I think her name is?) suggesting that leaving this cavity uninsulated is far more beneficial from a water management and building longevity standpoint than insulating for a minimal r value but potentially impeding waterflow if there was a breach in the flashing system.

So I am left astray-wondering to insulate or not to insulate, and if so, what is the preffered product, material, method. 

This is all given that the back edge of the window would be air-sealed to the RO wether it had insulation in the cavity or not.

Any advice would be appreciated!

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. plumb_bob | | #1

    Not sure where you are, but up in cold country we take the insulating and sealing of the gap between framing and window very seriously. If not, there would be terrible cold drafts, which would lead to condensation, which leads to water.
    In the old days the gap was stuffed with fiber glass insulation, which performed poorly. Now CCSF is the most common application, and this works well unless there is substantial movement of the frame, which will crack the foam and lead to air leaks (I have heard this is true but have never seen a bad example).
    Some of the best building science guys out of Vancouver now recommend backing rod and caulk.

    1. sunrisehomestead | | #2

      I'm in northern california, coastal. My understanding from passive house training is spray foam is not to be relied on for air sealing and either a backer rod + caulk form of air seal or tape should be used on the interior to seal the jamb to the framing or air barrier.

      I've heard alot of folks talk against using spray foam in this cavity but haven't found any next level solution that seems educated and building science savvy.

  2. Expert Member


    I think this will answer your questions:

    In the BS & Beer episode her comments start around the 18th minute.

    Christine is fine with some insulation, as long as you leave the outer portion of the gap empty as a drainage plane, and to equalize pressure.

    1. sunrisehomestead | | #4

      GOT IT! this is super helpful I will take a look


    2. sunrisehomestead | | #5

      I actually don't see her commenting at 18 minutes. Might have to watch the whole video...unless theres a chance this is the wrong video?


Log in or create an account to post an answer.


Recent Questions and Replies

  • |
  • |
  • |
  • |