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Community and Q&A

Window air sealing

JeffS_ND | Posted in Energy Efficiency and Durability on

Hello!  My insulation contractor used low expansion foam to insulate/air seal around the windows.  He recessed the foam 2-3 inches from the inside.  I am wondering, would it be a good idea to still install backer rod and caulk in the gap on the inside?  Basically, would this be a wise way to improve the longevity of the air seal by doubling up, or might it cause a risk by creating a cavity between the foam and backer rod, should moisture manage to find a way in?

I am located in North Dakota in climate zone 6.

I would very much appreciate any advice!


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  1. Trevor_Lambert | | #1

    I think it's pretty common to air seal both the interior and exterior sides of the window. That creates essentially the same scenario as you're describing.

  2. Expert Member
    Akos | | #2


    Depending on what your window is made of, backer rod and caulk might be worth the effort. Spray foam doesn't stick to most vinyl windows, as the building moves, you will get some air leaks over time.

    If it is around something that will be opened and closed a lot (say patio door caulk is definitely a good idea as it is more flexible than spray foam and will hold up much better in the long run.

  3. Peter Yost | | #3

    HI Jeff -

    I wrote an article on liquid sealants for that you might want to review (in search at type in "Sticky Business" - their search is down right now so I could not get you the link to the specific article).

    The key is to select the type of liquid sealant (and backer rod) formulated for the substrate(s) in the assembly.


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