Low-expansion foam
A window-and-door-rated spray foam used to insulate the gap around the frame — deliberately low-expanding so it fills without pushing.
What it means for you
The right product matters here. Standard high-expansion foam, or over-filling, bows the frame and makes the sashes bind and stop locking. A proper install uses low-expansion foam at a partial fill, not a stuffed gap.
Where you'll see it
In the insulation step of an install; the wrong foam shows up as sticking sashes later.
Don't be fooled
“We foam every window” is good only if it's low-expansion and not overstuffed — over-foaming is a real cause of a brand-new window that won't operate.
Related terms
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