Installer skipped the foam on new windows, saying it voids the warranty. Is that true?

The owner asked in Reddit's r/Homebuilding forum whether to pull out the fiberglass and use low-expansion foam or simply caulk. The installer's reason for skipping foam was the warranty.

Andersen's installation instructions for its 100 Series windows, dated September 2025, put low-expanding foam on the list of required supplies. They say the joint between the frame and the opening has to be sealed all the way around on the interior side, and that leaving it open will hurt the window's performance. Installers can either fill the inner third of the gap with low-expanding foam or set backer rod and sealant. The warning attached to both methods is about quantity: too much foam or overstuffed backer rod can bow the jambs.

Ply Gem gave installers similar advice in an Oct. 19 post. It says the AAMA 2400-21 installation standard allows foam from aerosol cans in the space between the frame and the rough opening if it is a low-pressure product tested under AAMA 812, and it recommends backer rod and sealant for the exterior joint, with a sealant joint at least 1/4 inch wide. Because low-expansion foam swells slowly while it cures, Ply Gem says, a properly applied bead is less prone to distorting the frame.

Neither manufacturer lists loose fiberglass as a way to seal that gap. Warranty terms differ by brand and series, so the owner's first step is the installation guide for the exact model, which shows the method the maker expects. If foam goes in, it should be a low-expansion product filling part of the depth, and the frame should be checked for bow before the trim goes back on.