Window installed out of square

[kbstat]

First I want to encourage anyone shopping for replacement windows to make sure you ask questions and evaluate the quality of the installer you select. I got Okna 800's installed this week. I am happy with the windows but not their installation. The installer only used fiberglass installation to fill some pretty large gaps between the new window the opening. I asked why they didn't use spray foam and was told that could warp the window. Do you think minimal expanding foam should have been used to fill air gaps?
My main question is I noticed that one window didn't latch as easy as others. I measured the diagonals and there is a difference of about 3/8". Opening the bottom sash 1/4" (measured on one side) reveals 1/2" on the other side. I assume that is considered an out of square window? How significant is this? I email the installer asking him to come take a look but he hasn't responded. Any suggestions on what I should do?

[HomeSealed]

Spray foam is ideal and fairly standard these days, but fiberglass is not wrong per se. You wouldn't be able to compel them to use foam at this point unless it was promised and not delivered.

As for the window, based on your comments it's sounds like some adjustments may be in order. I'd double down on your efforts to contact the installation company. If you've only emailed, give their office a call and try to move up the ladder to someone that can help. If somehow you still strike out, my next call would be to the manufacturer to add some pressure from the other direction, and then you certainly have recourse via social media, reviews, etc. I'd give them the benefit of the doubt and exhaust your other avenues prior to going that route.

[Windows on Washington]

+1.

FG is fine if done properly. Foam is just easier at this point.

[TheWindowNerd]

Window & Door foam is superior in at least three areas:
It water seals, if closed cell.
It insulates.
It air seals.
It acts like an adhesive.

[randy]

I would add:

It acts as a firm backer rod to support the caulking.

The WindowNerd’s point about air sealing is the critical one. Fiberglass insulation does nothing to prevent air leakage around the window since air will move right through it - which is why it is used in the cheaper HVAC return air filters. Unless it is in contact with the surface it’s intended to insulate, it’s of no value and a waste of time.

[Windows on Washington]

+1.

FG is fine if done properly. Foam is just easier at this point.

So we are clear about my previous statement...

In order for FG to be done properly, it needs to be sealed on both sides. So Randy and Window Nerd are both correct in that foam is vastly superior, but FG can be done if it has a seal on either side of it to stop air movement.

Again...begs the question...Why use it. Foam is better and easier.