Warped windows?

[a2mamaboose]

Imagine the single hung vinyl window (1996 builder's choice) closed and locked. At the bottom middle, the window has a tight seal and no air penetration. As you near the corners, a gap starts forming. At the corners, the gap is 1/4"! I can see daylight through this gap, not to mention the air I feel. I have 8 windows with this problem and all are on the north end of the house; 2 on the north side; 3 on the east side; 1 on the west side but all on the north end of the house. I wondered if it is a foundation issue, but nothing else is proving that theory. No wall cracks, no sticking doors, etc. I'm wondering if it's just poor craftsmanship. Not enough shims when installed?

I am almost to the point of opening up the wall under the windows to see if anything obvious jumps out.

I have plastic'd the windows with this problem for now. I would appreciate any and all theories or suggestions.

[masterext]

You need new windows, no way to fix a warped sash. Vinyl is a great building product as long as its a high grade vinyl material with a design that ensures structural integrity. The warping you are experiencing is very common in lower end vinyl such as the products sold at building supply houses and home centers.

[Window4U (IL)]

I've found that almost all old wood windows I've replaced have a crown in the middle of the old sill. That's why I always made it part of my procedure to put a 1/4" thick shim under the new insert window at each side to raise up the ends a little. That way the bow in the middle wouldn't transfer to the new vinyl window.
It's very likely that masterext is 100% correct and that you need new windows, especially if the bowing is caused by the vinyl sills or sashes being permanently warped.

[TheWindowNerd]

If you have frame construction your primary cause could be compression settlement.
Particularly if this was a problem that developed over time.