We get called to look at 'defective' windows regularly — drafty, leaking, hard to operate, fogging at the edges. The windows are usually fine. The installation is usually the problem, and the same handful of mistakes appears over and over, whether the installer was a rushed crew or a capable DIYer following a YouTube video.
Here are the seven failures we see most, and what correct practice looks like.
1. No sill pan or back dam
Water will eventually get past any window — good installation plans for it. A sill pan (formed flashing or flexible membrane with a back dam) catches that water and directs it out. Skipping it means the first seal failure drains into your wall framing. This single omission is behind an enormous share of rotted sills and stained drywall below windows.
2. Flashing installed in the wrong order
Flashing works like shingles: every layer must lap over the one below so water always moves down and out. Taping the head flashing under the WRB instead of over it — or sealing the bottom fin so water is trapped inside — reverses the drainage plane. It looks fine at final walkthrough and leaks in year three.
3. Out of level, plumb, or square
A window set even slightly racked never operates right: sashes bind, locks misalign, reveals run uneven, and weatherstripping can't seal evenly. Pros check level at the sill, plumb on both jambs, square across diagonals, and — critically — that jambs are straight, not bowed by shims or foam.
4. Over-foaming the frame gap
High-expansion foam blasted into the shim gap bows jambs inward — the classic cause of a brand-new window whose sash suddenly won't slide. Correct practice uses low-expansion window-and-door foam, applied in moderation, or backer rod and sealant. If a new window got harder to open after insulation day, this is why.
5. Missing or misplaced shims
Windows need solid bearing at specific points — under sill ends and mulls, at hinge and lock points on the jambs. Missing shims let the frame sag over time; shims in the wrong spots bow it. Long screws cranked through an unshimmed jamb pull the frame out of straight, with the same binding results as over-foaming.
6. Caulk as a substitute for flashing
A caulk bead is a maintenance item with a service life; flashing is a drainage system. Installations that rely on perimeter caulk alone — especially at the head, where gravity feeds water to the joint — are on a countdown. Correct details put flashing behind the cladding and use sealant as the finish layer, not the defense.
7. Ignoring the manufacturer's instructions
Every window ships with installation instructions, and deviating from them doesn't just risk leaks — it voids the product warranty in most cases. Fastener type and spacing, required clearances, foam restrictions, and flashing sequence all matter to a warranty claim.
The pattern in all seven: the mistakes are invisible at handoff and expensive at year three. JMB Builders installs windows and doors to manufacturer spec across the Twin Cities metro — for homeowners, and as the sub GCs trust when callbacks aren't acceptable. Call 763-204-2099.
