A window that leaks heat almost never announces itself. It shows up quietly: a fogged pane on a February morning, a curtain that stirs when the wind is from the west, a strip of paint lifting on the interior sill after heavy rain. By the time anyone notices, the mistake was made months earlier, usually in a single hurried afternoon with a tube of sealant and a screw gun. The ten failures below account for most of the heat and money that slips past a new window. Each one has a visible symptom you can check today, and a correction that does not require tearing out the wall.
1. No Head Flashing or Sill Pan
Water is the patient one. A window can survive a decade of wind-driven infiltration and still be destroyed in two seasons by a missing head flashing. Water runs down the wall, reaches the top of the window frame, finds the seam between the frame and the sheathing, and goes inside the wall — where it soaks insulation and starts on the framing.
Watch for a brown stain spreading on the ceiling below a window after a storm, softness at the lower corners of the frame, or insulation that feels damp when you pull back the interior trim. On the exterior, look for a bead of caulk doing the job a metal drip cap should be doing. Caulk is maintenance; flashing is construction.
The fix
Layer the weather barrier shingle-style, working from the bottom up. The sill pan goes in first, with a back dam at the interior edge and ends that turn up at the corners. Then the window, then the head flashing tucked behind the weather barrier above and lapping over the window's top flange. Tape the seams. Never rely on the flange alone, and never caulk the bottom flange — that traps water rather than shedding it.
2. Sealing Gaps With Spray Foam Alone
Expanding foam is a wonderful material used for the wrong job. Shot into a deep, narrow gap between frame and rough opening, it expands against the frame, bows the jamb, and leaves voids behind the cured surface that you cannot see and air can find.
The symptom is a draft that persists after foaming — you can feel it at the corners of the rough opening, especially on a windy day, or see the foam has pushed out and cracked. A simple test: hold a lit incense stick near the joint on a cold, breezy day and watch the smoke.
- Use a backer rod first. Push closed-cell rod into the gap to the correct depth, leaving roughly half the joint width for sealant or low-expansion foam.
- Use low-expansion window foam, applied in passes rather than one deep fill.
- Leave the bottom of the rough opening partly open to drain — a fully foamed sill is a bathtub for any water that gets in.
- Trim cured foam flush and seal the interior with a flexible sealant that will move with the frame.
3. Over-Tightening the Frame
This is the most common mistake on an otherwise competent job. The installer drives screws until they stop hard, then one more turn for good measure. The frame is now slightly out of square, and every consequence is downstream: the sash drags, the lock misses, the weatherstripping no longer meets its mate.
Symptoms show up fast. Try opening the window. If it sticks at one corner, or the reveal — the gap between sash and frame — is wider at the top than the bottom, the frame is out of square. Look for fasteners buried so deep they've crushed the flange, or a hairline crack near a corner weld. On a hot day, a compressed frame can crack a sealed unit; the fog between the panes is the receipt.
The fix
Shim at the correct points: under the sill at the ends and centre, at the head, and at the hinge side for operating windows. Drive fasteners through the shims, snug and no further. After every few screws, check the frame with a level and measure both diagonals from corner to corner — they should match. Then open and close the sash. If it drags, back the screw off before the trim goes on, not after.
4. Measuring the Old Window Instead of the Opening
Ordering to the size of the window you removed is a reliable way to end up with a frame that is either too small to seal or too large to level. Old frames were often out of square when they were installed. Measure the opening, not the thing that came out of it.
- Measure the width at the top, middle and bottom of the rough opening, and take the smallest figure.
- Measure the height at the left, centre and right, and take the smallest figure.
- Check both diagonals; a difference of more than a few millimetres means the opening is racked.
- Confirm the sill is solid and flat, and repair rot before ordering anything.
A gap wider than about 10mm on any side is difficult to seal properly and usually means the wrong unit was ordered. If the opening is badly out of square, fix the framing rather than forcing the window to absorb the error.
5. Sealing One Side and Not the Other
Interior sealing and exterior sealing do different jobs, and skipping either one costs you. Airtight interior joints stop warm indoor air from being pushed into the wall cavity by stack effect and wind. Exterior sealing and flashing keep rain out. Installers who seal only the inside get a window that feels tight but rots from the outside; those who seal only the outside get a drafty window with a wet cavity.
Feel along the interior trim on a cold, windy day. Cold air at the trim rather than at the sash is a sign the rough opening perimeter was never sealed. Look outside for gaps at the corners where the frame meets the siding — those are the ones that let water in sideways.
6. Ignoring the Sill Slope and Weep Holes
Every window with a sill needs a drainage path. Set the sill dead level, or worse, sloping slightly inward, and water stands on it until it finds a way through. Then it pools on the interior sill and stains the plaster below.
Symptoms: a dark tideline on the exterior sill after rain, water on the interior sill while the sash is closed, or peeling paint along the bottom rail. Check the weep holes at the bottom of the frame — they are frequently plugged by caulk, paint or a piece of trim the installer thought looked tidier.
The fix is to shim the sill with a slight outward slope so water runs to the exterior, keep the weep holes clear, and never caulk over the drainage path at the front face of the sill. If the window is already set level, a small trim sill on the outside, properly sloped and flashed, can sometimes redirect water without removing the unit.
7. Testing the Window Last
The final mistake is sequencing. Windows get installed, foam goes in, trim goes on, cladding closes the wall — and only then does anyone try to open the window. By that point, a frame that needs re-shimming is a demolition job.
Test in stages. Operate every sash after the first fixings. Check the reveal and the lock. Run the incense test before the interior trim hides the perimeter. Look at the exterior joints before the siding goes back on. Each of those checkpoints takes a minute and saves a day.
Fixing a Leak You Already Have
If the window is already in, work from the cheapest diagnosis outward. First, check operation and the reveal — a dragging sash tells you the frame moved. Second, inspect the exterior on a dry day: is the head flashing present, are the weep holes clear, is the caulk doing work that flashing should do? Third, test for air with smoke on a windy day and seal the interior perimeter with a flexible sealant and low-expansion foam where you can reach it. Fourth, look for water staining, which points to flashing rather than sealing.
If you find soft framing, widespread rot, or water reaching the wall cavity, stop and get a qualified installer or building surveyor to look at it. Rot does not reverse itself, and the repair cost grows faster than most people expect. Catching it in the first season is a weekend; catching it in the fifth is a wall.
Photo: stevepb / Pixabay
