Window Well Leaks and Wet Basement Walls

A window well leak lets water into the basement when the well fills faster than it drains and the water rises above the window sill or finds a gap around the frame. The usual causes are a clogged or missing drain, leaves and soil packed into the well, and ground that slopes toward it. The fix is almost always outside, and the wall below the window needs drying inside once the water has been stopped.

How a window well leak gets into the basement

A window well is a hole beside your foundation with a steel or plastic liner holding back the soil. It works only if water that lands in it has somewhere to go. Properly built, the bottom of the well sits well below the window sill, it is filled with clear gravel, and it either drains down through that gravel to the weeping tile at the footing or has a drain pipe that connects to it.

When that path is blocked, the well becomes a bucket. Water rises until it reaches the bottom of the window, then comes in through the joint between the frame and the concrete, through weep holes in the window, or straight over the sill.

The common reasons a well fills:

• Leaves, mulch and silt have covered the gravel and sealed the bottom.
• The drain pipe is clogged, crushed or was never installed.
• The soil in the well has built up to within a few centimetres of the sill.
• The ground around the well slopes toward the house, so surface water runs in over the rim.
• A downspout or a sump discharge line empties beside it.
• The liner has pulled away from the foundation and soil and water come in through the gap.

Signs inside the basement

In an unfinished basement it is obvious: a dark, wet fan shape on the concrete under the window, often with dirt streaks, and a puddle at the base of the wall after heavy rain. In a finished basement it is quieter. Look for staining or peeling paint on the drywall directly under the window, a stained or swollen window sill or stool, baseboard that has swollen below the window, damp carpet along that wall and a musty smell in that corner. Because the insulation and vapour barrier behind the drywall hide the water, the wall can be soaked inside while the paint still looks fine.

Timing confirms it. Window well leaks show during hard rain or fast snowmelt and stop soon after. Stand outside in a downpour and look into the well. If water is pooling in it, you have your answer.

Outside fixes, from simplest to biggest

1. Clean it out. Remove leaves, mulch and soil down to the gravel. Do this every spring and fall.
2. Lower the fill. The gravel surface should sit well below the sill, around 15 cm or more if the well allows. Dig out built-up soil and top up with clear stone, not pea-sized dirt-filled gravel.
3. Move water away. Extend downspouts so they discharge a couple of metres from the house, and clean the eavestrough above.
4. Fix the slope. Build up the soil around the outside of the well so the ground falls away from it. The rim of the liner should stand a few centimetres above the surrounding soil.
5. Seal the liner to the wall. Refasten a loose liner and seal the gap where it meets the foundation.
6. Add a cover. A clear sloped cover keeps rain, snow and leaves out. On a bedroom egress window it must be easy to open from inside.
7. Repair or add a drain. If the well still fills, the drain needs to be cleared or a new one installed down to the weeping tile. That is excavation work for a waterproofing contractor.

Drying the wall below the window

Once the well is draining, deal with what got wet. On bare concrete this is simple: mop up, run a fan and a dehumidifier, and let the wall dry. A finished wall is different. Fibreglass insulation holds water against the wood framing, and the bottom plate sits in whatever ran down to the floor. Left closed up, that cavity can stay damp for weeks.

The usual approach is to remove the baseboard and cut out the wet drywall and insulation under the window so the framing and concrete can dry in open air. Air movers and a dehumidifier run until the wood reads under 16% moisture content, with readings logged each day. If the well has leaked many times, expect some mold on the back of the drywall or the framing. Health Canada's guidance is that indoor mold should be removed whatever the type, and it is cleaned under containment so the dust does not spread through the basement. Basement mold jobs here typically run $1,200 to $9,500. New drywall and paint afterward are done by a contractor.

What to do this week

Go out and look into every window well. If you can see soil or leaves instead of clean stone, or the fill is close to the sill, clean it out now and check where the nearest downspout ends. Then press on the drywall and baseboard under each basement window. Anything soft, stained or musty deserves a moisture check before the next heavy rain.

Common questions