Mold around windows: condensation, flashing, and what it costs

Why windows grow mold

Glass is the coldest surface in a heated room. When warm, humid indoor air touches it in winter, water condenses on the pane and runs down to the sash and the sill. The sash corners, the bottom of the frame, and the sill stay wet through the cold months, and mold grows on the paint, the caulk, and the wood.

Single-pane glass and aluminum frames get colder than double-pane and vinyl. Blinds and heavy curtains trap still, cold air against the glass. A house kept humid for comfort, or a house with a humidifier on the furnace, condenses on every window.

The second source is rain. Flashing at the head and the sill of a window sends water out over the siding. When the flashing was skipped, installed backward, or the sealant failed, rain runs behind the trim and into the wall below the window. Weep holes at the bottom of a vinyl window that clog hold water inside the frame. Both cause growth on the sill and the wall under it that has nothing to do with indoor humidity.

A double-pane window that fogs between the panes has lost its seal, and the inside pane gets colder, which brings back the condensation.

Signs you can check at the sill

Look at the lower corners of the sash and the sill, and at the wall and the trim under the window.

• Black or dark green spotting on the sash corners, the caulk, and the bottom of the frame.
• Paint on the sill or the trim that bubbles, flakes, or stays dark.
• Wood sill or trim that is soft when pressed with a fingernail or a screwdriver.
• Staining, bubbling, or softness on the drywall below the sill.
• Trim that has swelled or pulled away from the wall.
• Water droplets on the glass on cold mornings, pooling at the bottom.
• Fog between the panes of a double-pane window.

The pattern tells you the source. Growth on the sash and the inside of the sill, with water on the glass in the morning, is condensation. Growth that is worse after rain, or stains on the drywall below the sill and down the wall, is a leak. Condensation is a cleaning and humidity fix. A leak is a flashing fix and often a wall job.

Press the sill and the trim with a screwdriver. Sound wood resists. Rotted wood gives.

What happens if it waits

Condensation mold on a painted sill is a surface problem that comes back every winter until the humidity drops. Left alone, it works into the wood. A wood sill that has absorbed water for several winters rots, the sash follows, and the window stops closing well.

A leak is worse. Water behind the trim wets the framing around the window, the sheathing behind the siding, the insulation, and the drywall below. The rough opening rots, the window sags, and the wall under it grows mold behind the paint. At that point the fix is a window replacement plus a wall repair, and the mold work is the smaller line.

Growth on a sill is small. Growth behind the wall below it is the behind walls problem. Mold can irritate people who are sensitive to it, and a bedroom with black sash corners smells on cold mornings.

Buyers’ inspectors check every window for condensation staining, soft sills, and failed seals. A soft sill and a stained wall under it go in the report as water intrusion.

What remediation involves at a window

Condensation mold is a cleaning scope. The crew removes failed caulk, cleans the sash, the frame, and the sill, treats the surfaces with an antimicrobial, and recaulks or repaints with a mold-resistant paint. Growth on a painted drywall surface gets cleaned and sealed. HomeWyse prices cleaning-only work under favorable conditions at $2.30 to $2.80 per square foot, and a window is a few square feet.

A soft sill or trim gets removed and replaced by a carpenter. That is a small repair priced on its own.

A leak turns it into a wall job. The crew sets containment, cuts the drywall below the window, removes wet insulation, cleans and treats the framing, and dries it. Outside, the trim or a section of siding comes off so the flashing can be redone, which is a window installer or siding contractor line. If the rough opening has rotted, the window comes out and goes back in with new flashing.

The bathroom mold page describes the same split between cleaning and demolition for another small, wet room.

What moves the price around a window

The band shown above, $500 to $1,500, is the small-area band. The engine applies it to window surrounds because a window job that stays on the sash, the sill, and the trim is a visible, contained, accessible scope, the same shape as a small bathroom job. This Old House prices that small-area scope at $500 to $1,500.

Within the band, the number of windows is the main driver. One bedroom window with black sash corners is the bottom. Every window on the north side of the house is the top.

Material moves it. Vinyl and aluminum clean. Painted wood gets sanded, treated, and repainted. Rotted wood gets replaced.

The conditions that turn it into a wall job, priced at $1,000 to $12,000 in the walls band, are specific: staining or softness on the drywall below the sill, a wet reading on a moisture meter in the wall under the window, growth that is worse after rain than after cold nights, trim that has pulled away with wet framing behind it, and a rough opening that gives under a screwdriver. Any one of those means the wall is involved, and the job is priced by the square foot at $10 to $25 plus drywall replacement at $1.00 to $2.10 per square foot finished.

Window replacement is a separate line and is often the biggest number on a leak job.

Get a planning range from the estimate tool. These are planning ranges from national data. An independent remediation company confirms scope and price on site.

Keeping windows dry

Lower the indoor humidity in winter. EPA’s target is below 60 percent, ideally between 30 and 50 percent, and a cheap gauge on the wall shows where you are. Turn the furnace humidifier down. Run bath and kitchen fans. Use a dehumidifier if the house stays above the target.

Open blinds and curtains during the day so air moves over the glass.

Add storm windows or replace single-pane glass, which raises the inside surface temperature and cuts condensation.

Wipe the sills on cold mornings and recaulk the sash when it stays dark.

Keep the outside sealed. Check caulk at the trim each fall, clear the weep holes, and look at the wall below each window after a hard rain. More in preventing mold from coming back.