Winter bathroom condensation: when your fan isn’t keeping up

A hot shower puts a lot of water into the air. In summer, most of it stays in the air until the fan or an open window takes it away. In winter, the bathroom is full of cold surfaces: the window, the outside wall, and a ceiling under an attic that may be well below freezing. When warm, humid air touches them, it turns back into water. That’s the fog on the mirror, the drops running down the window and the damp patch that appears on the ceiling every January.

Tighter houses make it more noticeable. Health Canada notes that energy-saving measures that make a house more airtight can raise indoor humidity when there isn’t enough ventilation. New windows, fresh caulking and weatherstripping are good for the heating bill, but they also close the small leaks that used to let moist air escape, so the bathroom fan has to do more of the work.

A little condensation that clears quickly is normal. Condensation that hangs around for an hour, drips from the fan or leaves marks on the paint means moisture is staying in the room, and that’s how mould gets its start. Mould behind old tile is a common find when an older bathroom is opened up, which is why we take the fan as seriously as the tile.

Watch for these over the next few cold weeks. One on its own may be nothing; several together usually point to a ventilation problem.

• The mirror is still fogged long after the shower ends
• Water drips from the fan grille, or a brown ring appears on the ceiling around it
• Paint is peeling or bubbling on the ceiling or above the shower
• Black or grey spots in the grout and caulking keep coming back after cleaning
• Water collects on the window sill or runs down the tile
• There’s a musty smell, especially in a bathroom without a window
• Towels never quite dry

Most winter bathroom condensation comes down to one of these causes, and often more than one at once. When we look at a bathroom with this problem, we work down this list.

None of these need tools beyond a stepladder and a little patience, and they tell us a lot if you later ask us to look at the room.

1. Test the airflow

   Hold a single square of toilet paper up to the grille while the fan runs. If the fan can’t hold it against the grille, it isn’t moving much air.

2. Find where it vents

   Go outside while the fan is on and look for the vent hood on the wall, roof or soffit. If you can’t find one, or its flap doesn’t move, the air may be going into the attic.

3. Look in the attic

   If you can reach the attic hatch safely, look above the bathroom for a duct that ends in the insulation, or for frost and damp patches on the underside of the roof.

4. Measure the humidity

   An inexpensive hygrometer tells you where the house sits. Health Canada recommends keeping indoor relative humidity between 30 and 50 per cent.

5. Change a few habits

   Run the fan for at least 30 minutes after showering, leave the door open a crack afterwards, and squeegee the shower walls, as Health Canada suggests.

Bathroom condensation isn’t always a bathroom problem. If every window in the house fogs on cold mornings, the whole house may be too humid. Check whether a furnace humidifier is set too high, and remember that drying laundry indoors adds a lot of moisture, as Health Canada points out. On very cold days, turning the humidifier down a notch is often enough to stop the windows streaming.

In the bathroom itself, Health Canada suggests keeping blinds and curtains open so warm room air can reach the glass, and drying the sills and frames so water doesn’t sit on them. A window inside a shower is a special case. It gets soaked every day and it’s often the coldest surface in the room, which makes it hard to keep dry no matter how good the fan is.

A cold outside wall can also collect moisture, especially behind a vanity or a linen cabinet where air doesn’t move. Leaving a little space behind furniture helps. If the wall feels cold to the touch and the marks keep coming back, the insulation in that wall may be missing or poor, and that’s something we’d fix when the room is renovated.

Health Canada sorts mould by size. A small area is three patches or fewer with a combined area under one square metre, and it can usually be cleaned with proper precautions and protective equipment. A medium area, meaning more than three patches or between one and three square metres, calls for an expert assessment, although it may still be cleaned with care. A single patch larger than three square metres needs an expert to assess it and clean it up.

Two points matter more than the cleaning. First, fix the moisture, or the mould comes back. Second, don’t paint over it. Health Canada is clear that painting over mould doesn’t kill it or stop it from growing; it only hides it for a while. If the mould is under the paint, the drywall needs to come out and be replaced.

Grout and caulking are usually the first places mould shows up in a bathroom. Health Canada recommends repairing or replacing cracked or damaged tile, grout and caulking around tubs and showers. Once water gets behind them, the problem is no longer just on the surface.

If the checks point to the fan, a better one is often a modest job. What makes the difference isn’t only the fan in the ceiling but the whole path the air takes. A fan sized for the room and quiet enough that nobody minds leaving it on, with a timer or humidity control, gets used properly. The duct should be as short and straight as possible, sealed at the joints, insulated where it passes through a cold attic and sloped so any condensation runs outside, ending at an exterior hood with a damper.

Where the fan sits matters too. One installed far from the shower, over the door or in a corner, pulls dry air from the hallway instead of the steam. Moving it closer to the shower usually means opening the ceiling, which is easier to plan when we’re redoing the bathroom anyway.

Wiring a new fan, a timer or a humidity control is electrical work. In Ontario, almost all electrical work has to be reported to the Electrical Safety Authority, and anyone you hire to do it must by law be a Licensed Electrical Contractor.

Sometimes the condensation is the symptom and the bathroom itself is the problem. That’s the case when tiles are loose or the grout keeps cracking and letting water through, when mould comes back after a proper clean, when the ceiling or the wall behind the tub is soft or stained, or when there’s no exhaust fan at all and no easy route for a duct to the outside.

A renovation lets us fix the causes together: a waterproofing system behind the shower tile, insulation in a cold outside wall, a fan in the right place with a proper duct, and finishes that cope with moisture. If you’re considering it, our bathroom renovation page explains layouts, fixtures and pricing, and our tile work page covers what goes under the tile.

Winter is a good time to plan the job, because you can see exactly where the condensation collects. Take photos on the worst mornings; they show us where the problems are better than any description.

More on bathrooms, tile and keeping a GTA home comfortable through the winter.