Ventilation, exhaust fans, and make-up air

Because the moisture generated inside has no exit. A household adds water vapour to the air all day through cooking, showers, laundry, and breathing. In an older, draughty suite that vapour leaked out through the envelope by accident. In a well-sealed suite it stays put, humidity climbs, and the coldest surfaces, usually window glass and frames, collect the surplus every cold morning. Tightness is good building practice; it simply makes deliberate ventilation mandatory.

Longer than anyone leaves it on manually. The steam a shower releases keeps condensing on mirrors, paint, and window glass well after the water stops, so the fan needs to keep clearing the room until surfaces are dry, not just until the person leaves. The practical fix is a timer switch or a humidity-sensing fan, so the run time follows the moisture instead of the light switch.

The tissue test: hold a single sheet of tissue paper against the grille while the fan runs. Working exhaust holds the tissue firmly against the grille. If it flutters or drops, the fan is underperforming, and the cause is as often the duct as the fan itself: crushed flex duct, a stuck backdraft damper, a clogged exterior hood, or a grille packed with lint and dust.

Make-up air is the replacement air that flows in while a fan pushes stale air out. Air leaves a suite only as fast as new air can enter. In a tight suite with no planned inlet, the exhaust fan builds negative pressure working against itself, and its real flow drops far below its rating. Trickle vents, door undercuts, or a dedicated supply give the fan something to move.

For moisture control, yes. A recirculating hood pulls steam and grease through a filter and blows the same air, moisture included, back into the kitchen. It removes odours and grease particles while every gram of water vapour returns to the room. Boiling pasta under a recirculating hood loads the suite air exactly as if there were no hood at all. Ducted-to-exterior hoods are the only kind that export moisture.

Grilles and fan blades collect lint and dust that choke airflow, so they need periodic cleaning, bathrooms especially. Ducts need checking for sags, kinks, and disconnections, and exterior terminations need their flaps and screens cleared of lint, nests, and paint. A fan that was adequate when installed can quietly lose most of its delivery to a decade of buildup, which is why moisture complaints often trace to ducts nobody has looked at since construction.

It fixes the share of the problem caused by excess indoor moisture, which in our experience is a large share. It cannot warm a cold surface: an uninsulated slab edge or an old aluminum frame will still condense whatever moisture remains, because the surface sits below the dew point of even modest room air. Buildings with chronic cold-surface problems need thermal fixes alongside ventilation, and diagnosis tells you the split.

A heat recovery ventilator exhausts stale indoor air and supplies fresh outdoor air continuously through a core that transfers heat between the two streams, so the suite gets ventilation without losing most of the heating energy. Newer airtight construction leans on HRVs because it has no accidental air leakage to fall back on. In older buildings, adding one is a meaningful retrofit that usually enters the conversation during larger renewal work.

Both, split along the property line. Owners control how the equipment gets used: running fans during and after moisture events, keeping grilles clean, reporting failures. Councils control the systems themselves where they are common property: rooftop exhaust fans, shared ducts, and the building-wide question of whether the original ventilation design still matches how tight the building has become after window replacements and draught-proofing.

Every litre of water that leaves the clothes enters the suite air, and a full rack carries litres. Spread over an evening in a closed suite, that is a heavier moisture event than a shower, and it lands during winter when windows stay shut and cold surfaces are everywhere. Suites that must dry laundry indoors should do it in a room with a working exhaust fan running, door closed, and never against an exterior wall.

Many have less than owners assume. Mid-century buildings often relied on openable windows and envelope leakage, with modest bathroom fans added later, and some corridors were pressurized to push air into suites under the doors. Decades of window replacement and draught-sealing have made those buildings much tighter than their ventilation ever anticipated, which is why condensation complaints often begin the winter after an envelope upgrade rather than before it.