Caulking and mould: cause or solution?

How leaking air creates condensation

In winter, the air in your home is warm and holds moisture. When it escapes through a poorly sealed joint around a window or door, it passes through the wall and cools down. As it cools, it leaves its moisture inside the wall.

The Government of Quebec explains that these warm air leaks carry moisture. They can cause hidden condensation, damage insulation and lead to mould and rot in the structure. The problem often develops out of sight.

These same leaks also cost energy. According to the Government of Quebec, they alone can cause 25% of a home's heat loss. A continuous joint solves two problems at once.

A blackened joint: clean or replace?

A surface stain can sometimes be cleaned. But if the joint is blackened deep down, cracked or pulling away, cleaning isn't enough. Moisture and mould settle in the cracks and under the joint, where you can't reach them.

In that case, we remove the old joint completely. We clean the substrate, let it dry, then apply a new joint. The Government of Quebec also recommends cleaning joints well before caulking, whether of paint, dust or old caulking.

• Joint blackened deep down: replace
• Cracked or loose joint: replace
• Soft or crumbling wood frame: have it repaired before caulking
• Stains that come back despite a new joint: look for the source of moisture

Apply on a dry surface

A joint applied to a damp surface traps the water. This Old House points out that the surface must dry completely, or mould can form under the joint.

Manufacturers agree. The data sheet for DOWSIL™ 791, one of the silicones we apply, states that it must not be applied to a frosty or wet surface. We caulk year-round, but we always wait until the substrate is dry and free of frost.

The choice of product matters too. On the outside, we mainly apply DOWSIL™ 791 and CWS silicones, designed for the weather. On the 791, we offer a lifetime warranty on materials and labour, under the terms set out in the contract.

Order of work: inside first

The Government of Quebec recommends caulking the inside first to control humid air, then the outside to protect against rain. It's the indoor air that carries moisture into the wall, so that's the side to stop it on first.

The same page advises working from top to bottom, starting with the upper floors and humid rooms like the bathroom and kitchen. These are often where the first signs of mould appear.

For high exterior work, hire an equipped professional. There's no need to climb a ladder for a second-floor joint.

Airtightness and ventilation: finding the balance

You sometimes hear that a house that is too airtight grows mould. In fact, the goal isn't to stop air from moving, but to control where it goes. Stale, humid air should leave through the ventilation, not through the walls.

A better-sealed house needs ventilation that works: bathroom fans and range hood in use, ventilation system maintained. If you have doubts about your system, have it assessed by a ventilation specialist. Caulking and ventilation work together.

Be careful not to block what needs to breathe, either. A vent, a dryer exhaust or a ventilation grille should not be caulked. If mice or insects get through, we install steel mesh instead, which keeps the air moving.

When to call us

Call us if your joints are blackened, cracked or pulling away, if you feel drafts around your windows, or if you see water marks under a frame. We assess on site, give you a written quote and redo the joints cleanly, with short lead times.

If the mould covers a large area or comes back after the work, the cause may lie elsewhere: water infiltration, a ventilation problem or damage inside the wall. We'll tell you, so you can consult the right specialist.