Moisture problems & air barriers

We mostly think of air sealing as a critical step to prevent heat and cooling losses; but air sealing can also be an important means to prevent moisture and mold damages.

Air barriers & Moisture
Air barriers prevent moist air from entering into the wall cavities, attics and floors, where it condenses and can cause the growth of mold and mildew, and structural damages.

Air sealing can be an effective way of preventing or fixing moisture problems, and is largely based on air barriers materials.

Air barriers & moisture

To prevent moisture problems, modern buildings should have a continuous air barrier all over their envelope, that is, over their walls, ceilings and floors. That's critical to avoid moisture problems.

But what's an air barrier?

Specific air barriers

The term air barrier is often used to refer to specific materials, designed to resist air infiltration such as peel-and-stich rubber membranes, or elastomeric coating paints for masonry walls.

Continuous set of some construction materials

Anyway, there is a broader technical meaning of air barriers.

An air barrier as an assembly of construction materials - an assembly able to provide the sealing of all the enclosure of the house.

These materials include 1) poured concrete, 2) gypsum drywall, 3) rigid and spray foam insulation, 4) glass… and also 5) materials such as plastic house-wraps or polyethylene properly installed.

To be effective, the air barrier has to be continuous (without any interruption).

The parts of the air barrier should spread all over the envelope of the house and have to be properly caulked in all the transition areas where there is seams and penetrations.

Flashing, caulking and membranes are critical to the effect.

Keep in mind this meaning of air barriers, and what it means in practice.

If you are going to build a new home, your should try hard to implement an air barrier, in this technical sense.

Your designer should indicate the location of all the air barrier parts in the section drawing. It’s very important, especially in wood-frame homes. The location shouldn't be improvised in site by roofers and, builders.

Attic, moisture and air sealing

The attic is a good illustration of the air-tight approach as a means of controlling moisture. The attic floor should be part of the home's air barrier.

Leaks provide a path for moist air (coming from people, and kitchens, bathrooms or laundry rooms, or from leaks at the bottom of the house) to escape to the attic, where it condenses and becomes a habitat for mildew and mold, or a cause of damages on rafters and sheathing.

A continuous layer of air barrier materials - and a proper sealing of the air gaps and holes in the attic floor - is critical to control moist air and prevent damage.

See: Attic Sealing

Basement air sealing

And the same goes with basements and crawl-spaces, which should also be part of the air barrier system.

Many old codes recommended ventilation to prevent moisture damage; but that's not the best approach to avoid moisture from getting into the house through the basement. Ventilation can help, in some climates, but is not a remedy and can make things worse, in moist climates.

In cold climates, crawl spaces and basements should be airtight, that is, they should be thoroughly sealed and properly insulated. Ice dams
Sealing air leaks is also a way of preventing ice dams.

In wood frame homes, leaks in the basement walls and sill plates should be sealed using appropriate materials, to prevent moist air from getting inside.

But there are other features to be taken into account: a comprehensive air barrier, ground-moisture barriers and water control around the home's foundation are key elements of an airtight approach, intended to transform basements and crawlspaces into healthier and more comfortable spaces.

See: Basement Air Sealing



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