Moisture Control in Your Home

Properly controlling moisture in your home can improve the effectiveness of your air sealing and insulation efforts—in turn helping to control moisture. The best strategies for controlling moisture in your home depend on your climate and how your home is constructed. Proper ventilation should also be part of a moisture control strategy.

Keep in mind that this section aims to provide a general overview of moisture control, and should not be taken as definitive. For moisture issues related to thermal or acoustic insulation, contact your selected insulation supplier. For other moisture issues, contact a qualified building professional.

What contributes to moisture in your home

Before you decide on a moisture control strategy, it helps to understand that moisture or water vapour moves in and out of a home in three ways:

  1. With air currents—accounting for more than 98% of all water vapour movement in buildings
  2. By diffusion through materials
  3. Through heat transfer

Moisture transfer by air currents happens quickly

Air naturally moves from high-pressure areas to lower pressure areas through the easiest pathway available, such as holes or cracks. To effectively control moisture, you must carefully and permanently air seal any unintended paths for air movement into and out of the house.

Moisture movement by diffusion through materials and through heat transfer are much slower processes. Most common building materials slow moisture diffusion to a large degree, although they never stop it completely. Insulation also helps reduce heat transfer or flow.

What causes condensation?

Air’s ability to hold water vapour increases as it warms, and decreases as it cools. Once air has reached its dew point—the temperature and moisture concentration at which water vapour condenses—moisture in the air condenses on the first cold surface it encounters. If this surface is within an exterior wall cavity, the result is wet insulation and framing.

How you can help control moisture

Because moisture is transferred predominantly by air currents, air sealing your home is essential. You can also control temperature and moisture content. Installing insulation reduces heat transfer, so it also moderates the effect of temperatures across your home. In most Canadian climates, properly installed vapour diffusion retarders can be used to reduce the amount of moisture transfer. Except in deliberately ventilated spaces, such as attics, insulation, and vapour diffusion retarders work together to reduce the opportunity for condensation in a house’s ceilings, walls, and floors.

Foundation Moisture Control

The potential for moisture problems exists anywhere building components are below grade, whether you have a basement, crawlspace, or slab-on-grade foundation. Most basement water leakage results from water flowing through holes, cracks, and other discontinuities into the home’s basement walls or water wicking into the cracks and pores of porous buildings materials, such as masonry blocks, concrete, or wood. These tiny cracks and pores can absorb water in any direction, even upward. To create an energy efficient and comfortable living space in your basement, you’ll need to insulate as well as properly control moisture.

Preventing foundation moisture

The best approaches for preventing foundation moisture problems will depend on your local climate, type of insulation, and style of construction. If you need to correct moisture problems in your existing home, consult a qualified builder, basement designer, and/or insulation contractor in your area for specific basement moisture control measures tailored to your climate, type of insulation, and construction style.

In a new home

If you’re building a new home, pay particular attention to how water will be managed around the foundation. The following guidelines will apply in most circumstances:

  • Keep all untreated wood materials away from earth contact
  • Install well-designed guttering and downspouts connected to a drainage system that diverts rainwater completely away from the house
  • Slope the earth away from all sides of the house for at least 5 feed at a minimum 5% grade (7.5cm in 1.5 m, or 3 inches in 5 feet). Establish drainage swales to direct rainwater around and away from the house
  • Add a gasket under the sill plate to provide air sealing
  • Install a protective membrane, such as rubberized roofing or ice-dam protection materials, between the foundation and the sill plate to serve as a capillary break and reduce wicking of water up from the masonry foundation wall. This membrane can also serve as a termite shield on top of foam board insulation
  • Damp-proof all below-grade portions of the foundation wall and footing to prevent the wall from absorbing ground moisture by capillary action
  • Place a continuous drainage plane over the damp-proofing or exterior insulation to channel water to the foundation drain and relieve hydrostatic pressure. Drainage plane materials include special drainage mats, high-density fiberglass insulation products, and washed gravel. All drainage planes should be protected with a filter fabric to prevent dirt from clogging the intentional gaps in the drainage material
  • Install a foundation drain directly below the drainage plane and beside (not on top of) the footing. This prevents water from flowing against the seam between the footing and the foundation wall. Surround a perforated 10cm (4-inch) plastic drainpipe with gravel and wrap both with filter fabric
  • Underneath the basement or on-grade slab flor, install a capillary break and vapour diffusion retarder, consisting of a layer of 6- to 10-mil polyethylene over at least 10cm (4 inches) of gravel
  • If your new or existing home has a crawlspace, you can also install a 6-mil polyethylene vapour diffusion barrier across the crawlspace floor to prevent soil moisture from migrating into the crawlspace. Overlap all seams by 30.5cm (12-inches) and tape them, and seal the polyethylene 15cm (6 inches) up the crawlspace walls. As an option, pour 51 mm (2 inches) of concrete over the vapour barrier to protect the polyethylene from damage

Moisture control in walls

It’s a myth that installing vapour barriers is the most important step for controlling moisture in walls. Vapour barriers only retard moisture due to diffusion, while most moisture enters walls either through fluid capillary action or as water vapour through air leaks. Most climates require taking specific moisture control steps.

Steps to control moisture in walls

To control moisture within a wall, you must seal any pathways that allow for air movement. Depending on the building code and local climate zone, you may also have to install a vapour retarder along with insulation. Without a vapour retarder, warm indoor air moves into the colder building envelope, where it could condense.

Additionally, you must control heat transfer. The purpose of insulation is to retard the flow of heat from one place to another, and to maintain temperatures such that condensation doesn’t occur inside a wall’s surface.

What to do with wet insulation

In Natural Resources Canada (NRCan)’s publication, Keeping the Heat In, if insulation is wet, remove the source of moisture and leave the insulation uncovered until it is dry. However if it contains mould, it should be discarded and replaced. If in doubt, err on the side of caution and replace all wet insulation.