IT'S essential in our wet North Shore environment to carefully consider moisture mitigation strategies when building a home.
Moisture is the single greatest source of damage to the exterior walls of a house with structural deterioration, rot, mould and mildew, visual blight and thermal envelope breakdown all attributed to moisture issues being left unchecked.
For homeowners not versed in building construction technology it may seem a fairly straightforward task to keep moisture out of their walls: build the wall lock-tight and thus keep the water out. But building a simple water-tight envelope on the outside of a home won't suffice in maintaining a healthy home. I like to make the analogy to keeping oneself dry in a rainstorm when out for a run. Anyone who has done this recognizes that being shielded from the rain is the easy part, letting perspiration out is the challenge. Your home, very much like your body, creates its own moisture and if not adequately addressed can create a watery mess.
Moisture can develop in a wall assembly in three ways: First, water from the outside works its way into the wall system; second, water from the inside - held in the warm, moist air of the interior environment - migrates into the wall assembly and condenses in the structure; and third, water from within the wall assembly itself becomes trapped in the construction.
In an exterior wall, mitigating water issues coming from the outside is done by using so-called rain-screen construction, a construction technique that is now required on all new construction by the National Building Code of Canada.
In rain-screen construction the outer layer of siding or cladding is separated from the inner wall assembly by an air space. This space is created with vertical furring strips or rain-screen drainage mats and is vented to the outside to provide pressure equalization that allows any moisture that makes it into the cavity to escape at the bottom. In simple terms, if moisture makes it through the exterior siding then it has a place to go that won't damage the home.
Moisture can move from the inside of a dwelling into the exterior wall assembly if moisture-laden air from interior spaces is allowed to make its way outwards.
Wind loads, temperature gradients and mechanical systems will create pressure differences between the inside and outside of a building, forcing warm air from inside the house outwards. As the warm air migrates through the assembly it will find a cool surface and it will condense. Think of your lawn the morning after a hot, humid summer day. The water drawn out by the cold will create a moisture problem within the assembly and will result in all sorts of problems from rotting and structural deterioration to fungal growth and mould. The way to prevent this problem is to ensure that the warm air from the interior of the house is prevented from migrating into the cavity of the wall. This is typically done by installing a continuous membrane (commonly a 4-6 mm polyethylene film) on the warm side of the insulation that acts as a vapour barrier.
One must also ensure that the lumber used for framing is considered dry before it's used for construction. The National Building Code of Canada deems this to be a moisture level of 19 per cent or less. Green lumber (wood that has been freshly cut) will have a moisture level of 100 per cent. As green wood dries it shrinks and undergoes dimensional changes. It's critical that the bulk of drying has already happened before incorporating the lumber into the wall assembly. Not doing so will create all sorts of deformation issues as well as problems of rot and fungal growth as excess moisture is trapped in the assembly itself.
The well-designed exterior wall addresses moisture from the outside, moisture from the inside and moisture from within.
Understanding this concept will go a long way to ensuring your exterior walls remain healthy and moisture free for the life of your home.
Kevin Vallely is a residential designer in North Vancouver. Follow along Kevin's "small house" design at cliffhangerhouse.com.
