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Rain-screen controls moisture

THE exterior walls of a home are fascinating moisture control devices that are far more sophisticated than most of us realize.

THE exterior walls of a home are fascinating moisture control devices that are far more sophisticated than most of us realize.

Moisture is the single greatest source of construction problems in the wall assembly of a home and its mitigation is a crucial component of good building envelope design.

When left unattended, moisture within a wall assembly will produce mold and mildew, decay and rot, thermal envelope breakdown and structural damage.

There are three distinct ways that moisture can develop within a wall system with the first being from the outside in as outdoor water and moisture works its way into the wall assembly through imperfections and breaks in the exterior skin.

The second way moisture develops is from the inside out as warm, moisture-rich air from the inside of the house migrates into the wall assembly and condenses when it meets a cold surface within. The third and, for me, the least obvious way is the development of moisture from the assembly itself as changing moisture levels in the framing material that make up the assembly produce moisture problems within the wall.

Controlling external moisture from entering the wall assembly has been greatly enhanced in recent years with the required installation of rain-screen walls for new construction by the National Building Code of Canada.

In rain-screen construction the outer layer of siding or cladding of a wall assembly is separated from the inner wall 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. This new method of construction ensures that if moisture makes its way through the outer protective layer of the wall assembly it will naturally migrate out again. Lack of rain-screen detailing was the primary culprit for the "leaky condo" debacle that plagued homeowners some years back.

Moisture can move from the inside of a dwelling into the wall assembly if moistureladen air from the interior spaces of a home 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 outwards. As warm air migrates through a wall assembly it will find a cool surface at that magic temperature and it will condense. Think of your lawn after a clear night. The water drawn out of the warm air will create moisture within the assembly and with it all sorts of problems from rotting and structural deterioration of the assembly to fungal growth and visual damage.

Installing a continuous membrane on the warm side of the wall insulation - typically a 4-6 millimetre polyethylene film - prevents warm, moist air from the interior of the house from migrating into the wall assembly and is the standard way to prevent this insideto-outside moisture problem from occurring.

When constructing a wall assembly one must ensure that the lumber used for framing is considered dry

- The National Building Code of Canada deems this to be a moisture level of 19% or less. Green lumber (wood that has been freshly cut) will have a moisture level of 100%. As green wood dries it shrinks and undergoes dimensional changes. It's crucial that the bulk of drying has already happened before incorporating the lumber into the wall assembly. Not doing so will create deformation issues as well as problems of rot and fungal growth as excess moisture is trapped in the assembly itself.

It will come as no surprise to North Shore residents, who understand the destructive effects of water and moisture more than most, that unwanted moisture in a wall assembly will lead to a glut of construction problems and needs to be prevented at all cost.

Kevin Vallely is a residential designer in North Vancouver. His website is

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