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.

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.

Photograph by: Terry Peters, North Shore News

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.

Region's housing costliest in Canada

In my last column I discussed the present state of our real estate market and how economic indicators suggest we're not in the midst of a housing bubble at all.

I made the argument that limited buildable space and our desirable location make for an environment that will maintain high housing prices. What I didn't address was the repercussions of this lack of affordability.

The Royal Bank of Canada's report on housing affordability in Canada issued in August indicated that most housing markets across the country are affordable or slightly "unaffordable" with one major exception: Vancouver.

The L41 ultra-compact home by Vancouver architect Michael Katz and

designer Janet Corne is a mere 220 square feet.

Photograph by: Jon Benjamin, for the NEWS

"By and large, the share of household budgets, taken up by the costs of owning a home at current market values, remains close to historical norms," said Craig Wright, senior vice-president and chief economist with RBC. "However, extremely poor and rapidly eroding affordability in the Vancouver-area market is somewhat skewing the national picture."

RBC's report lists the affordability of owning a detached bungalow as a measure of the percentage of pre-tax household income required to service the costs of owning a home. Vancouver's index came in at a whopping 92.5 per cent. What this means is that the homeownership cost of owning a typical detached bungalow in Vancouver, including mortgage payments, utilities and property taxes, takes up 92.5 per cent of a typical household's monthly pre-tax income.

What this outrageous number unequivocally states is that if you're a typical household in Vancouver not already in the housing market, you might as well forget about owning a typical detached home in the city.

I believe a solution to this conundrum can be found in the so-called small house movement that has gained considerable momentum in recent years. Architect Sarah Suzanka has been credited with starting this movement with the publication of The Not So Big House in 1997.

Her thesis is a simple one: build smaller but build better. She believes that the quality of a living space is not related to its size but rather to the efficacy of its design.

Buildable land on the North Shore is all but gone and we're left with few options. We can go up, of course, and this is a reasonable solution in some cases but densification by building towers is very different than the densification created by low-rise structures knitted more tightly together.

The sense of community and neighbourhood created by these low-rise models are far more intimate, and in my mind far more successful, than the general anonymity of tower living.

A small home could be a stand-alone residence on a smaller lot or be an additional structure on a lot with a home already on it (the coach house concept). Either way, the small home increases density and affordability without deteriorating the quality of a community.

For any of this to happen municipalities will need to amend their existing zoning requirements to permit smaller scale construction. The City of North Vancouver has bravely moved forward in this direction, recently permitting the addition of coach houses on residential properties.

Smaller houses are a logical, economic and environmentally sound solution for a city that is feeling growing pains. There's no question that the singlefamily suburban model is being transformed. We live in a new era where new models for living need to be entertained for the betterment of our communities.