February, 2007

Five D’s for Moisture Control

Better building practices lay a solid foundation for durable exterior trim. 

By Gary Katz

Close your eyes and imagine that your store has no computer system, and that you’re still writing all your sales receipts, purchase orders, and invoices in longhand. Pretend that you’re still keeping track of profit and loss and payables and receivables on legal-sized ledger paper.

Just remembering those days probably makes you want to close your eyes.

New building materials have had a similar fast-forward effect on contractors and installed sales programs. We can’t go back; we can no longer build houses the way our fathers and grandfathers built homes.

Architects, contractors and carpenters—your customers—must learn new installation techniques, and certification programs must be introduced to support better building practices. Educational programs are new to our profession, but they’re essential to our bottom line.

And the best report card from any construction program should show the “Five Ds” for moisture control—design, deflection, drainage, diffusion, and durability.

Design

Failure or success occurs before the first board is cut or the first fastener is driven.

The longevity of exterior cladding, trim, and paint is no longer a given. Modern materials require more care than the siding and trim my father installed, which was all made from old-growth timber.

Designing exterior cladding systems that address the five Ds is the only way to ensure a durable, claim-free job, and a long-lasting reputation.

Deflection

Excessive water absorption in exterior cladding and wall systems leads to mold, mildew, rot, ruined reputations, and even lawsuits. Always design exterior cladding and trim details to shed water and deflect moisture quickly. As Norm Slavik from Norcon Forestry Ltd. is fond of saying, “a 5-degree slope on window sills is a thing of the past. Today, 10 degrees works better, but 15 degrees gets the water off fast. The same approach applies to all exterior trim installations.”

Successful deflection also means flashings. Every horizontal and raked or angled surface—window and door heads, roof trim, aprons, water tables and bands—requires a flashing that’s integrated with the housewrap system and that rapidly deflects water away from a house.

Drainage

Provide a means for moisture to drain away quickly from wall and trim assemblies. Don’t inhibit or block the potential flow of moisture. Weep holes and drainage designs must be incorporated into every cladding system.

Diffusion

This is probably one of the most overlooked yet critical areas in construction. Diffusion through evaporation is the means by which we encourage moisture vapor and liquid water to dry. According to building scientists, a minimum 4 mm (3/16-in.) space is required to break the surface tension of moisture caught between cladding and drainage plane materials. In Canada, the building codes in coastal regions call for a larger gap of 10 mm (7/16 in.). Some building scientists and manufacturers say for a true rainscreen wall, the gap between the siding and the housewrap must be ventilated with openings at the top and bottom to neutralize the forces that draw water behind the siding, like capillary action, which drives and draws liquid water through cracks, crevices, nail holes, and up the back side of lap siding.

I’m no scientist. I’m only a carpenter. I like to keep things simple. From experience I know that wood products need breathing room. Diffusion is achieved by providing ‘breathing room’ between exterior cladding and housewrap systems.

Durable Exterior Trim

Whatever housewrap system you use—felt paper, plastic housewrap, textured drainscreen, or rainscreen products—allowing exterior cladding an opportunity to drain and dry is the secret to installing durable exterior trim and ensuring long-lasting paint applications.

Today’s wood building materials are cut from younger trees with wider growth rings, which are trees that haven’t aged enough to convert sap wood to heartwood. As Stephen L. Quarles wrote in a recent article in Jan. 2007 issue of Fine Homebuilding, “Because of the extractives in heartwood, both mold and decay fungi prefer sapwood. …Today’s houses have more potential mold and decay food than earlier houses built with…lumber cut predominantly from the heartwood of softwood species.”

Wood movement tears up trim

In my last article, we discussed wood movement and how it affects miters. Because wood moves more on the outside of a home than on the inside, miters should be avoided on exterior trim. That explains why nearly every manufacturer of exterior trim doesn’t recommend the use of miters with their products. But we should also avoid another old-fashioned construction building habit—cross grain laminations—stacking boards or mouldings on top of each other and creating peanut-butter-and-jelly sandwiches.

Walk through the simple steps required for constructing a caulk-free column and you’ll see the Five D’s in practice.

Construction today is different than yesterday, but that’s a good thing for building product professionals. Educated lumber and building material dealers and installers are able to better serve their customers. And service, not low pricing, is the key to long-term profits for our industry.