We usually think of our homes' exterior sheathing - like the brick or siding - as its weather protection, like our winter coats.
But what's more important in protecting your home from damage caused by moisture is house wrap.
House wrap is a breathable membrane installed directly over your home's plywood or OSB (oriented strand board) sheathing, underneath the exterior finish.
House wrap has been around for over 30 years - since the 1970s energy crisis. It became a hot commodity back when R-2000 homes were introduced to the market in Canada in the 1980s and is now required by code in many cities.
The most commonly used house wraps are made of threads of vinyl on the inside surface of a synthetic sheet. There are also newer types, in a spray-on elastomeric version, that aren't as common in use.
House wrap works in several important ways. The membrane stops direct moisture, like rainwater from penetrating the plywood or OSB sheathing on the outside of your home, yet allows water vapour from within to migrate outside. Don't forget, we create a lot of moisture inside by living in the house - breathing, showering, doing the laundry, cooking. Moisture vapour that gets into your wall cavities must be allowed to escape back out through the house wrap, or it will rot the studs and wood inside and lead to mould.
House wrap also stops outside air from penetrating the home. But it is not airtight: It is a breathable membrane. House wrap has no insulation value, but it improves energy efficiency by preventing air leakage. So, it helps retain heat in winter and retains cool, air-conditioned air in summer.
The synthetic house-wrap system replaces the old-fashioned tarpaper membrane used on some older homes, though many older homes only had sheathing without tar paper. Wrap is much more effective and durable and comes in larger rolls than tar paper, making this step of the building process much more cost-effective for a builder.
House wrap comes in a nine-foot-long roll, which helps to avoid seams and gives you a continuous sheet with minimal seams and overlaps. It takes two men less than a day to wrap a small house, with only a stapler and a roll of Tuck Tape. But it isn't a job for just anyone. It takes experienced contractors to do the job properly and efficiently.
There must be a minimum six-inch to 12-inch overlap at all joints. Every - and I mean every - hole and overlap is sealed tight with tape. If the taping isn't done properly, the system won't be effective and you will have air movement and the possibility of water damage.
Details like wrapping the membrane around window and door openings are important, too. Taking the membrane into the inside edge of the frame, which will be covered by window and door trim, guarantees an airtight envelope - providing your contractor does a good job of spraying foam insulation around all windows and doors. The same applies for any accidental tears or holes.
Let's face it, on a busy construction site, accidents happen.
Don't let your contractor start covering the house wrap with the external sheathing until you are satisfied every opening is taped over.
Once the house is properly wrapped up in its new windbreaker, it's all right to apply the finishing layer, the outside coat.
Without this breathable membrane, moisture would collect on the underside of your wood, brick, vinyl or metal siding, as well as on the outside surface of the plywood. In time, this can lead to mould and wood rot, and very expensive repairs.
Mike Holmes is the host of Holmes on Homes on HGTV.
For more information, go to http://www.holmesonhomes.com