Protecting your attic with a vapour barrier

Hey there, time traveller!
This article was published 21/07/2002 (8521 days ago), so information in it may no longer be current.

Question — In one of your columns, you replied to ice buildup in older homes by recommending adding proper vents (if possible) and checking the attic for proper insulation. I have similar problems as well in my older home.

I was wondering: If I reinsulate the attic, can I add a vapour barrier first? I’ve talked to different trades people and home-improvement stores, and every one has a different idea.

My question is: Can I add a vapour barrier to an older (50-year-old) home by laying a vapour barrier up and over the joists? Would moisture get caught between the joists? Should I cut the vapour barrier in strips so that it fits between the joists? Will this help keep the ceiling from getting water stains in the spring, but not provide an air seal?

My original eaves sit right at the edge of the wall, so there are no soffits to add vents to for better air flow. I only have two gable vents at either end of the peaks, plus four roofing vents near the top. I used to have a whirlybird vent, but on non-windy days rain would get in, or in the winter it would freeze up and have ice around it that would melt.

Is it safe, but more labour-intensive, if I staple a vapour barrier on the ceiling, then add new drywall and paint? About 18 years ago I repainted the house with a “vapour sealer” first on the ceilings before adding the final coat of paint.

— Howard Kazuska, e-mail

Answer — Installing an air-vapour barrier in the attic of an older home always poses a difficult, but not insurmountable situation. Ideally, proper polyethylene sheathing should be installed beneath the ceiling joists just above the drywall or ceiling sheathing. It should be sealed with acoustical sealant to the wall vapour barrier at the edges and at each joist, light fixture and protrusion. This is difficult to accomplish perfectly in a new home and impossible in a retrofit situation. Striving for a reasonably installed vapour barrier is more realistic.

The purpose of the air-vapour barrier is to restrict air and moisture movement into the cold attic from the warm living space below. Heated house air contains water vapour, which may be released in the form of condensation when the air is cooled. If ceilings are not well sealed, excess moisture can enter the attic, get trapped due to inadequate ventilation and cause moisture damage to wooden structural components. With vapour retarders, even a partial installation will be somewhat effective at reducing moisture intrusion. Even if it is not perfect, it will stop some of the moisture from escaping the living space. This may be accomplished by trapping moisture on the warm side of the barrier or simply by reducing air movement. Restriction of air movement has been found to be the most effective method of preventing heat and moisture loss from a home. This is why caulking or sealing around windows, light fixtures, outlets and other openings is so important.

In the case of Mr. Kazuska, the multiple layers of paint applied to his ceiling in the past will provide a moderately effective air-vapour barrier, and the addition of extra poly in the attic will have questionable added benefit. Installing poly over the old ceilings before new drywall is normally done only when the ceilings are damaged or deteriorated and smooth new ceilings are desired. Installing the plastic sheathing in the attic is the less expensive and easier alternative.

Placement of the plastic sheathing over the joists is acceptable, depending on the thickness and location of the insulation in the attic. The currently accepted rule of thumb is that the vapour barrier may be installed in the first third of the insulation. To clarify, if 12 inches of insulation is to be placed in the attic, the air-vapour barrier may be installed over the two-by-four ceiling joists and still remain in the first third of the insulation. This will be much simpler than cutting and caulking poly between each joist cavity and will provide a more continuous membrane.

When attic insulation is increased, additional vents are normally required and should be installed at the same time. The current amount of attic ventilation described by Mr. Kazuska sounds reasonable, dependent on the size of the home, but the location of the vents may not be ideal. Gable vents are excellent in allowing air in and out of attics and are normally installed near the peak. The other roof vents are also high up, so upper attic ventilation sounds excellent. The problem may arise from trapped moisture in the lower part of the attic due to the absence of soffits and vents. Additional gable or roof vents may be installed lower down in place of soffit vents, but care must be taken to prevent snow coverage and leakage in the lower roof vents if the roof pitch is low. There is still much debate about proper methods of insulating and ventilating homes, and much to learn as new systems and methods stand the test of time. Much research is ongoing by organizations such as CMHC as well as private construction material manufacturers. The key to all of this is to reduce air and moisture movement from the warm house air into the attic, and getting rid of the moisture when it does sneak in.

Ari Marantz is owner/inspector of Trained Eye Home Inspection Ltd. and is the public relations representative for the Canadian Association of Home Inspectors-Manitoba (www.cahi.mb.ca).

Questions can be e-mailed or sent to: Ask The Inspector, P.O. Box 69021, #110-2025 Corydon Ave., Winnipeg, Man. R3P 2G9. Ari can be reached at (204) 291-5358.

trainedeye@iname.com