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ASK THE INSPECTOR: Rigid foam insulation needs covering up

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Spray foam insulation.

QUESTION: We want to add insulation to the north walls in our home. The walls currently have fibreglass insulation, a vapour barrier and drywall. Can we attach foam insulation and new drywall on top of the existing interior walls? If so, can this be done with glue that won't break up the foam and that is safe for interior use? We installed a new picture window last fall and allowed for the increase in wall thickness.

-- Joyce Reimer

We would like to insulate our basement with Styrofoam insulation and are wondering if it has to be covered with drywall or some other material? Is there something else you would suggest we insulate it with? We want to make sure we are abiding by fire regulations. We will not be finishing the basement. We only want to insulate it for warmth.

-- Cherry Karpyshin

ANSWER: While your excellent questions are not identical, they both address the issue of insulating walls from the interior using rigid foam insulation. Both of these situations pose challenges that may be tricky and not often addressed in typical literature designed for the do-it-yourselfer. The answer to both of your main questions is yes and I will offer suggestions for proper methods for insulation in either case.

Rigid foam insulation, whether it is expanded or extruded polystyrene, does have to be covered if it is installed in the interior of a building. This wall covering must have a minimum fire rating to protect the insulation from quick combustion during a fire. Because this type of insulation is made from plastic, it is combustible and may give off highly toxic fumes when burning. These fumes can overcome occupants of the home during a fire, even before the fire itself.

Drywall, either Standard or Fireguard, is normally used for this purpose because of its low cost and fire-resistant qualities. Other wall coverings may be available that will meet the criteria for fire protection but they will likely be more costly and sometimes more difficult to install.

Because you do not have the desire to fully finish the basement, you may be able to install the drywall directly over the rigid insulation and air/vapour barrier without taping or painting it. I'm not sure if that will be acceptable to the municipal officials in your area, so check with them to ensure it will meet their criteria. If not, the drywall taping and painting could always be completed later.

Also, if you use adequate thickness of high-density extruded polystyrene sheathing and tape or glue the seams, you may not need a typical polyethylene air/vapour barrier on the interior. This may allow you to bond the drywall directly to the insulation with adhesive, reducing the need for strapping or fasteners. Special adhesives, designed for foam sheathing, should be available at building supply centres and most often are applied with a typical caulking gun.

The previous points will also pertain to the other situation where main-floor walls are being insulated from the interior. It may be possible to install the rigid insulation directly over top of the interior drywall, but that's not the ideal method for one major reason: Because you already have a polyethylene air/vapour barrier installed on the exterior of the existing drywall, your proposal could trap some moisture in this area after finishing your new interior wall.

As already noted, a decent thickness of extruded foam insulation may create a pretty good air/vapour barrier, especially when covered with taped and painted drywall. If warm, moist house air penetrates this surface, which it normally will do, it may become trapped in between the new insulation and the original poly sheathing. If the temperature of the air in this cavity drops too low, partially because of the new insulation, it could condense. If this happens, it will wet the old drywall sheathing, which can become damaged and mouldy. Because this drywall is encased in air/vapour barriers on both sides, it will not easily dry or allow the moisture to escape the cavity.

While damage to the old drywall is possible, it may not occur if the new insulation and drywall are very well sealed at the interior or if the temperature of the old drywall is not allowed to drop to the dew point of the interior air. Determining this, however, may be difficult. Doing the calculations to figure out whether this will become a major issue is quite complicated, so it may be best to err on the side of caution. Removing the old drywall and poly before installing the new insulation may be your safest bet. This will not only eliminate the double air/vapour barrier issue, it will allow for additional thickness of rigid foam insulation to be installed, improving the energy efficiency even more. It will also allow shorter fasteners to be used and no guesswork about stud locations when installing the new insulation and wall coverings.

Rigid foam insulation has many advantages for use in insulating exterior walls in that it is highly moisture-resistant and will not become mouldy. Unfortunately, these same properties pose certain challenges when trying to use it in a retro-fit situation. These challenges are often easily surmountable, but care must be taken to properly plan the renovation to avoid creating a moisture issue when striving to improve energy efficiency in your home.

Ari Marantz is the owner of Trained Eye Home Inspection Ltd. and the President of the Canadian Association of Home & Property Inspectors-Manitoba (www.cahpi.mb.ca). Questions can be emailed to the address below. Ari can be reached at (204) 291-5358 or check out his website at www.trainedeye.ca.

trainedeye@iname.com