Trained Eye


Styrofoam on Outside Walls 1


I read your articles in the Free Press regularly but I believe that I missed the one about applying rigid insulation properly. I have concerns that if I apply an extruded insulation to the out side of my home I will create a moisture barrier and therefore potentially rot the siding underneath. My home is from the 1940's and does not have a proper vapour barrier. If I apply Tyvec or some other air barrier underneath the 1.5 inches of insulation board I intend to apply, will this prevent any moisture that comes through the walls and cannot pass through the extruded board from condensing on the siding and rotting it? Is there any extruded board that I can purchase that will allow the moisture to pass through it and not do any harm to my walls? The reason I am concerned is that I did some renovations to my home and found areas of rot where I think the moisture could not pass through the 1 inch of stucco that was applied to the front of my home. Do you have any suggestions?


You appear to have a fairly good grasp of the potential problems that may be created in attempting a retrofit insulation job on an older house. Applying extruded foam insulation on the exterior may create an air-vapour barrier in the wrong location, and may cause moisture build-up within the wall cavity. Applying a housewrap or Tyvec underneath the extruded foam will do little to prevent the trapping of moisture and is not a good idea. This rolled sheathing is designed to prevent air movement, but is permeable to moisture and will not prevent trapping moisture in the wall cavity if installed on the wrong side of the new insulation.

A properly installed polyethylene air-vapour barrier should be installed on the warm side of the walls studs in an insulated exterior wall. This is difficult to accomplish in an older home, without removing all of the old plaster and lath from the inside of the house. Renovations of this kind are very disruptive and require very labour intensive removal and repairs. It is for this reason that adding insulation from the exterior is the more common method, unless extensive interior renovations are planned. Care must be taken to apply an exterior insulation designed for this purpose.

Many types of rigid, extruded foam insulation are designed for interior or exterior use, but are not ideal for exterior wall installation above grade. Most of these insulation materials have a reasonable vapour barrier rating and may provide a decent air-vapour barrier, if seams are sealed. They are more suitable for below grade installation, and are quite moisture resistant and may be used for insulating foundation walls. In this area trapping of moisture is not a concern, as little will pass through the foundation wall and damproofing to the exterior.

The only time that rigid foam insulation would be acceptable on the exterior of a wall is if a vapour barrier is installed on the warm side and is within the first third of the overall insulation. For example: If an older wall is poorly insulated to R10 overall, to add rigid foam to the exterior, the level of additional insulation must be at least R20. To accomplish this would require several inches of insulation and would make this difficult or impractical, at best.

There is rigid or semi-rigid insulation designed specifically for this purpose that should be readily available at local building centres. This may be rigid fibreglass or other material that has a vapour permeable membrane or sheathing attached to the exterior. This insulation may not have as high an R-value per inch as extruded foam, but will be much more breathable.

The reason that visible rot was seen when the old stucco was removed is more likely due to the condition of the exterior wall rather than the impermeability of the old stucco. The old plaster may have cracks or openings that allow warm, moist air from the home to enter the wall cavity, and gaps in the poorly insulated wall that let cold outside air in. Once the warm air comes in contact with the cold exterior air, condensation will occur. This will create frost that will melt in the spring, wetting the old sheathing and creating ideal conditions for rot. Cracks in the old stucco and a deteriorated paper air barrier behind the stucco may also allow wind to force cold air inside the wall.

After the new insulation is installed, checking the inside walls for gaps around windows, baseboards and mouldings should be done and these areas should be sealed with a good quality caulking, before painting. If these areas are well sealed and high quality paint is installed over the many old existing coats of paint, a reasonable air-vapour barrier on the inside may be achieved. This will help, along with the new exterior insulation, in preventing excessive air movement through both sides of the exterior wall of the home.