Trained Eye


Styrofoam on Outside Walls 2


Thanks for your column in the Winnipeg Free, read it all the time and look forward to your suggestions. We currently have stucco on three walls and wood siding on one. I would like to add Styrofoam SM on all the outside walls of our forty four year old house, then either install vinyl siding to the whole house, or just on the one side and stucco the remainder. The reason for this is that the walls only have R8 fibreglass with a paper vapour barrier, and the stucco is very worn looking with old paint and cracked. I was told by an insulation company that, because of the existing insulation in the wall cavity, they would not be able to properly blow in insulation. Therefore, my options are either remove the wood siding only and apply one to two inches of Styrofoam SM over top of the stucco or remove the stucco and the wood siding then apply the Styrofoam SM.

Is it a worthwhile endeavour to apply Styrofoam SM to the entire house? Other than decreasing the thickness of the wall, what are the benefits to removing the stucco, and are there any downsides to doing this? Finally, what is the proper way to install Styrofoam SM to the outside walls with vinyl siding and stucco on top? If I was to forgo adding the insulation to the stucco portion of the house, can new stucco be successfully reapplied to my old, painted and cracked walls?


I must commend you for your proposed attempts at adding additional insulation to your house walls to reduce heat loss, although your methods are somewhat misguided. Adding insulation to the exterior walls of a home is possible but your method of using Styrofoam SM, or similar products from other manufacturers, may lead to problems down the road. Styrofoam SM is simply the brand name of this insulation manufactured by Dow. I will answer many of your questions and steer you toward the correct method for this insulation upgrade.

The exterior walls of your home are part of an overall system currently known as the “building envelope”. This system includes any component in the home that separates the conditioned, or heated, living space from the outdoor, or unconditioned, environment. This includes the walls, ceilings, windows, doors, and often the insulated basement walls over the foundation. Proper construction of this area of the home is becoming more and more complex as our homes become more energy efficient and as we learn more about moisture problems. You currently have a relatively loose component of this envelope in the poorly insulated and sealed walls.

To make an initial point, the paper backing on the insulation is not a vapour barrier, but may act as a partial air barrier. If not covered with polyethylene sheathing, it will not prevent moisture diffusion into the exterior walls from the home. The real concern with adding insulation to the exterior of the home is the possibility of trapping moisture within the wall cavity. If done incorrectly, this may cause major rot and mould growth in the walls. Styrofoam SM is a type of extruded polystyrene insulation that provides a very good vapour barrier when properly sealed at the overlapping joints and edges. When installed on the exterior of the home, this material may prevent proper “breathing” of the walls.

While your home may not have a polyethylene air-vapour barrier behind the drywall or plaster walls, the numerous coats of paint on the drywall may provide a reasonable job in this regard. Unfortunately, air and moisture from the interior of the home can still leak through this area in various locations such as electrical outlets, windows and joints. The wood or stucco siding on the exterior of your home is designed to allow moisture that penetrates this part of the building envelope to eventually evaporate to the exterior. If you add extruded polystyrene to the outside of this wall, whether you remove the siding or not, you may prevent this evaporation from occurring, trapping the moisture in the wall and creating the problem identified above. By applying this rigid insulation to the outside of the wall, you are essentially adding a second air-vapour barrier. Exterior walls are only supposed to have one, normally on the interior of the studs in cold climates like ours.

Removing the old stucco or siding may be an advantage for a couple of reasons. Firstly, this may allow you to add insulation without building out your window and door frames to accommodate the added thickness. This may simplify the retrofit, considerably. Secondly, the extruded polystyrene may be usable in some instances if the inside face is within the first third of the insulation value. Given your statement that there is at least R8 insulation in the walls, we will disregard this second point, as too great a thickness of foam insulation will be required.

My recommendation would be to remove the stucco and wood siding to allow installation of rigid insulation designed for exterior use. There are several product options, many manufactured with semi-rigid fibreglass covered with an air barrier material. This material will provide a moderate increase in insulation value, while allowing moisture to exfiltrate the wall cavity. Once installed, the walls could be strapped with thin strips of wood followed by vinyl siding, stucco wire and stucco, or other types of siding.

Extruded polystyrene is an excellent product for many insulation purposes, especially those below grade where moisture is a big problem, but may not be suitable for use on the exterior part of house walls. Care must be taken to allow the building envelope to breathe, as it was originally designed, when upgrading insulation. If you chose not to add insulation to the stucco walls they may be painted or re-stuccoed, but we will leave that topic for another column.