I read one of your articles with interest since my basement is not insulated and I’m planning to do so. I was advised to use the Manitoba Hydro Power Smart booklet #2, Basement and crawlspace insulation. In that booklet, they describe applying a moisture barrier. This is to protect the insulation and framing from dampness or occasional water leaks in the basement walls. My walls are not insulated yet, but I want to do it. Last spring was my first spring in the house and almost all of the tie holes in the walls leaked when the snow melted. With a moisture barrier, the water would have run down the walls and under the plastic, without damaging the insulation and framing. The booklet describes how to put up this barrier and the rest of the wall, with insulation and air-vapour barrier.
I asked a number of people about this method of insulating basement walls, and although they said they never heard of it, it sounded pretty good to them. A couple mentioned that if you used 2×3 studs instead of 2×4 and didn’t touch the framing or insulation to the cement wall, and left about 6 inches at the bottom free of insulation and vapour barrier, that effectively you were creating the same protection against moisture.
I’d be interested in your comments about what I have just written. Thanks.
This is an issue that I have wrestled with since seeing the pamphlet you have mentioned a few years ago. I have fielded many calls and inquiries on the need for this second “moisture barrier” when insulating basement walls. There is more than one school of thought on this, but my own personal view is that it is wrong. I will go into some detail about why I think this is an improper method, while trying to avoid getting too technical or complex.
Firstly, I am basing my response on a combination of personal experience with many years of renovations, in tearing apart and rebuilding homes, and numerous seminars and courses on indoor air quality and building science. I have seen several basements insulated in this manner that have had big problems in the wall cavity and others that have not. I don’t know the reason for this discrepancy, but current thinking about air movement in homes says that a double air-vapour barrier is wrong.
The purpose of a Polyethylene air-vapour barrier on the warm side of the insulation is to prevent warm, moist air from the home penetrating into the wall cavity. As any homebuilder or renovator can attest, no matter how diligent you are, it is impossible to have a perfect air-vapour barrier installation. In our climate, the warm house air that gets through this plastic sheathing may condense inside the wall, when it hits the cooler air within the insulation. In a basement, this condensation is very likely due to the cold concrete wall that is below grade. In some cases, frost can form on the concrete surface, if it is cold enough. When the weather warms in the spring, this frost will melt and dribble down the foundation wall and out the bottom of the insulated wall. In this situation, the second moisture barrier on the foundation wall may help prevent this frost formation, but this is where it gets a little more complicated.
We want the moist air that penetrates the wall cavity to have some means of escape, especially if it is cool enough for frost to form in the winter and melt in the spring. If there is a second barrier on the back of the stud walls, it will prevent the moisture from leaving the wall cavity at the concrete wall side. This condition is much more likely to promote rot in the wood studs and mould growth in the wall cavity. This is the reason that I think a double barrier method is harmful.
The inverse thinking is that any moisture that seeps into the foundation through rusted snap ties or small cracks in the concrete will not make the insulation wet if it protected by a Polyethylene sheet. While this is true, my view is that the foundation wall should not be leaking and one that is should be patched or damp proofed before insulating. If there is an isolated case of seepage due to unusually wet weather, like this past spring, the wall cavity will eventually dry if there is allowance for moisture to escape.
Now to address the second part of your question, leaving a portion of the air-vapour barrier off the bottom of the wall or a space behind the insulation is a very wrong method of installation. This is where it gets really complicated, and somewhat technical. To address this we must know a little about convective currents and the “stack effect”.
In a very simplified explanation, we all know that heat rises by convection, and the stack effect is simply the result of that basic physical law, which may be increased by a flow of cool air at the bottom of the “stack”. In this case, our wall cavity is the stack, and by leaving a gap at the back of the insulation we are creating a lovely chimney that will draw in cool, moist air under the wall plate and force it upwards by convection. This moisture will surely condense on the cold foundation wall, while trying to exit at the top of the wall, between the floor joists. Leaving a section of the air-vapour barrier off at the bottom of the wall will allow more air to infiltrate the wall cavity, making the problem much worse. The results of this convective space at the back of the insulated wall may be moisture damage to the floor joists above and water leaking under the wall in the spring when the frost on the foundation wall melts. This can mimic a leak through the foundation wall and often is confused with seepage by the homeowner.
In a nutshell, the conventional insulation method for basement wall cavities, with a single, well sealed 6-mil Polyethylene air-vapour barrier over the warm side of the wooden studs with no space behind is the way to go. Care should be taken to ensure installation of the insulation and air-vapour barrier up between the floor joists and sealing the Poly very well to the joists and floor sheathing above with acoustical sealant.