Moisture under cottage floor a common woe

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This article was published 02/11/2003 (7986 days ago), so information in it may no longer be current.

QUESTION: Dear Mr. Marantz,

Back in the summer of 2001, you wrote a couple of terrific articles in the Free Press dealing with insulating cottage floors and crawlspaces. For various reasons, which I won’t bore you with, I am only now attempting to solve my moisture problem in my cottage crawlspace. The following are my cottage particulars and then my questions.

My cottage was built in 1979 on Sonotube pilings at Sandy Hook, on the west side of Lake Winnipeg, 16 kilometres south of Gimli. The floor was insulated using R20 batt insulation set on buffalo board with a vapour barrier installed over floor joists and insulation and covered with plywood sheathing. Six years later, a hot water tank, pump and plumbing were installed under the cottage.

To winterize the cottage and protect the pump, I built a two by four, three-foot-high (one metre) insulated knee/pony wall around the entire perimeter of the cottage (8 m X 8 m) and installed a small baseboard heater that I keep about 5 to 10 C. I also installed five small vents around the perimeter plus an opening to get into the crawlspace from outside, which I leave open during the summer and shoulder seasons. Some years later, to help reduce electricity charges, I enclosed the pump area, about a quarter of the crawlspace, with some Styrofoam.

Excessive humidity became a problem when I closed up the vents for the winter. Over the last 10 years, the water table has been high in this part of Manitoba and only this year dropped. Thus, the dirt floor of the crawlspace and the cottage floor insulation was always damp to the point where some mould formed on the underside of the floor joists. Rot, if any, seems to be minimal. Obviously, I am very concerned with damage I may have already caused through my procrastination. My wife and I hope to retire to the cottage in about six years.

I have been offered varying types of advice. This ranges from: “after 25 years don’t worry about it,” to “lift the story and a half cottage and place it on a concrete foundation.” Based on your articles, I am going to cover the dirt floor of the crawlspace with heavy poly, staple it up the inside of the pony wall and caulk it as tight as I can to prevent moisture from the ground rising into the crawlspace. The three interior posts will be tricky.

Now the question is: before I lay the poly should I remove the old buffalo board and floor insulation? Also, what should I do about the floor vapour barrier? I am afraid to slit it between the floor joists as it has protected my plywood floor from the damp crawlspace for many years now.

Thank you very much for taking the time to read my e-mail.

Cheers,

Ken Porteous, e-mail

Answer:

Questions about crawlspaces for seasonal homes are among the most common that I receive and I attempt to address this issue at least once or twice a year. I hope my response to you will answer some of the other queries that I have not been able to address. The first thing to mention is that I am assuming the cottage in question is used periodically in the heating season and that the crawlspace will be heated year- round. This should be treated somewhat differently from cottages that are closed for the winter, with the plumbing drained and the heat shut off.

I would say that, without a doubt, you should remove the buffalo board and insulation from between the floor joists. Seeing the signs of mould in this area and damp insulation shows that moisture is being trapped in these cavities and in the insulation. Mould may be a health concern to sensitive individuals and care should be taken when removing the damp insulation and other building materials. Anyone working in the crawlspace should be wearing a proper respirator designed to minimize inhalation of small airborne particles. The insulation should be thoroughly dried out, if it is to be reused in another location, or discarded if any visible mould is seen. The floor joists may be treated with a fungicide, after removal of insulation, to help prevent further deterioration and mould growth.

After removal of the K. B. board and insulation, I would take a utility knife, cut and remove the polyethylene air-vapour barrier from below the floor sheathing. In my opinion, this should be done for two reasons. The first is to release any buildup of moisture between the poly and the plywood floor sheathing. Well-sealed plywood provides a moderately good vapour barrier and may serve to trap moisture underneath, allowing an excellent area for mould and rot growth. The second reason for removal of the poly is to increase the chances for moisture in the crawlspace to escape during the heating season.

The dampness observed in the crawlspace in the winter is partially due to this air-vapour barrier underneath the floor sheathing. The moisture in this area may be substantially increased by the exposed dirt floor, but it will also be trapped by the existing insulation and air-vapour barrier below the plywood floor sheathing. Now that the dirt floor and knee walls in the crawlspace will be relatively well-sealed to prevent moisture intrusion and heat loss, we can allow crawlspace air to escape into the living area of the home. The moisture in this air will do much less damage to the larger living space, and will have much better circulation and ventilation due to doors, windows, exhaust fans and fireplaces. Air movement is critical for drying of damp building materials and, contrary to your beliefs, removal of the poly below the floor sheathing should decrease the chances of moisture damage.

Ari Marantz is the owner of Trained Eye Home Inspection Ltd. and vice-president of the Canadian Association of Home & Property 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