Renovation & Design

Cold-air ventilation will fix moisture problem in camp's crawl space

Question: My camp is up in Sudbury, Ont., and I have a problem with moisture in the crawl space. It is mostly enclosed, due to the previous owners living up there full time, so they required this for the plumbing.

The ground is made up of dirt and rock under the building. The moisture issue has been there since purchasing. I asked a friend, who is an architect, to have a look and he recommended putting up eavestroughs, as there were none. I also raised the soil around the crawl space walls, so that it is angled away from the camp.

I also added about four vents to the crawl space this past summer and found that there is still plenty of moisture, to the point where little droplets of water are actually forming on the wood above. The wood itself has some white mould on it, too. I do not see any vapour barrier, unless it is under the hardwood flooring inside the camp.

Would you suggest I build a hatch from the main living area and leave it open in the winter so that air circulates better under the floor?

I would install good screening, of course, for critters. Or should I put vapour barrier on the entire ground covering the dirt floor in the crawl space? Would there be any consequences if I did that?

I am currently running a fan to help keep it dry, but it is too big a space to cover it all.

Any suggestions would be very welcome.

Thank you,

Mike Francone

Answer: Moisture buildup in an enclosed crawl space, especially in lake country, is to be expected.

The keys to minimizing it are related to improvements in both grading and ventilation. Concentrating on these two areas may provide considerable relief before moisture damage gets too excessive in the floor system.

Damp conditions in a seasonal home crawl space is something that most owners will have to deal with. This is due to high humidity levels, which in lake country can be present all year round.

Enclosing the area below the floor to grade can be an effective way to prevent pest and cold air intrusion, but will make the conditions more favourable for moisture damage to floor components. That is because any moisture building up in this crawl space will be prevented from easily drying, by the walls of the crawl space.

In the winter this area may be fairly tightly sealed, and if not heated, any moisture should remain frozen in the soil. Once the weather warms, in spring, the crawl space may become very damp, or even partially filled with water due to runoff from melting snow.

Placing a layer of polyethylene sheathing on the floor of the crawl space, as you suggest, is recommended mainly for crawl spaces that are heated.

That is because warming the soil during cold weather will draw water vapour from the soil, which can condense inside the crawl space and cause moisture damage.

The thin plastic layer can help prevent this but may also prevent easy soil drying or absorption of other crawl space moisture when the weather warms.

So if the crawl space is not heated and the building is shut down for the winter, I would avoid covering the dirt floor.

Also, I would not put a big opening in the floor to vent the crawl space into the main building, as this may just cause moisture damage in that area.

The proper approach would be to allow some cool, dryer winter air to access the crawl space during this season.

While it is not critical to allow ventilation in the colder months, due to the frozen moisture, it may help vent some moisture left from the previous warmer months.

Whether you cover your vents for the winter or not — to prevent blowing snow intrusion — the key will be to address this issue when the weather warms.

The vents you have added to the crawl space skirting walls are exactly the right approach to help remove some moisture in the area. I would suggest increasing the number and size of these openings to improve passive ventilation considerably.

Normal winds often provide sufficient airflow to an area like yours, to properly ventilate a damp crawl space. If there is excessive vegetation or other items obstructing airflow, your continued use of fans will be a good alternative, especially during and after the snowmelt.

The next area to address is the grading, not only outside the crawl space, but inside, as well.

Your architect friend was right on the money to suggest building up the soil outside the crawl space and directing the roof runoff away from the dwelling. But the grade level inside the area should also be checked. If it is significantly lower than the average soil height in the area around the home, then it will be subject to what I call the "swimming pool effect."

Any depressed area of soil may be subject to water accumulation, simply from moisture seeping in from the adjacent higher graded areas.

All you have to do is dig a small hole in any yard after the surface snow has melted, or after a heavy rain, to see this in action. So if this is an issue in your crawl space, shovelling in a few yards of fill to raise the grade should make a noticeable improvement. I would caution you not to overdo this, as too much soil against the inside of the skirting may accelerate any moisture damage already occurring.

Since you are already seeing the signs of mould or wood rot on the surface of the floor framing, it is past due time to prevent further moisture in your seasonal home’s crawl space. A combination of raising the grade, in and around the crawl space, and increased summer ventilation may be the winning formula to minimize further damage.

Ari Marantz is the owner of Trained Eye Home Inspection Ltd. and the past president of the Canadian Association of Home & Property Inspectors — Manitoba (



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