Renovation & Design

Sealing crawlspace prevents excess moisture

Holmes Group

A home inspector inspects a crawlspace. Properly insulating and sealing the perimeter and dirt floor of a crawlspace prevents excess moisture.

Question: I have recently acquired a house with a crawlspace about four feet deep. The crawlspace walls and under the floor have been spray-foamed, but with an open dirt floor. The crawlspace is currently not heated. As you can imagine, there was a significant amount of moisture in this space earlier this year. In fact, it was dripping off surfaces in a few places. I have since been able to open vents and start to remove the moisture. My first priority will be to install an airtight poly vapour barrier on the dirt floor of the crawlspace, which I believe will significantly reduce the accumulation of moisture.

While I have yet to spend a winter in this house, the floors did not appear too cool earlier this spring. I assume that should I want to increase the warmth of the floors, I should do my best to remove the spray foam from underneath the house floor? Then, should additional warmth be necessary, consider heating the crawlspace.

Thanks, Paul Runnels.

Answer: Properly insulating and air-sealing a crawlspace may be the most misunderstood and critical part of a good crawlspace, but adding heat will be required to make the floor above comfortable for the home occupants. Both of these systems will need to be addressed to prevent further moisture related issues and have a warm floor to walk on above the crawlspace.

You appear to have grasped the important concepts in my previous article, well. While I have written numerous times on this subject, the two main issues are likely always the same in whichever version you found. As with any building system, moisture control is the most critical component to prevent deterioration to the building products and growth of mould or rot.

You are correct that some of the moisture in your crawlspace is likely due to evaporation from the uncovered dirt floor, but the improper location of some of the foam insulation is also to blame. Because the previous owner has had spray-on foam insulation installed not only on the inside of the grade beam foundation or skirting, but also between the floor joists, the space is fairly well sealed. While the foam does a proper job of sealing the perimeter walls of the crawlspace, its improper installation on the underside of the floor sheathing also stops air from leaking out of this area. That may do little to prevent heat loss from the building, but will certainly prevent air and moisture from escaping the crawlspace.

Since the crawlspace temperature will drop significantly during the winter, it will be much lower than the building above and the previous months. This will undoubtedly lead to condensation of the humid air inside, which may even freeze in the coldest months. Once the ambient temperature rises in spring, the frost will melt, and that is the likely source of the dripping you observed.

You have taken the perfect measures to minimize the effects of this moisture by adding summer vents to the area. This natural ventilation should help dry out the crawlspace in the warmer months, but care must be taken to ensure the soil is dry inside before installation of the air/vapour barrier, over top. Once the 6MIL or thicker poly is in place over the soil, the vents should remain open until the temperature begins to drop in the fall. Next, the foam insulation should be completely removed from the underside of the floor, except for a small amount at the perimeter above the crawlspace foundation or skirting. Before the outside air temperature reaches the freezing mark, insulated covers should be placed over the vents. This can often be done most easily and economically by insulating the covers with a couple of layers of extruded polystyrene and weather-stripping. Once these items are in place, the final piece of the puzzle can be addressed.

Since heated air inside the building will naturally rise by the "stack effect", the temperature of the unheated crawlspace will remain much colder than the rooms above, despite the insulation. To prevent the floor from being ice cold, a heat source should be installed prior to closing the summer vents. This could be done by adding ducts from the existing forced air heating system, if that is the primary heat source. Otherwise, installation of electric baseboard heat, with a thermostat or sensor located in the crawlspace, is your best bet. This thermostat may be set at a lower temperature than the one for the living space, but high enough to warm the crawlspace air to prevent a cold floor. Since you have removed the insulation from under this area, any residual heated air should rise up through the floor sheathing, which may also help to further prevent moisture buildup in this space.

Properly insulating and sealing the perimeter and dirt floor of a crawlspace, and removing improperly located insulation under the floor, will help prevent excess moisture issues in this space. But installation and use of a proper heat source in the colder months will be required to prevent a cold floor above and help further prevent condensation and mould growth in the enclosed space during the heating season.

Ari Marantz is the owner of Trained Eye Home Inspection Ltd. and the past president of the Canadian Association of Home & Property Inspectors — Manitoba ( Questions can be emailed to the address below. Ari can be reached at 204-291-5358 or check out his website at


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