Renovation & Design

Insulation, heat will help prevent crawlspace condensation

Questions: I really appreciated an article you wrote on heating a crawlspace. I live in Ottawa and am currently debating how to make a cottage warmer and more comfortable. We have a cottage that is on piers/posts that we mainly use in three seasons, but enjoy visiting for a handful of weekends in the winter. We’ve noticed quite a bit of condensation in the cold days under our couches, etc., where air circulation does not happen. I was planning on having the floor sprayed underneath with foam insulation. After reading your article I am doubting if I should continue with this expensive plan. I can see the benefit of sealing off the crawlspace. Since I only plan to use the cottage a few times in the winter, I typically shut off the power. Do you see any issue with heating the crawlspace and letting all of this go through freeze/thaw cycles?

Regards, Bob Botti.

Answer: Insulating an open floor of a seasonal home, even with high density spray foam, will do little to prevent condensation or minimize issues with an infrequently used building. Enclosing, insulating, and heating the crawlspace is a better way to go to make it more comfortable when you are able to use it in the winter months.

Your submission comes at a time where I annually address this frequently confounding issue, for people with seasonal homes. Many cottage owners and contractors have battled this question about whether to insulate an open floor of a cottage, or install an insulated skirt to enclose a crawlspace below. I fit in both of these categories, being both a summer home owner and ex-contractor, and will tell you that there should only be one choice that will provide any sort of satisfaction. Having attempted the open, insulated floor method on more than one occasion, all have later had the insulation and air/vapour barrier removed and replaced with an insulated crawlspace. Admittedly, none of these floors were done with spray foam, as it was not readily available a few decades ago. But, in hindsight that may not have made much of a difference. For any residential property in our climate, home or cottage, a heated crawlspace will lead to a much more comfortable living space above the floor.

The reason for this is due to the simple scientific principal that tells us that warm air rises. In a building, this phenomenon has been termed the “stack effect” by building scientists and engineers. Understanding how much this affects air movement inside and outside the entire building enclosure has changed the way buildings are designed and built in the last few decades. Because of the stack effect, almost no heat loss from air leakage will occur from the living space through the floor, insulated or not. Insulation may help prevent radiant or conductive heat loss through this area, which should only be a minor amount. High density foam should prevent cold air infiltration from below the floor, which may help keep it from being really cold in the dead of winter, but will not make it much comfortable to walk on. Only adding heat to an open airspace underneath will provide a noticeable warming effect on the floor joists, sheathing, and flooring above.

Skirting in and insulating the walls of the crawlspace with high density polyurethane spray foam, or extruded polystyrene sheathing, will ensure the space may be heated either continually or periodically over the cold months. The dirt floor should be covered with a layer of 6MIL polyethylene sheathing, or equivalent, to prevent drawing excessive moisture into the enclosed space from the moist soil. A few screened summer vents, at all sides of the enclosed space, will help remove excess moisture, which can be replaced with insulated hatches in the fall.

Another benefit of heating the crawlspace will be to allow year-round use of plumbing supply and fixtures. Even by heating the entire cottage to normal levels, there may be no way of ensuring water supply pipes don’t freeze in or below an insulated floor system. As long as the heat source is reliable, typically electric heaters or ducts from a forced air furnace, a warmed crawlspace should prevent frozen plumbing supply pipes. These will still have to be drained if you do plan to shut the heat entirely, but leaving the thermostats set low should prevent frozen pipes. That will also save energy and money, preventing the need to keep it at normal room temperatures when no one is there.

If you do leave the plumbing shut off and drained during the cold weather, there should be little problem with turning of the heat for the crawlspace, for long periods of time. This may allow some frost to build up, from frozen condensation inside the enclosed crawlspace, but that should melt when the heat is restored, sit on the surface of the floor poly, and evaporate in the spring when the summer vents are installed. That method may not do much for the condensation you have observed on the floor, which may still return every time you shut of the power and subsequent heat in the building. For that issue, leaving a couple of ceiling or portable fans on when the place is vacant may help with air movement. Also, if you have a wood burning fireplace, leaving the hearth doors and flue open should provide a natural vent for the living space, as long as you have a proper rain cap and screen to prevent pest and snow infiltration.

Foam insulating a cottage floor may do nothing to prevent condensation in the unheated, enclosed living space above. Providing some air movement and ventilation, through use of fans and fireplace flues, may be a better approach. Either way, enclosing, insulating, and heating a crawlspace below the floor will make it much more comfortable and prevent condensation when you are using the building during the cold months.

Ari Marantz is the owner of Trained Eye Home Inspection Ltd. and a Registered Home Inspector (RHI)( 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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