Renovation & Design

More than one way to warm a floor

Question: I was browsing the Internet looking for good information and recommendation on a particular topic and I ran across an article you had done for the Free Press back in 2003. "Moisture under cottages a common woe" which had referenced from an article from 2001.

I have a cabin in the old campground in Riding Mountain National Park. We tore down an old one and built a new cabin in 1982. It is 16 by 24 feet and we recently added a eight-foot square bedroom addition on the back. We are now getting water and sewer service to the cabin area and leases which run from early April to late October. So, we are in renovation mode again, with putting in a bathroom. What I have noticed, since we are now using the cabin more in the shoulder season, is that the floor is cold on the feet. We have laminate rather than carpet, due to pine needles and dirt tracking in, and want to keep cleaning easy. We wish to stay with laminates and vinyl flooring.

The cabin is on cinder blocks, just high enough that I can shimmy under most of it. The floor joists are uninsulated, and no vapour barrier on the ground or under the subfloor. We currently heat the cabin with small electric space heaters, but I will be adding electric in-wall heaters once we get the plumbing upgraded.

So my question is; how best do I warm up the floor? I can get the air temperature toasty warm inside, but the floors and the air layer at the floor remain cold, as the cavity underneath is cold. I have considered laying in heating mats and redoing all the underlay, vinyl and laminates, which seems expensive and labour intensive. I have also considered putting in cork floors, as it feels a bit warmer on the feet, but does not solve the cold-floor issue.

I was wondered about insulating the joist cavities and covering the bottom of the joists with plywood so the rodents don't get at it. Although a lot of people are doing that with new cabins, I wonder how effective it is. The cavity under the joists is still cold. Plus I would likely have to raise the cabin a bit to effectively work underneath in order to insulate the floor like that.

My old construction days tell me if you want a warm floor, you have to put heat under it. So I was thinking of this and wanted your opinion. If I could find a small down draft electric furnace, could I use it to heat the cottage? I have enough space to run ductwork to each of the two bedrooms, the living room, kitchen and bath, plus direct a couple of heat vents into the crawl space area. In the shoulder heating season I would close in the skirting around the perimeter, and in the non- heating season, open it up for ventilation.

I am concerned about putting down a heavy vapour barrier on the ground, as it is just one more thing for the rodents to get into, and I doubt I could effectively cover it with rock. Plus, it will trap any moisture that does get under the cabin. As the ground appears naturally dry and if I am heating it in the shoulder season and opening it is the summer season I wonder if the vapour barrier is even needed. I would appreciate any thoughts, recommendation or other ideas you might have.

Jeff Sim


Answer: You have noticed one article where I annually address this common question, because it is one of the most misunderstood and frequently inquired about topics related to building issues.

You appear to have an above-average grasp on the concerns and proper techniques with this issue. Your suspicion that simply insulating between the floor joists will not be enough to provide a warmer floor is correct. While this method is commonly used, it is not effective without a heat source. In many cases, this will prevent excessive heat loss, lowering heating bills, but will often make the floor colder. This can happen because this reduced heat transfer from the living space to the joists prevents slight warming of the floor relative to the cold air outside this area. In an attempt to warm the cold floor surface, sometimes the opposite is the result. Installing an electric heating grid would warm the floor, but would require extensive insulation in the cavities, often combined with costly reflective insulating sheathing to prevent excessive hydro bills.

Unless you are planning that type of major floor renovation, your other idea will be much more effective for warming up the flooring. The only thing I would suggest to improve upon your idea is to install a permanent grade beam below the perimeter of the structure, rather than an insulated skirting, as you suggest. That way, you will be able to insulate it more effectively and won't have to remove and replace it every year. Another benefit of this method will be to improve the support structure for the cottage, preventing the possibility of uneven shifting between individual concrete block supports. This grade beam could be constructed out of concrete, block, or pressure treated lumber and sheathing. Any of these materials would allow permanent insulation inside the grade beam with rigid expanded polystyrene or blown-in high density polyurethane insulation. Using either of these materials will allow easier air sealing and minimal use of polyethylene sheathing for your air/vapour barrier.

Installing large screened vents, with removable winter covers, in several areas around the perimeter of the new grade beam should be adequate for summer ventilation of the crawlspace and also double as access hatches. These, combined with six-ml polyethylene sheathing over the entire crawlspace floor, sealed to the perimeter insulation should be installed to prevent moisture into the living space. Adding heat to the crawlspace and living space with an electric, forced-air furnace, will ensure you have a warm, comfortable cabin floor for the future.

Ari Marantz is the owner of Trained Eye Home Inspection Ltd. and the 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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