How to Insulate your Basement like a Pro

To make your older home more sustainable and cut your energy bills, sometimes you have to look down. In this article, you will learn how I insulated my basement in Halifax, Canada, and give you some tips to consider when insulating your basement.

An uninsulated basement in an older home can easily lose 20% of your hard-earned heat energy in winter. In my own house, according to my Home Energy Assessment, it was a whopping 50% of the heat loss. My basement heat loss was a higher percentage because I had already improved the insulation in the rest of the house. It was time to dig deep and deal with the basement!

Why is basement heat loss so much of the total?

  • A basement is a large surface area of the home, often with little or no insulation.
  • Heat is lost in all directions (not just up), including through the walls and floor coming into contact with the ground
  • Foundation materials like concrete and stone have almost no insulating value.
  • Cold air leakage through the headers (where the foundation meets the upper walls) is a huge energy loss.

BEFORE — Here is my basement before insulation:

Photo by Wayne Groszko.

The foundation walls are stone with mortar, and the floor is dirt with a layer of gravel. This basement is relatively dry, with no standing water and no evidence of water running down the walls. The floor drains well through the gravel to a sump pump located in the other corner. Experience shows me the basement does not regularly flood with water, an important point to consider.

Some basements can be exposed frequently to water, so avoiding water damage is the most important thing to think about when choosing insulation for a basement. If you have a relatively dry basement like mine, you are fortunate to have a few different options. But if your basement regularly has groundwater running through it, you must make sure your drainage system is effective before you install any insulation. Talk with a professional in basement drainage.

What’s the quickest, most effective way to insulate an unfinished basement?

Closed-cell spray foam: Having closed-cell spray foam insulation applied to the entire basement (walls, header, and floor) is the quickest, most effective way to insulate a rough stone basement like mine. I had a continuous 4-inch layer applied in a single day. It’s triple-purpose, providing high insulation value (R-28), sealing all the air leaks around the stones and headers, and stopping dampness and humidity. A layer of closed-cell spray foam at least 2 inches thick works as a vapour barrier.

One catch is you have to leave your house for few days while the insulation cures. But when I went back into the basement after a couple of days to check it out, the air quality was way better than before the insulation job. Ther was no more damp, musty basement smell. And it’s so clean down there now!

AFTER — Here is my basement after spray foam insulation:

Photo by Wayne Groszko.

Dense spray foam like this is hard enough to walk on once it is cured, but you still have to cover it with either a fire-retardant paint if it’s in a non-inhabited space, or a protective surface like gypsum board or wood if it’s inhabited. My basement is not occupied, but I covered it with wood anyway, as you’ll see later.

If you are going to use spray foam, choose a brand of spray foam with a low Global Warming Potential (GWP). I found foam with GWP = 1 (one), which is the most climate-friendly kind available in Canada.

What’s a good option for a dry basement that you want to finish?

Mineral wool boards and batts: If your basement is smooth and dry, like a concrete floor, you can insulate with rigid mineral wool boards against the concrete. You can then frame a wall and a new floor over the rigid insulation. Mineral wool is a good choice in this case because it is not damaged by exposure to dampness. Include mineral wool batt insulation in the cavities of the new framed wall.

In this method, it can help to install a polyethylene vapour barrier, preferably heavy-duty 6-mil poly, to reduce moisture evaporation into the basement. The wood framing should not be under the vapor barrier, because it could get too damp in that position, so install the vapour barrier either against the concrete or against the rigid boards.

In my basement, once the spray foam was done, I decided to finish it by building a wooden floor and wall coverings to conceal the spray foam. I included mineral wool batt insulation in the floor and wall construction. I didn’t need to add a vapor barrier, because the foam is already a vapor barrier, but I added breathable house wrap to contain the mineral wool insulation fibers. Here is a photo of the insulated wood floor under construction:

Photo by Wayne Groszko.

The completed floor and walls will have an effective insulation value of R-40, rivaling a top-performing new Passive House.

My Basement Insulation Project by the Numbers:

  • Cost: $5,300*
  • Area covered: 740 sq. ft.
  • Rebate from Efficiency Nova Scotia: $1,180
  • Estimated energy savings: $625 per year

*Cost includes $3,700 spray foam installation, $400 prep work, and $1,200 for insulated wood floor and walls to cover the spray foam.

What about insulating the outside of the foundation?

Yes, insulating the outside of a foundation can be a good approach, especially if your basement is already finished, and you don’t want to tear out the interior work. You can have a contractor dig around the outside of the foundation and add expanded polystyrene (EPS) insulation to the exterior.

Insulating the exterior has advantages, especially because you can damp-proof the foundation at the same time, solving any drainage and water leakage issues you have.

Final Observation

One thing that surprised me about the spray foam job was how much overspray there was on everything in the basement afterward. Luckily there wasn’t much equipment down there because anything down there would have been covered in foam.

I took out the hot water tank temporarily for the job, and the contractor taped over the water meter and the main water valve. We covered the sump pump with a protective sheet of plastic, which is a good thing, because this is what it looked like afterward, totally encased in spray foam:

Photo by Wayne Groszko.

More resources: For a complete guide to basement insulation, check out Keeping the Heat In, from Natural Resources Canada.

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