Foam Under Concrete

After compacting the crushed rock in the basement, it was time to do something I’ve been waiting years to do: put styrofoam under the basement floor.

Does that sound crazy to you? Well, it’s one of those things that soon everyone will be doing, especially in colder climates. Let me put your mind at rest on a few points:

A stack of 3" Terrafoam sheets sit on the Terrafoam floor. Note the perimeter of foam around the walls and post bases. We made long cuts with a hand saw. Easy.
A stack of 3″ Terrafoam sheets sit on the Terrafoam floor. Note the perimeter of foam around the walls and post bases. We made long cuts with a hand saw. Easy.

1. Too bouncy? Worried the concrete floor will sink or shift if it’s resting on foam? The rigid foam we used, which is called Terrafoam and is made by Beaver Plastics, has a greater compaction rate than the soil that is under it. It is solid. That’s not just me saying that, that’s our engineer. Even standard, white, EPS which is that white stuff that we see in packaging all the time, is used in big engineering projects like embankments under highways. In fact, I learned too late that we could have put foam under the footings of the house for maximum effect with no structural issues. Oh well, next time!

We used a table saw to cut the edge pieces to separate the concrete slab from the wall.
We used a table saw to cut the edge pieces to separate the concrete slab from the wall.

2. What if water gets under the house and floats the floor up? That was a story someone told us. Well, if you put blocks like they put under highways–great 2 meter by 8 meter ones–under your house, maybe this could be a problem. However, even as much as 12 inches of styrofoam under your house should be fine.

3. Why bother? Some people say the energy savings aren’t going to pay for the extra time and money to put foam under your house. In this housing market when every homeowner is thinking about selling, I understand this perspective. But when you think 40 years down the road and you remember what we’ve learned about the greenhouse effect, it is totally worth it. An efficient house uses less of everything that is warming the planet.

This whole idea came from Monte Paulsen, our energy advisor from Reddoor Energy Design. In February, 2013 he gave us his notes on the HOT2000 energy model he had created.* I’ve had that long to get used to the idea. He wrote:

So, if you’re going to build a conventional foundation, what’s the most cost effective way to insulate it?

Consider adding a “U” of foam insulation that runs down the wall, under the slab, and back up the opposite wall. This arrangement separates the slab from the wall, so the slab can work as thermal mass inside the envelope.

The upgrade model assumes six inches of EPS foam (white Styrofoam) under the slab (R-24) and two inches of XTPS (pink or blue foam) along the perimeter of the concrete (R-12). Inside the XTPS, the model assumes a 2×4 @ 24” wall with R-14 batt (eg, Roxul) insulation and GDW.

DSC03089Can you picture that? You have to remember that your footings and foundation walls were poured first and your basement floor is going to be a separate chunk of concrete poured on a different day. First walls, then floor.

Concrete transmits heat quite well, so if you pour your basement floor without separating it from the walls, any of the heat that is in the room can go into the floor and escape through the walls and footings into the ground.

If the floor is suspended on a bed of styrofoam and doesn’t touch the walls, any heat that is in it can’t escape so easily.

Monte’s model assumed six inches of EPS rigid foam under the slab (floor). Discussing it later, we upped that to 12 inches. Now you know why I was disappointed that we could only fit 3 inches because that was as deep as the storm sewer would allow.

Everyone said that 3″ was plenty. The temperature doesn’t vary much that deep in the earth, they said. You’re only required to put foam around the edges of the slab, they said. You don’t have to do that, you know, they said. The most helpful of these comments was from the Maple Ridge Building Dept:

Not to sure where the trade off is but 12″ of insulation under the slab strikes me as being excessive for no real gain. Below frost the ground maintains a consistent temperature which I believe is around 5 degrees so thermal conductivity through the slab‎ is minimal. Unless you are doing radiant in floor heat the better places to increase R value are the walls and ceiling plus looking at ways to eliminate thermal bridging.

Well, as you know, we did end up doing radiant in-floor heat, but that is another post.

Meanwhile, Monte was working on sexy new Passive Houses where that much under-slab insulation is routine. It really is a case of short-term thinking. If you are not going to profit off it in the next five years, conventional wisdom says you shouldn’t do it. Conventional wisdom has its limits.

Upon further discussion with Monte, I settled on 9″ of white EPS or 6″ of Terrafoam, which is a newer EPS product and insulates better. Since his 2013 report, Monte has pointed out that XPS, the blue or pink rigid foam you see around a lot, has a very high carbon-footprint in its manufacture, and he has stopped recommending it from an environmental standpoint.  For this reason I decided not to use it in the walls.

I settled on 3″ of Terrafoam because every extra inch meant raising the house by that much more. This was one of the compromises we made between heritage, economic and environmental considerations. I still feel like I didn’t get the accurate information from our contractor that I needed to make this decision. I mean I have a sneaking suspicion we could have put more foam under the slab somehow.

Among our graffiti is Dave's birthday wishes to his wife, Leanne's Mom, who grew up in this house.
Among our graffiti is Dave’s birthday wishes to his wife, Leanne’s Mom, who grew up in this house.

With 3 inches of Terrafoam and its insulation value of R-5 per inch, we ended up with R-15 under the slab. The BC Building Code says you must have a minimum of R-13 under a heated slab, so we were okay.

With no contractor working with us, I set about laying our 3 inches of Terrafoam, keeping in mind we wanted a 4 inch deep basement slab. We always intended to do this type of work ourselves, but it was a little nerve-wracking not having professional advice as we proceeded toward our first independent building inspection.

Our contractor had left a chalk line on the walls showing where the top of the basement slab should be. I decided to trust this line was accurate which saved me renting a laser level and figuring out how to use it.

It was frustrating to discover that often the space between the chalk line and the top of the footing was closer to 6 inches instead of the 7 we needed for foam + concrete. Can I blame our contractor for that? I don’t know. I do know it took more time to lay the Terrafoam because I trimmed some of the edges to fit the uneven and too-high footings. (You can see me doing this in the time-lapse videos.) Would it matter if the concrete were a little thinner around the edges of the slab? Probably not.

The sanitary sewer sump was a challenge. You can't use the canned spray foam to fill in gaps because it dissolves EPS foam like Terrafoam.
The sanitary sewer sump was a challenge. You can’t use the canned spray foam to fill in gaps because it dissolves EPS foams like Terrafoam.

Tips for laying Terrafoam:

It cuts super easy. You can cut it with a table saw if you need thinner pieces, but use a hand saw (or a circular saw, I suppose) if you’re cutting a big piece.

Wear a dust mask because, you know, there’s dust, but don’t worry because it’s not toxic or itchy.

Cut the pieces a little big so they snug together nicely with no gaps.

Don’t compromise! Separate everything from that slab! Put foam around any plumbing or structural stuff that pokes up through the floor.

Umm, that’s it! Any questions?

Ready for the next step: plastic vapour barrier!
Ready for the next step: plastic vapour barrier!

*I shared Monte’s complete report in this post if you’re interested in the whole thing. It’s near the bottom of the post.

Published by

James Rowley

James lives in Maple Ridge, BC, Canada with his amazing partner, Leanne Koehn, and their two amazing kids in their beautiful house. He studied Science and English Literature at the University of British Columbia where he met Leanne. He also studied acting for a while at Studio 58 in Vancouver. He works as a teacher of English and curriculum writer for new Canadians.

Leave a Reply

Your email address will not be published. Required fields are marked *