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Questions about installing EPS above concrete.

I'm in climate zone 6A, I think. Halifax, Nova Scotia. I've read that the best approach to insulating an existing basement floor is to install a continuous layer of 1" thick EPS foam over the concrete, with taped seams, and two layers of either plywood or OSB on top.

1) If EPS is vapour permeable why use tape to seal the seams?

2) I have existing partition walls that are screwed to the concrete with foam gasket in between, please tell me I don't need to remove them and put the foam under the walls?

3) I have a couple of areas that I don't think I want to cover with foam, the mechanical room where I don't want to impede water flow toward the floor drain in the event of a leak, and my work shop with the rolling tool box that weighs ~1000#. Am I wrong about those areas, would the foam handle the weight of the wheels and is there a better way with the floor drain?

4) If I leave those areas uninsulated how should I transition from insulated to not insulated? I'm guessing just tape the edge of the foam to the concrete?

5) Is there any reason not to use 2" of foam for more insulation?

6) I presently have insulated floors with a single layer OSB over dimpled plastic and have seen no issues with the OSB warping. Is there any reason not to do the same again, especially if I can save what I have here already?

As always, thanks guys!!

Asked by Calum Wilde
Posted Aug 12, 2017 11:24 AM ET


5 Answers

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I'd use higher density foam in the shop area. I've seen enough plumbing leaks, toilet overflows, external bulk water entry, etc that I'd prefer a design that allows bulk water to move under the floor. Or even better, one that doesn't involve wood.

Re #1 - to prevent air movement from moving moisture to the cool concrete.

Answered by Jon R
Posted Aug 12, 2017 12:00 PM ET


In order,

1: Vapor permeance allows a tiny amount of water vapor diffusion, but is not the same as an air leak. The seams are large air leaks, that allow both air and bulk water (not just water vapor) to move freely. Air tightness matters.

2. Don't sweat the small stuff. While a bit of EPS would be preferable to none, the foam gasket is still WAY better than nothing. It could take 100 years to become a problem. As long as the slab is dry and the humidity in the basement air is reasonably controlled it'l never be a problem. The other reason to move it above the foam (and this may be more important) would be to be able to nail the wallboard & kick boards to the bottom plate of the studwall. In many ways it's easier to put all the foam & subfloor down, and put the bottom plate of the partition wall framing on top of the subfloor.

3. Foam on top of concrete, covered with sub-floor can handle a LOT of weight. Woudn't sweat the tool box. (It's common in some areas to up 50 gallon water tanks directon on an EPS pad, without a subfloor to distribute the weight That's over 400lbs.

4. It may need a ramp/wedge or a threshold to not be a trip hazard, depending on the depth of the total stack up.

5. It's still financially rational in a 50 year analysis to put R8 between the conditioned space and subsoil in locations with subsoil temps as cool as NS, but it's more than needed just for summertime dew point control. If you have the headroom and access to cheap EPS, go for it. Bigger lifts complicates issues around stairs & doors though.

6. You can't argue with success! If it worked in one part of the basement it's a good sign that it'll work everywhere.

Answered by Dana Dorsett
Posted Aug 12, 2017 4:30 PM ET


Thanks Dana!

I was planning to drop the level of the insulation by an inch to allow my exterior door to open. The interior doors I'll likely have to raise. The stairs I'm hoping I can just ignore the codes for now... Not my favorite thing to do, and maybe it would be better to replace them and run EPS under the stairs, but for budget constraints I'm just gonna leave that for now.

Answered by Calum Wilde
Posted Aug 12, 2017 5:51 PM ET
Edited Aug 12, 2017 6:42 PM ET.


Q. "Is there any reason not to use 2 inches of foam for more insulation?"

A. No.

Answered by Martin Holladay
Posted Aug 13, 2017 5:29 AM ET


Thanks Martin.

Answered by Calum Wilde
Posted Aug 13, 2017 8:47 AM ET

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