Where do I place vapour barrier in relation to basement slab insulation?
sikasmith
| Posted in Mechanicals on
I realize this question has been answered in numerous posts previously; however, our mechanical design involves using Insulworks insulation in the basement. Where and how do you suggest I place the vapour barrier if it is to be added on top of the insulation?
Here’s the Insulworks product which involves using staples and is designed for the tubing to snug into grooves. http://www.amcfoam.com/PDFs/Insulworks%20general%20spec_web.pdf
Also, please forgive my ignorance, but why is it important to put the vapour barrier above the insulation and not immediately below it?
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Replies
Jane,
Q. "Where and how do you suggest I place the vapour barrier if it is to be added on top of the insulation?"
A. In most cases, builders install a layer of polyethylene (or a more rugged material) between the rigid insulation and the concrete.
Q. "Why is it important to put the vapour barrier above the insulation and not immediately below it?"
A. You don't want any moisture to be trapped between the polyehthylene and the bottom of the concrete. This can happen, for example, when it rains after the foam is installed but before the concrete is placed. For more on this issue, see http://www.buildingscience.com/documents/insights/bsi-003-concrete-floor-problems/.
Q. "Here's the Insulworks product which involves using staples and is designed for the tubing to snug into grooves."
A. It looks like this product won't work unless the polyethylene is under the rigid foam. That's not ideal, but it will work as long as you don't get any rain during construction, and as long as you wait for the concrete to cure completely before installing any flooring. If you have further questions on the Insulworks product and installation methods, you should contact Insulworks.
Jane,
More information from the manufacturer of Insulworks can be found here:
http://www.amcfoam.com/PDFs/Insulworks%20spec%20sheet_web.pdf
The installation instructions state, "A polyethylene barrier is not necessary either under or over lnsulworks, as EPS is in itself a capillary break to moisture movement."
Now why on earth couldn't I find those when I looked! Thank you so much for your help. Is it also your view that a vapour barrier is not necessary? I know you have recommended it in conjunction with 'rigid foam'. Clearly not all rigid foam is made equal!
A follow up question. The maximum regular foam thickness appears to be 3" giving R 12 (I'm actually being quoted the R6). I'm in central Alberta. Zone C (sorry I don't know if that compares to 7 or 8 on your scale). Would you add a second layer of rigid foam beneath the Insulworks to increase R value?
Jane,
If you have a layer of compacted crushed stone under your EPS, you will probably have an adequate capillary break.
A layer of poly may have a few other virtues, however, including (possibly) limiting radon migration, and acting as a barrier to liquid water. Whether these possible virtues really matter for most slabs is debatable. Assuming that you include footing drains to keep your crushed stone dry, and assuming that you install a PVC riser to allow the easy retrofit of an exhaust fan to address future radon problems, you'll probably be fine.
Jane,
Increasing the R-value of the sub-slab foam beyond R-12 isn't likely to be cost-effective. But how much foam to include depends on your goals. Some builders of superinsulated homes (for instance, a Passivhaus builder) might argue for more foam. It all depends what target you are trying to reach.
My initial thoughts had been to have crushed stone, sand, vapour barrier (and pvc riser for radon possibility), EPS, tubing, concrete. We do have weeping tile around the stepped footing which includes a 4' frost wall on the walk out side.
The mechanical drawings show the EPS ending flush with the top of the footing and then a 4" concrete layer over top. So concrete will be touching concrete footing on 2.5 sides of the house (before the step footing disappears below grade. At that point concrete slab touches icf block insulation on the wall. Should I be incorporating some kind of break over the internal footing such as a 1" piece of rigid foam over top of the footing thereby decreasing the depth of the slab to 3" on the 2.5 sides?