I intend to finish a part of my basement following this guideline from Fine Homebuilding and Building Science. In this configuration, the foam board completely contacts the concrete. My conern is that should a small water leakage event occur, such as a washing machine or a water tank leak, the water will have nowhere to go and gradually seep through the foam and eventually lead to the entire floor needing to be replaced.

I've noted some products on the market make a point in having air space in their floor contact area, for example AmDry, DriCore RPlus and ThermalDry. See sample photo below.

I'm thinking it would not be too hard to use a table saw and dado blade to cut channels in the back of the panels. I was thinking I would cut half inch deep, quarter inch wide, on a 4 inch grid.

The down side is that this will decrease the PSI capacity of the foam. I was thinking I might have to increase the thickness of the plywood over the foam. I'm not sure if this effort would even be worth it without the water-proof layer that the commercial products offer - however, I assume anything is better than nothing. I'm not sure if this will weaken the insulation and cause it to crack along all these lines.

Image Sample floor product

  • Why not just use one of the products you named that's intended for this application and already has the features you're looking for? – Sean Dec 8 '16 at 18:03
  • Price and availability. The panels of AmDry work out to $3.10 per square foot, shipping will likely bring that cost up to $4 per square foot. There's also lead time for delivery. XPS foam and plywood work out to $1.50 per square foot and are at every hardware store. There's also the ability to add hydronic radiant floor heat to a site-built option – kavisiegel Dec 8 '16 at 20:58
  • I'm sure your project is long completed. but in case someone else is interested, you can always install a drain pan for the washer and water heater. – SkottyG Dec 31 '18 at 18:14
  • I am curious. Would the following, placed underneath the foam, suffice to mitigate your concerns? amazon.com/MODUTILE-Interlocking-Perforated-Drain-Floor – peinal Oct 22 '19 at 1:29
  • @peinal Interesting idea, but I think the contact area between the foam and the floor drain mat would be too small. Cutting 5% of the foam out for drainage keeps 95% of the contact area, but layering it with a floor drain would put only 50% of the foam in contact with the layer below it. If there were floor mats with a flat top surface and only say 20% open air, that would probably be suitable – kavisiegel Oct 23 '19 at 16:51

I did almost what you propose, but used a commercial dimpled tile mat between the foam and the slab floor. It worked out great, no issues after 6 years.

That said, the go to place for detailed energy questions like this is Green Building Advisor http://www.greenbuildingadvisor.com/ (I have no affiliation with them).

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  • I believe there is expertise on this site to cover this but sending others to different sites sounds wrong , but may have been good advice since there are no other answers. – Ed Beal Dec 25 '18 at 18:39
  • This is the first I've seen of this Q&A but this answer sounds very spammy to me. The saving grace is the score of the answerer... – FreeMan Jun 24 at 17:15
  • Sometimes it makes sense to refer people to where deeper expertise lies, in this case on energy topics. – Bryce Jun 25 at 20:00
  • That's true, @Bryce, but SE rules require that you include quotes or summaries of relevant info because links die. If the link dies (or changes), the answer is no longer useful. Also, you should indicate your affiliation with the site you're linking to - Owner, employee, happy user, first place I found when searching... Just so we know. – FreeMan Jun 26 at 13:37

I went through this last fall: you do not need air moving with this foam board. I used the pink soft fiberglass and put the moisture barrier outside it... In a month I checked and the insulation wasl all wet. I had it backwards. So the vapor barrier, which is what you ahve there, needs to be air tight and it does not matter if you have a gap. I'm very sure of this. Good luck

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