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

First off, the go to place for questions like this is Green Building Advisor http://www.greenbuildingadvisor.com/ (you can become a GBA Pro member).

I did almost what you propose, but used a commercial tile mat between the foam and the slab floor. It worked out great, no issues after 4 years.

  • 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

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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