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
    Commented Dec 8, 2016 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
    Commented Dec 8, 2016 at 20:58
  • 1
    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
    Commented Dec 31, 2018 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
    Commented Oct 22, 2019 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
    Commented Oct 23, 2019 at 16:51

4 Answers 4


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).

  • 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
    Commented Dec 25, 2018 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
    Commented Jun 24, 2020 at 17:15
  • Sometimes it makes sense to refer people to where deeper expertise lies, in this case on energy topics.
    – Bryce
    Commented Jun 25, 2020 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
    Commented Jun 26, 2020 at 13:37


  1. You should not have any cavities between your thermal planes (consider the manufacturing requirements for eliminating humidity in the fabrication of insulated glass units).

  2. Likewise, it is critical that you do not provide means of airflow.

    • Airflow will provide thermal bridging, 'short circuiting' your insulation.

    • Airflow will cause degradation of your building materials, mold and so on.

That is, natural convection will pull warm air into the cold cavity and spill the cold air into your conditioned space, not only will this fail to reduce your energy costs to their full potential for a given insulation value, but also it will cause the humidity in the warm air to precipitate on the cold surfaces of your thermal plane(during the cold seasons; and vice versa in the hot seasons). This can be gallons of water and is a common problem with basement weatherization improvements.

As for what do you do about water damage (be it from appliance failure, burst pipes, ground water pressure, flooding spilling over the foundation, or whatever), you will need to determine whether your conditions ensure reasonable certainty of the success or failure of your project. First, you'll need to determine if any of these are known, unpreventable, recurring issues with your specific home site. Next, you'll need ensure you have the means to protect your investment (preventative maintenance) and insure it against unforeseeable-unpreventable damage events to the portion of the replacement cost you are comfortable with and/or financial capable of covering (Homeowner coverage against the particular water damage types you are concerned about; partial or full replacement cost and in either event you'll either need to and then you need to catalog the actual cost, down the the individual screws - no joke - or you'll have to pay someone to do that work for you after the fact if and when the damage occurs). (A note on flood insurance, some types of flood insurance, especially FEMA backed flood insurance, are notorious for never getting paid out even with a solid claim).

The products you've listed all require a means of capturing moisture and expelling it. Often times that's a perimeter trench cut into your basement slab, channeled to a sump pump. This is critical for flooring applications for several technical reasons - for example, if you don't get the water out, it can will literally float your flooring as ground water pressure of percolates up through the slab.

Closed cell foam is the only appropriate material for your project (unless you want to go nuts and get into using an exterior rated waterproofing system inside your foundation, which would be hilarious).

  • ok ok I know super users on this site have a hard on for telling us "people don't have to explain why they downvote" but someone tell me what's up. This answer is building science 101. Why the down vote? And why the down votes on all the answers? This is a grumpy little question. And, I come across this failure of thinking logically every time I renovate a basement because mold or water or.... Not usually people rabbeting, this is a first, but folks always try to put a cavity behind basement furring walls like, "don't let the studs touch or they'll rot." They rot anyway with so much as vapor Commented Feb 19, 2021 at 5:05

No, don't cut grooves. If you were following correctly the board is taped to provide air seal, you defeat the purpose of what you read in articles. Your defense to outside water is on outside. Keep check of your fixtures and piping. Do not put a vapour barrier if wall is below grade, vapour drive is towards the interior in these conditions.

Even if you have grooves and its a flood water will lift the insulation anyways. Grooves will only delay the inevitable as water will be trapped. You are better off making sure you slope to drain with the plywood and finish floor as best effort. Personally would not use insulation on top of the floor, i would just live with the heat loss in older home or put in radiant floor.


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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