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Under the Slab: What Kind of Insulation?

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By Steven in Colorado Springs, CO on 5/4/2010

Okay, I've done some poking around the Web here, and definitely seem to be getting some different opinions, so I thought a new thread was in order here.

We're getting ready to (finally!) start excavating out our ground floor and getting it ready to lay in gravel, insulation, and the radiant heat. The questions I'm hoping folks here might help me with are:

  • What kind of underslab insulation do you recommend? The foil "bubble wrap" stuff or rigid foam? If rigid foam, how thick?

  • How thick would you make that slab? Most of the radiant heat references I've seen recommend that it be about 2" thick here, with the tubing no deeper than that.

  • The walls are ICF, but the foundation itself (the footers around the perimeter of the house) isn't, so I'm assuming that a foam (or foil insulation) break between the main slab and the footers is a reasonable idea. Do folks agree or am I completely bonkers?

  • My builder is recommending that I not install a radiant-heat system in the garage, since the house is half-buried in the hillside already, and this will serve to keep the garage much warmer than originally expected. His experience with his garage (with 2x6 stud walls) is that it's been warm enough he's never had to turn on his heating system, though his house is about 3,000 feet lower than mine. I had always intended to only run those zones in the coldest of cold periods, so that my cars would be at, say, 40 degrees rather than below freezing--makes a huge difference in the morning--so these zones would not be a huge "hit" to the house heating budget since I won't run that system all the time. My thinking is that it's better to spend a bit more to put the tubing in now and not use them than to wish I had them two winters from now--what do folks here think? Have you done this?
Hoping to gain some wisdom and knowledge from Those Who Have Gone Before, as it were... thanks all!

Steven in Colorado Springs


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By Kenneth in Lees Summit, MO on 5/5/2010

I’ll share my experience here. Let me preface this by identifying I am not in your locale, but I am in Kansas City so you can compare climate data.


I would not get the foil bubble-wrap insulation underslab. This is radiant barrier, which is effective for radiant insulation provided you have an air space or void. What of conduction or convection? Put a cold ground surface in contact with the radiant barrier (no air space), and you simply won’t get insulation. I guess this is where the bubble-wrap part of the insulation is supposed to come into play compared to simply radiant, but I think you will find that this is insufficient airspace. Now get down to Texas and use this in your attic; I think it would prove its value. In an underslab install in a cold climate, I don’t think so.


As to underslab insulation, I bucked conventional wisdom and didn’t install any. Whoa!!! Let me reiterate this, I have no underslab insulation. What I do have is ICF walls, crushed stone inside my footers (level excavation, footers are not in trenches) to bring the bottom of the slab where I wanted it, and then the slab over a vapor barrier. In my previous house, there was a thermal connection between the foundation and the slab, and that slab was cold all the time (like wool-socks-and-boots cold). In this house, there is no thermal connection to the foundation (ICF walls, no continuous connection to the footers) and only to the ground underneath. This is barefoot-in-the-wintertime comfortable.


However, my plans did call for underslab insulation. It was my ICF subcontractor who suggested a cost savings by eliminating it. There is some discussion of the technique in this thread – I would tell you that this works. If I wasn’t so sure and wanted some insurance, I would have put one course of insulation board only at the edges of the slab to thermally insulate it. I do not have radiant floors though, and can see some value in that you want that heat to go up and not down to the ground. However I can tell you that I can heat my house with basically no forced air, so I am not sure how much hot water would be pumping through the system anyway.


As to garage, again I have no insulation. My garage temp never drops below about 44-F in the wintertime. It also never gets above about 80-F in the summertime. No HVAC intervention. I wish I had this data when I was originally sizing my HVAC, because it’s critical to know how much tempering takes place with ICF construction. As to radiant in the garage, cheap insurance is perhaps that you space your tubing much farther apart that you might expect? Or how much do baseboard radiators cost in comparison to radiant tubing in the floor? Perhaps I would stub out for a future connection to a baseboard radiator instead of tubing in the floors (if needed at some point in the future)? Now if I had a workshop in the garage and intended to use it year round, I can see where heated slab would be much nicer – but I don’t.


I would never pour a slab 2” thick. Perhaps 2” on all sides of the reinforcement, or 2” thick above the radiant tubes. But 2” total thickness is a very thin slab.


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By Mary in PA on 5/5/2010

Well, I haven’t technically “Gone Before” yet, so I’m light on the wisdom at this point. But I’ll share my research info in case it helps.


My research agrees with what Kenneth has said regarding foil bubble wrap stuff. I would stick to rigid foam if you’re going to do under-slab insulation. Here is an interesting site that talks about the bubble wrap:


For the rigid-foam type, I believe XPS is the choice due to the ability to NOT absorb moisture. Here was a resource I found useful:


Should you insulate under all of the slab, just the perimeter or not at all? From what I’ve read, a lot of that depends on the construction type, location (climate, deep-soil temp, soil type, soil moisture, etc.), radiant or not, interior-temperature goal, and so on. To me, this is a complex topic and I’ve only got the roughest beginner idea of it all, and only for my location. I found reading through the archive of posts over at GreenBuildingTalk helpful to try to get a handle on the topic. I haven’t been able to manage it yet, but apparently there are simulations available to help model the performance of the structure and these can be helpful in determining the cost/benefit for the under-slab insulation.


I can’t make a mental picture of your footing/wall/slab detail – probably due to my inexperience in construction. I would think that the slab edge would be up against the ICF, not the footing. (?) If so, the ICF insulation provides a thermal break for the slab edge. If the slab is up against the concrete footing, then slab-edge insulation – from what I’ve read – is very important. In the same way, depending on location and design goals, perimeter insulation is sometimes used instead of full under-slab insulation.


For my project, I’ve been looking at getting recycled rigid-foam insulation to save costs (and it’s good to recycle too). One source is Insulation Depot, but they want to ship by the tractor-trailer load (for really good prices) and that is just way more than we’ll need on our build. If I recall correctly from your Construction Journal, (a fun read, BTW), you may be needing a large quantity of the material. Perhaps a recycled source could be a significant savings. I’ve never worked with the company above and have no comments good or bad about their service, I just list them as a possible source.


If you go with under-slab insulation and if you have heavy loads on portions of the slab (masonry stove?) consider the compression strength of the rigid foam in that area (not to mention the strength of the slab). Apparently XPS comes in a variety of compression strengths. I think the basic stuff would suit in almost all residential cases, but if you have some special circumstances, it might be worth taking a look at this issue. In my case, the under-slab insulation (perimeter only) for our shop did raise some questions for foam-compression strength. Here is a source I found helpful for looking into this issue:


Happy building!


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By Jeff in Hartland, WI on 5/25/2010

Hi Steven:

I built a conventional poured foundation, with 1" foam panels (Owens-Corning FOAMULAR 150) covering the walls, and 1" foam panels (FOAMULAR 250) beneath the slab. I also used Spancrete panels on my garage floor, and put 2" Foamular 250 on top of the Spancrete panels and beneath a 4" slab.  I park three cars on that slab every day with no problems. 

The foam panels sit on top of the footings and gravel base. I didn't line the interior walls with foam panels prior to having the slab poured, but I probably should have. The only area where it's a problem is beneath two large egress windows. The floor gets pretty cold beneath those windows. Everyplace else, the floor stays at the normal 55-degree temperature all winter.

I used 1" foam beneath the slab, rather than 2" for two reasons:

1. Cost

2. The average soil temperature here is around the 55-degree mark. I figured I didn't need a whole lot of insulation to protect against 55-degree temperatures. 



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By David in Ocoee, FL on 11/10/2010


What you are saying makes sense to me. Not having insulation under your radiant-heat floor would allow your entire slab along with whatever is just below it to act as a thermal mass. The alternative would be to insulate and isolate your floor thermally, thus eliminating quite a bit of thermal mass. In such a case, your floor would heat up relatively quickly, but would also cool back down relatively quickly, because air has very little capacity to retain heat. With the uninsulated floor, there is more thermal mass to retain heat. The more heat the floor is able to retain, the longer it is going to stay relatively warm.


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