Insulating monolithic footings

shmealMay 29, 2012

DH & I are going to install hydronic heat in our new home as part of our sweat equity. We are laying down 2" foam board as insulation below the basement slab. Our builder has specified that we do not put the foam board on top of the monolithic footings. He wants us to put down foil to create a thermal break between the concrete and the stud wall. He was very clear that he did not want us to use foil bubble. He wants something that has no potential of being compressed.

I asked him where to get this foil insulation and he rattled off a couple of places in town. I've called around and they only carry bubble foil or nothing. My F-I-L, who designed and installed radiant heating before he retired, said I should just go to Sam's Club and get a box of their heavy duty foil and put that down.

This would simplify my life incredibly if that would really work to create the thermal break. Is this idea feasible? I don't want to have the inspector come out and laugh his head off and then say "NO" you can't pour the floor today.

If this is the wrong forum please let me know. I looked in the heating forum but they seem to be talking more about the actual systems there.

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foil as a thermal break? no way. Metal is a fantastic conductor of heat...why would you use it as a thermal break. You can use sill seal if really needed.
Heat loss through your stud plate and your floor will be minimal, or next to nothing with the 2" of XPS below grade. For the monolithic footing, I am assuming you mean a thickened slab under an interior load bearing wall? If so, you can continue the insulation under that too. You would just have to enlarge your trench area to continue the insulation down into it and back up again.
If you have an insulated slab and then a large piece that is uninsulated...that will work to wick the heat from the slab into the ground. Highly recommend you keep the insulated plane as continuous as possible. Your house has 6 sides...not 5 like most people think. All need to be addressed to be efficient. You wouldnt leave a big open, uninsulated space in your exterior wall would you?
some things to chew on.

    Bookmark   May 29, 2012 at 2:17PM
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Thank you for your answer. I didn't understand how foil would work as a thermal break but I seemed to be the only one involved in this discussion (between our builder, DH and FIL) that was concerned by the idea.

Your understanding of what our builder calls a monolithic footing is correct. Unfortunately they have already been poured so it will be impossible to insulate underneath them. Our builder wants them on undisturbed earth and he wants nothing above them that can be compressed. Do you have any suggestions as what we can do or use to minimize the heat loss through those slabs?

I understand the concept of the heat wicking into the ground through those large uninsulated slabs. I was concerned about the heat loss in those areas and brought it up at one of our building review meetings. That is when I was told to use the foil.

I've spent hours reading every forum and link I can find regarding hydronic heating systems. But I can't find an answer about how to deal with the heat loss that will occur through these large masses of concrete.

I could cover the sides of the footings with the XPS and just leave the top side open. Would that be worthwhile?

    Bookmark   May 30, 2012 at 12:54AM
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I am not sure what you mean by covering the footings with XPS. I assume they are already poured and possibly covered? You can always install XPS on the exterior of the footings and basement walls.
what is your climate zone? It sounds like your contractor has very little knowledge of thermaldynamics. I would be curious to know what you plan on doing for the rest of the shell as far as insulation, air sealing details, etc. Maybe you can catch some things before you get to them.

    Bookmark   May 30, 2012 at 6:20PM
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I'm sorry I'm not explaining this clearly and I appreciate your patience. The monolithic footings were poured at the same time as the exterior footings but they aren't covered yet.

Once the plumber has finished all his underground work the gravel will be spread and then we will lay the XPS on top of the gravel and staple the PEX to the XPS. His goal is for the XPS to be level with the top of the monolithic footings so they will all be covered equally by the 4" concrete pour for our basement floor.

It's the top side of the four interior footings that concerns me. We'll run the XPS up the side of these footings, right where the forms are showing in this picture.

We are in the foothills in Northern Utah, zone 3-4. One of the reasons we actually went with this builder was their focus on the building envelope. Also, our builder has hydronic heat in his own home which is why I'm so confused about the "just throw down some foil" comment he made at our meeting. i don't think they have very many clients who ask to DIY it though and that may be where the confusion is coming from because they just let the heating subs handle all the decisions and details.

    Bookmark   May 31, 2012 at 6:04PM
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