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Community and Q&A

Detailing an Insulated Slab for In-Floor Hydronic Heat

mark_h84 | Posted in Green Building Techniques on

I live in Minnesota in climate zone 6.  I’m building a two story with a slab on grade foundation (no basement).  I plan on doing in-floor hydronic heat.  I’ll have some bearing points coming down from upstairs, so I’ll need some interior foundation walls.  Do I just not put insulation under the slab where the foundation wall is and pour the slab thicker there?  How would you detail that?  Should it be thermally broken from the rest of the slab?  This is new territory for me, so any advice would be great!

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  1. brandons | | #1

    I think your situation requires a structural engineer. What's the square footage and footprint you are planning? What style of roof? The project could potentially clear span with joists and beams for the second floor, the same for the roof.

    It's possible to insulate under footings and have them formed similar to the slab edges. Engineer should run the load calculations and spec what you need. I'm assuming you want to monopour, meaning the entire slab/footings are poured at once.

  2. Expert Member
    Michael Maines | | #2

    Mark, there are various ways to do it, depending on several factors including your performance goals.

    For Zone 6 Pretty Good Houses on slabs, I like to run at least an inch or two of foam over the footing so the columns (usually Lally columns) are the only things puncturing the thermal control layer.

    With Passive Houses we fully wrap the footings with foam so there is no thermal bridging, because every Btu matters when trying to meet that standard in a cold climate.

    If you don't have too many point loads and you're ok with spending a few extra dollars a year for energy, you could just skip the insulation at the footing. But I find that it takes little extra effort to run insulation over the footing. I'll attach the detail I like to use.

  3. BirchwoodBill | | #3

    Are you planning on using something like Logic heat sheet in the slab for PEX tubing. Something like R16.

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