Shorts-Sub-Slab Assembly

What needs to be under a slab?  Well, it kind of depends.  Is the slab for a home or an accessory building?  Is there heat in the slab?  Is there a radon requirement?  This quick post is all about what I like to see included under a slab and the reasoning behind the sequencing.

Whether the slab is a frost protected shallow foundation, the floor system for a basement, or a stem-wall foundation system, many of the required elements for a sub-slab assembly will be the same.  The first item that is required will be well compacted soil.  Soils directly under a slab will often need to be excavated to accommodate foundation work, plumbing, foundation drains, sump pits, etc… these soils will need to be backfilled and compacted to a required grade.

Once the soil is at a required grade, the next step is to install the radon rock bed.  Four inches of clean 1/4-to-2-inch stone across the entire floor area.   This stone serves two purposes, the first will allow any soil gasses to easily accumulate in all the small air spaces between the rocks under the slab where it can be evacuated by the radon pipe, either passively or actively vented.  The second, the rock will act as a capillary break, keeping any potential moisture from moving up through the sub-slab insulation system by way of capillary action.

Next would be any insulation that is required (or desired) to be under the slab.  I usually like to see at least 2 inches of sub-slab insulation for my climate.  I would increase the insulation level if the slab were part of a heating system, such as hydronic in-floor heat.  There are several different insulations that can be used, EPS, XPS, Rigid mineral wool, and even closed cell spray foam are all options.

The next part of the assembly is where I often see a product out of order.  On top of the insulation is where the vapor/soil gas membrane is required to be located.  This is typically 6-mil or thicker polyethylene sheeting lapped and taped at each seam and sealed around any penetrations (such as plumbing waste and drain lines).  I usually see the poly under the insulation which is the wrong location.  The reason it needs to be above the insulation is to prevent the insulation from absorbing the water from the concrete during the pour.  The wet insulation will take a long time to dry, which would only happen upwards and into the home.  The poly could be eliminated if using a thick enough application of closed cell spray foam.

This frost protected slab assembly is out of order, the insulation needs to be installed under the sub-slab vapor/soil gas membrane.

What about tubing for an in-floor heating system?  The tubing can go either above or below the sub-slab vapor/soil gas retarder.  It depends on how the tubing is fastened.  When using staples, the tubing should be under the poly, this way the staples aren’t poking a bunch of holes in the membrane.  If the tubing is attached to a concrete reinforcing system, such as rebar, then the tubing will be above the poly.

This tubing is stapled to closed cell spray foam, there is no polyethylene sheet present.  Had this insulation been EPS, XPS or rigid mineral wool, the poly would be installed on top of the tubing.

Last in the order, add any concrete reinforcing products, rebar, mesh, or don’t, order fiber mesh as part of the concrete mix, and pour.  In my experience, the “pour” is when the work begins!

6 Replies to “Shorts-Sub-Slab Assembly”

    1. Hey Doug, I prefer to see expansion joints in most cases. The code minimum house did not have any cut, the concrete pour happened in very warm weather, and we ended up with a crack between two inside corners. Not a big deal, the crack was hairline, no lifting or separation. Still, something we had to look at until the floorcoverings were installed. I prefer not to have expansion joints in small areas where tile will be installed. We have to isolate the crack from the tile or create an expansion joint in the tile at that location just in case the crack should move a little, it may cause a crack in the tile if it’s not addressed properly. The Barndominium project had the entire 8700 square foot slab cut, it was cut when the concrete was still green. From what I understand about creating expansion joints, its best to cut before the concrete cures completely. I’ve never asked why, just what I was told.

  1. Seems like the concrete sawing companies want to cut the day after the pour. I think you get a better, more clean cut if you wait several days. The concrete has had a chance to harden a bit and the cut is better.

    1. The concrete guy I used to use happens to be my brother-in-law. He would cut either way, a day after or several weeks after. He said the thing to be aware of is you need a different blade for cutting green concrete than for cutting cured. I think the risk of waiting too long is to have the concrete crack before it’s cut.

  2. If you will be using multiple layers of 2” rigid insulation, can the vapor barrier go between the insulation layers to still allow the PEX tubing to be stapled to the top layer?

    1. Hi Jason,
      The reasoning behind the poly on top is to prevent the rigid insulation from absorbing moisture, then slowly releasing that moisture back into the living space over the coming years. Having it between is better than below, not as good as on top. You could install the Pex, then cover with the poly. On top of that, install any rebar. The risk is tearing holes in the poly. If it’s for vapor control, a few small tears and holes won’t hurt, if it’s for radon control, the holes need to be taped. Another option is to use a better poly, such as the 15 mil Stego Wrap. I did this wrong for years.

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