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!