Insulation & Installation of Pex Tubing in Concrete Slab

Lay a base of sand or fine gravel 2 to 4 feet thick across the area where you plan to pour the concrete slab. The depth depends on how deep frost goes in your part of the country. Spread the base material with a rake to level it. Use a plate compactor to pack the base firmly into position.

Unroll 6 mil polyethylene vapor material across the base material. This acts as a moisture barrier between the ground and the concrete slab. Extend the vapor material to the top of the foundation walls on all sides. Overlap the vapor material and seal joints with duct tape.

Set 1-inch extruded polystyrene sheets on top of the vapor barrier to act as insulation. Cut the sheets with a utility knife, as necessary. Butt edges together and tape with duct tape. Place insulation sheets along the inside of the foundation wall as well and hold them in place with duct tape.

Cut pieces of 1-by-6 inch lumber, using a circular saw, to create four sides of a form large enough to hold the manifold of the Pex tubing. Nail the form together. Drill holes in one side of the form large enough for the Pex tubing to pass through. The number of holes depends on your system, but you'll have at least two. Set the form box in the location specified by your heating contractor.

Cover the insulation with 6-by-6 inch reinforcement bar, or rebar, panels set on rebar stanchions that elevate the rebar above the insulation. The height of the stanchions depends on the thickness of the concrete slab you are pouring. A concrete slab is usually 4 to 6 inches thick, depending on the function of the floor. The rebar and Pex tubing need to be in the middle of the concrete. Wire the rebar panels together with wire ties.

Mark where to place the Pex tubing on the rebar with spray paint. Follow the scale drawing provided by the heating contractor. In most cases, the scale drawing corresponds with the 6-inch squares of the rebar, so it’s easy for you to mark.

Thread one end of the Pex tubing into one of the holes in the form you built. Loop the Pex tubing over the rebar following the paint markings from the previous step. Secure the Pex tubing to the rebar every 2 to 3 feet with wire ties. Thread the other end of the Pex tubing through the opening in the form.

Connect the two ends of the Pex tubing to the manifold. Set the manifold inside the form. Connect the manifold to the test kit that comes with the Pex tubing. Increase pressure in the tubing to 100 psi. Leave it at that setting for at least 30 minutes to ensure you don't have any leaks in the system. If leaks are evident, use a repair kit, following manufacturer’s directions.

Pour the concrete slab using an overhead pumper truck. Work the concrete into corners with a shovel. Smooth the concrete as it begins to dry with trowels and floats. Allow the concrete time to cure before walking on it. Once the concrete cures completely, the heating contractor can complete the connections of the Pex tubing and its manifold with the heat source.