Construction Design-Concreteless Slab on Grade-Walls

The concreteless slab on grade home was in the planning stage for over a year.  One of the first conversations I had with the owners was they wanted a home that was comfortable, healthy and had low operating costs.  Extra insulation and good air sealing is how we planned on accomplishing these goals.

The framing for the home started much like a traditional build.  2 x 6 walls on 16 inch centers.  The biggest difference in the framing is we decided to build directly off the ICF (insulated concrete form) foundation walls where it’s more common to start framing off a slab or floor system.  There is a double bottom plate, which helps with the final height of the interior walls.  The floor system and insulation over the ICF foundation raised the finished floor height, requiring a second bottom plate to achieve an eight foot finished ceiling.

Click here to see how this home’s floor system was constructed.

We used Zip sheeting for both the air control layer and structural sheer of the framing. All seams were taped per the manufacturer’s instructions. None of the sheeting was cut for the installation of the windows until after the first blower door test was completed.

After wall framing was completed, roof trusses were set and the roof, soffit and sub-facia completed.

The wall framing was set back from the ICF form by 2 ½ inches. A decision was made during the planning stage to flush the ICF form with the continuous exterior insulation. The pic shows this detail. A bent piece of metal flashing was installed to cover the foam ICF form. This flashing extends up the Zip sheeting a few inches and is taped. The metal flashing also extends down the ICF and becomes the ground breaker/foam protection.

Windows openings were cut into the Zip sheeting after the initial blower door test. The rough framing for the windows were made one inch larger than the required window rough opening. This was to allow for the installation of a window buck. A window buck pushes the window frame out to the siding plane. Anytime you are installing continuous exterior insulation, a decision needs to be made about where the windows will be installed. In our case, they could have been installed directly to the Zip sheeting. They would have been inset into the siding by nearly three inches, making them an “innie window”. This would have complicated the flashing detail so we choose to use a window buck.

ThermalBuck is a foam window buck with a hard plastic covering, similar to a truck bedliner. The product has a slight pitch to allow water to move outward if there was ever a water leak around the window. The ThermalBuck is sealed and fastened to the rough framing and the window is sealed to the ThermalBuck. Windows are fastened through the buck and into the framing using long screw.

After the windows were installed, we then could install the continuous exterior insulation. Rockwool’s Comfortboard 80 is the product we used. At two inches thick, R-8 does not have enough R-value to assure the interior side of the sheeting will be warm enough not to reach a dew point in my climate zone, so an interior class I or II vapor control layer will be needed. The insulation was temporarily secured to the sheeting using a few cap nails. It is fully secured when the 1 x 4 pine strapping, which is supplying a vented rain screen for the wall assembly, is installed over the Comfortboard insulation.

We chose Comfortboard because of some of its properties. It is vapor open, with a perm rating of 31 perm. It is also hydrophobic, meaning it doesn’t care if its wet or dry. Water will bead up and run off the surface of the insulation. Lastly, it is fire resistant. A very unique insulating material.

The interior cavity insulation is R-21 fiberglass insulation. Total gross wall insulation is nearing R-30. Not only is the home easy to heat, we are finding the interior very quiet.

The final two parts of the wall assembly include the vapor control layer, which is Pro Clima’s Intello, a smart vapor retarder and 1/2’” drywall. Most homes in my area use polyethylene sheeting as the vapor control layer, which has a perm rating of .06, or vapor closed. I prefer allowing the wall assembly to dry both inward and outward. Intello has a variable perm rating, from .13 perm, which is a class I vapor retarder to 13 perm, which is a class III vapor retarder. Should a wall cavity become wet during the summer months, when vapor drive is inward, the Intello would “open” to allow for inward drying.

With so many new to us products and the different building techniques required, this home has been both fun and challenging to construct. When completed, I’m expecting an efficient, durable, comfortable and healthy home which should last for generations.