Once the underslab insulation was installed we proceeded with installing the foundation formwork, another process that was rather simple and quick. For the foundation of the building we will be using a grade beam system, similar to a slab on grade.

To create the grade beam, we used prefabricated, insulated formwork called: insulated concrete forms (ICFs). While the system costs are comparable to an ordinary wood-framed formwork, the thermal performance is significantly greater.

ICFs are made of clipped together insulated panels, in which the concrete is poured. The plastic clip system that holds the panels together also supports the rebar, holding it securely in place during installation.

The benefits of the prefabricated, clip together sections are the reduced cost and improvement of energy efficiency with fast installation.

The flowable fill discussed previously has created a quick, even pad on which a layer of high-density expanded polystyrene insulation is installed. The insulation sheets come in large sizes – 4’ x 16’ x 6” thick – making them quick and easy to install. Our total installation time with only two people involved was about an hour.

Typically a building sits right on top of a concrete foundation, without any separation from the ground. Imagine standing outside with a heavy wool coat on and no shoes — your coat is trying to keep your body warm, but you are losing a lot of heat through your feet. When you put boots on, you are separating your feet from the ground and providing a better insulation. Adding a layer of insulation on top of the flowable fill creates a barrier between the foundation and the ground. This separation provides a complete thermal and moisture break between the earth and the building’s concrete foundation, just like boots prevent your feet from getting wet and cold. Supplying the building with a highly insulated foundation allows the house to retain more heat than in a conventional construction practice where there is no separation between the house and ground.

As with any building, creating a solid foundation is important as it provides the base for the rest of the structure. In order to improve the thermal performance of the building, as well as reduce construction costs, a slab on grade foundation was designed for the prototype. Typical residential foundations consist of concrete foundation walls that are installed below the frost line on undisturbed soil or compacted gravel. An alternative to excavating and installing foundation walls below the frost line is to install a layer of rigid insulation horizontally under the entire building. This layer of insulation thermally isolates the building from the ground, as well as maintains the earth’s geo thermal heat under the area of the building, and thereby prevents frost heaves at the building’s foundation during the winter months.

To ensure the thermal performance of the foundation, we installed 6” of rigid EPS insulation under the entire building. The potential liability of installing this thickness of rigid insulation, is that the structure of the insulation can bridge over voids in the compacted layer beneath the building during the foundation installation, but then settle with the weight of the completed construction. To ensure that the substrate is completely smooth and compacted, a layer of concrete and sand called “flowable fill” was installed. This layer of highly aerated concrete is very easy to install and manipulate to create a level and fully compacted substrate. The result of these construction layers and systems is an extremely well insulated and quickly installed foundation.

Flowable fill is a mixture of coarse sand and cement that is heavily aerated to make it – you guessed it – flowable! It came out of the truck like a frothy milk shake and was easily placed inside shallow forms. When the flowable fill cures it is crumbly and easily raked or dug up which allows for fine tuning and leveling of the pad.

A layer of high-density insulation will be placed on top of the flowable fill, providing a complete thermal and moisture break between the earth and the building’s concrete footing. The combination of the flowable fill and the high-density insulation are fundamental details that provide the prototype house with a highly insulated foundation at an affordable cost.