Figuring Out Foundations

There is an expression, “green from the ground up,” which is often used to describe a home that is thoroughly green. However, many homes have a substantial portion that is below the ground, and it is here that many buildings do not carry through on their green promise.

Basement foundations have typically used large amounts of concrete (8-12 inch thick walls that are 8 feet tall, with a 4-6 inch slab floor over the entire footprint of the building), and concrete is a material with a high amount of embodied energy and very high carbon output. In recent years, attempts have been made to make basement foundations more energy efficient, and these attempts have largely relied on foam insulation products. Foam also has very high embodied energy and a whole host of environmental issues, including greenhouse gas emissions, toxic production emissions, and all the negative aspects of petroleum exploration, extraction, production and disposal. Basement foundations made from the combination of these two materials are often thought to be a “necessary evil,” even by green builders.

With Canada’s Greenest Home (as with all our projects), it is our intention to greatly reduce, or potentially eliminate, all concrete and foam use. For most of our projects, we have chosen to build grade-based foundations, and have used materials like earthbags and hempcrete and used carpeting to provide the required structural and insulative values.

Canada’s Greenest Home has many ambitious goals, one of which is on-site treatment of human waste via a composting toilet system. Another is a high degree of solar thermal contribution to the building’s domestic hot water and space heating needs. Both of these elements require storage systems, and the composting toilets require storage below the level of the main floor toilet. Creating basement space for these and all the other mechanical systems makes sense in this scenario. But how to make an energy efficient basement with low embodied energy materials?

We have been considering three different strategies for creating this basement space:

  1. Autoclaved aerated concrete block

    Autoclaved Aerated Concrete blocks (AAC) – This material is also known as cementitious foam. Unlike typical concrete, quartz sand is the only aggregate and the cement is mixed with aluminum powder, causing a chemical reaction that creates hydrogen. These hydrogen bubbles aerate the mix, creating up to 80% void space in the mix. The mixes can be created to have specific densities for particular uses. The wet mix is then steam pressure hardened (autoclaved) for up to 12 hours, giving the mix its structural properties. The combination of structural and insulative properties is highly desirable, as most foundation materials require two separate elements to achieve thermal resistance and structural integrity. AAC blocks are bonded with a purpose made thin mortar, and holes in the blocks are fitted with rebar and filled with concrete for reinforcment. A concrete bond beam is also created at the top of the wall. See Safecrete.

  2. Durisol block

    Durisol Insulated Concrete forms – Most insulated concrete forms (ICFs) are made with foam, and therefore not within our desired range of options. Durisol ICFs, however, are made from cement bonded wood fiber cast into light-weight hollow core blocks that are stacked together, fitted with rebar and filled with concrete. The blocks can contain varying amounts of insulation to create walls of a desired R-value. See Durisol.

  3. First courses of an earthbag foundation

    Earthbag foundation – Rammed earth is a low-impact material, and using polypropylene tubes to act as forms for the rammed earth makes the system quite simple to use. Typically, site soils from the excavation can be used in the tubes, and in the best case scenario will not require any amendment. If the soil is less than ideal, a small amount of cement or slag can be used as a hardener/stabilizer. Because rammed earth has little insulation value, we would need to build a double-wythe wall and fill the middle with insulation.

Each of these three options has strengths and weaknesses. So how can we make the best decision?

First, we think about our goals and priorities. If we’re clear about these, then we know what level of performance we’re asking of each option. We then need to research to find out how each option measures up against each of our goals.

The decision-making process looks something like this:



* Figures come from the Inventory of Carbon and Energy (ICE) 2.0

Reaching a final decision is a matter of knowing the facts and being able to rank the importance of the criteria. For our foundation, earthbag is the clear winner if we’re only concerned about the lowest environmental impacts and costs. If we’re concerned about energy efficiency, the Durisol is easily the winner. If we want to set a precedent for using an innovative material that has a lot of potential, the AAC blocks win.

In the end, we’ve decided to build with the Durisol. We like that the factory is local, and that the producers use a lot of recycled material in their production process. We like the R-value, and the relatively simple installation process. What we don’t like is the fairly high use of concrete (lower than many other ICFs, but still much more than we usually use). However, we will use the highest possible recycled slag content in our concrete mix and offset some of the environmental impacts.

Earthbag was the leading contender for a long time while designing. However, we are going to be building an earthbag root cellar this summer as well, and felt that the students in the program will get their fill of earthbagging on that project, and would benefit from working with another system (Durisol). We still love earthbag… nothing costs less or has lower impacts, and it’s fun, too!

If AAC were produced locally, there’s a good chance it would be the leading contender. But the closest distributor is in Georgia, and they’ve moved their production facility to Mexico. It’s just too much transportation energy, and the fact that we’d need to add additional insulation sealed the decision.

At Endeavour, we think it’s important to fully explore these questions and options. And equally important to be open about the process, to carefully try and quantify the decision and to reflect honestly at the end of the project about how we feel our goals were or were not met.

We hope you’ll continue to follow the project as we make more decisions! We’ll be starting a dedicated site/blog for the project soon…