The last couple of weeks, we’ve discussed the hidden cost of traditional building practices, specifically looking at unstainable concrete and steel. Both materials emit huge amounts of carbon dioxide during the production process. If we care about being green and sustainable, we must think about using less concrete and steel.
During the previous blog, we very briefly discussed the possibility of using less of these materials. In this week’s blog I want to dive into practical solutions to use less concrete and steel in our homes. It turns out that the topic is large enough to warrant a two-part blog series.
According to several studies, the concrete in our houses represents about 1/3 of the embodied carbon in our homes. The steel, aluminum, glass and other materials are the other 2/3. But the concrete is the largest single source of embodied carbon. So, let’s look at how much concrete is in a typical home, how much does it cost, and the function of that concrete.
Lots of Concrete in a home
First, let’s consider how much concrete is in a typical 3000 SF home. According to our blog on Unsustainable Concrete, there are about 40 cubic yards in a typical slab-on-grade home. For our purposes here, let’s assume that the slab is 1500 SF (does not account for the garage or driveway). Depending on the style of house, ranch, 2-story, etc., that number will be higher or lower. The slab alone accounts for about half of the concrete needed (about 20 cubic yards).
Concrete is expensive and not sustainable
So how much does that 20-cubic-yard slab of concrete cost? First there is the environmental impact of producing 20 cubic yards of concrete. The cement production process will release more than 14,000 pounds of CO2. Mixing up concrete and transporting it to the building site will release still more CO2. Second there is the dollars and cents cost of creating that slab. Expect that slab to cost anywhere from $15,000 to more than $20,000. There are a lot of variables here, for example, the distance from the concrete plant to the building site, the terrain at the building site (flat or mountainous), and whether a concrete pump truck is required.
Given these costs, its fair to ask, is a concrete slab even needed? The short answer, it depends.
Not a water barrier
Very often builders (and homeowners) will specify concrete slabs because they think that a concrete slab will keep water out of the house. But concrete is a porous material and water will absolutely penetrate a concrete slab. To keep water out, competent builders, architects, and engineers will pour the concrete over a bed of gravel. The builder will then embed a set of drainage pipes into the gravel to prevent water from ever reaching the concrete slab. Additionally, the builder will install a layer of plastic sheeting under the concrete to further separate the concrete from the water. Additionally, the builder will install rigid foam insulation under the concrete slab to help the floor stay warm.
Structurally Needed?
The other major reason for using a concrete slab is to provide structural support to house. While it is true that in many parts of the country this design approach is reasonable, it’s problematic here on the Colorado front range. The most significant geological hazard in Colorado is expansive or swelling soil. Soils containing Bentonite clay or montmorillonite underlie many of the most populated areas of the state. The clays can expand by as much as 20% in the presence of water and can exert forces up to 30,000 pounds per square foot – more than enough to destroy any structure. See this report from the Colorado Geological Survey for more details. As a result, engineers will typically design the home’s foundation as either spread footings or piers and allow the concrete slab to float up and down depending on soil moisture levels. So, if the slab is not supporting the house and it’s just floating there, what service is it performing?
Here are examples of homeowners who might want to opt for a concrete slab: Those who expect to have very heavy point loads (think large safe or home gym) in the house may want to have a concrete slab at least under the heavy load. Also, the garage probably does need a concrete slab due to the heavy weight of cars.
So, with the exception of the examples above, we’ve established that a concrete slab doesn’t really do much for us, and omitting the concrete becomes a real option for many homeowners. A structural engineer will need to perform an analysis taking into account the soil conditions and the anticipated use by the homeowner, before we can finalize that decision.
Alternatives to Concrete Slab
If we can eliminate the concrete, what are alternative solutions? We still need a flooring system of some sort. One solution is a so called “concreteless” or concrete free slab, see Fig 2 for more details. The design concrete free slab is similar, in that we start with bed of gravel with drainage pipes embedded. Then cover the gravel with rigid foam insulation and plastic sheeting to keep the floor dry and warm. Finally, we’ll install a double layer of interlocking tongue & groove OSB sheeting, which replaces the concrete. The floor will be sufficiently strong and stiff to support all of the normal household items and activity. The builder can then cover the OSB with whatever type of floor covering the homeowner desires; carpet, tile, hardwood, etc. With this flooring system, the homeowner saves the considerable financial investment of a concrete slab – $15,000 to $20,000. Plus, we save the environmental impact of all that CO2 emitted during concrete production.
Traditional concrete slabs have one other significant disadvantage; very often plumbing drains must be installed under the concrete. In the event that repairs are required the concrete must be broken out before the repair can be started, and then it must be replaced. That is potentially an expensive proposition.
Other places to reduce concrete
In next week’s blog, we’ll discuss options for using less concrete in the foundation walls.
At Delta Energy Solutions, we are always on the lookout for new ways of doing things. For more information about how your dream house could be designed that is more sustainable and perhaps even less expensive to build, please contact us.