As part of our building project, we have designed a frost-protected stub foundation. This would be built conventionally out of concrete, but we have decided to make it out of fieldstone and mortar. Although we made this decision in order to source our materials from as close to the building site as possible (check out our 10-mile building challenge), it also emits five times less carbon than concrete. Let’s break it down and see why.
Concrete Foundation Emissions
A typical foundation in our area must go down at least 4 ft to avoid frost heave and provide a solid base for a building. We are using a special design that allows for a shallow foundation that is frost protected (more on this in a future post). Accordingly our foundation is only 2 ft 4 in deep. The perimeter of the foundation measures 116 ft and would be 1 ft thick if built with concrete. This would require 271 square feet of concrete, or almost exactly 10 yards.
Concrete is made up of 10-15 percent Portland cement, 15-20 percent water, and the rest is aggregate, by volume. Each pound of Portland emits 0.9 lb carbon dioxide into the atmosphere. Those emissions are from the energy used to produce the cement (ca. 40 percent) and calcification over the lifetime of the material (ca. 60 percent; source). Each yard of concrete accounts for 400 lb of carbon emissions.
If we had poured our foundation out of concrete, we would have been emitting 4,000 lb of carbon. A full-depth foundation would have been double this amount.
Stone Foundation Emissions
Stone foundations were standard in this area before the easy availability of concrete. Many barns and old homes, including our own, stand on stone foundations. We have designed a hybrid foundation system using the old-style stone foundation with the newly developed frost-protected stub foundation to reduce the amount of materials needed. Unlike the 1-ft-wide concrete version, we must build a stone foundation a little wider: 1 ft 4 in. This results in a total volume of 13.4 yards compared to the 10 of concrete.
Stone foundations are laid dry and then mortar is added to fill the air space within the wall. A typical estimate is that 20 to 30 percent of the wall volume is mortar. We’ll use 25 percent for this calculation. Below-grade foundation stonework requires type-M mortar by Wisconsin code. This is made of a mix of sand, Portland, and lime putty in a 12:1:1.5 ratio (if powdered hydrated lime were used instead of putty, it would be 12:1:1). Hydrated lime (used to make the putty) emits 0.94 lb carbon per pound (source), compared to Portland’s 0.9 lb carbon per pound. Since the mortar makes up a quarter of the foundation volume of 13.4 yards (3.4 yards), and Portland and lime are one-fourteenth of that volume (0.24 yards or 6.5 sq ft each), we can calculate a total use of 611 lb of Portland (at 94 lb/sq ft) and 260 lb (at 40 lb/sq ft) of lime.
Our total emissions for the lime and Portland in the mortar for our stone foundation works out to 794 lb, or about a fifth of that of concrete.