In this post, I’ll share the first of two concept houses showing examples of how to incorporate low-carbon building strategies. While somewhat more complicated and more expensive than code-minimum assemblies, they provide much better comfort and energy savings. They also allow for smaller heating and cooling systems without much increase in embodied carbon or changes to conventional practices.
Concrete has high embodied carbon, but a slab on grade with sealed concrete as the finished floor is usually the least expensive approach, and up to 50% of the carbon-polluting Portland cement can be replaced with Portland-reducing SCMs (supplementary cementitious materials), such as pozzolans.
With most slab-on-grade homes that have insulation under the slab, the insulation is located directly below the concrete, but this makes it hard to do a good job with the insulation, and there should be insulation below the turned-down footing anyway. A simple approach is to grade and compact crushed stone, then form the perimeter with foam, add a layer of foam, and top with compactible fill. I spec borate-treated EPS insulation for use below grade; recycled XPS is another option. Don’t use polyiso because it absorbs too much water, and don’t use new XPS because the energy savings will never make up for the climate impact of its manufacturing process compared to using EPS. Expanded glass aggregate is another option.
For the form edges, consider WarmFörm by ByggHouse. Compact the fill in 6-in. lifts, cover with a heavy-duty vapor retarder, add reinforcing steel where necessary, use a concrete mix with microfibers to reduce fine cracking, and allow the mix to damp-cure slowly to improve the strength—most concrete needs to be kept damp for a week to reach 90% of its design strength. In cold climates, consider adding a frost wing even if it’s not…
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