The “Bathtub” Basement

Most of the work recently has shifted to the basement as the contractor prepares to pour the basement slab.  Like everything else in this home, a lot of detail has gone into the design.  The first photo shows the basement prepared for insulation.  Under the gravel is an extensive network of drain piping to route any sub-slab water into the sump and also capture any radon gas so the external exhaust fan can remove it.  The gravel itself helps the basement drain and prevents moisture from leaching up into the concrete slab from the ground.

The visible pipes are for the various drainage, septic, and plumbing systems in the basement. Once they were placed, the contractor finished filling the basement with gravel and then our insulation contractor, Green Cocoon, came in to install a layer of closed-cell spray foam.  In our system the foam serves several functions: it is an air barrier between the house and the ground, so radon is trapped and drawn into the exhaust system instead of entering the home; it is a water barrier, helping prevent any underground water from getting into the basement or wicking into the slab; and it insulates the concrete slab, making the house more efficient and reducing the heating load.

The technique we’ve used here is called a “bathtub” because the slab is surrounded by foam on the bottom and sides.  After the slab is poured we’ll extend the spray foam up the foundation walls to the rim, creating a tight, dry, and well-insulated basement.

This particular closed-cell foam, from Demilec, is an attractive material for this project: besides having a high R-value (which means it insulates well), it uses a very environmentally-friendly blowing agent, and the material itself is GreenGuard Gold certified, meaning it has very low levels of volatile organic compounds (VOC) and is approved for use in schools and healthcare facilities.  We are committed to using only low-VOC products (adhesives, flooring, paints, etc.) to provide the best possible indoor air quality in this home.