Since my last blog post, the house has taken on recognizable form, and is in fact nearly closed in. The exterior walls are framed and the roof SIPs are on. In another week or two, the roof will be sealed, and the walls will be enclosed with windows installed, giving the building crew a warm, dry environment in which to work. The home is designed for minimal input to keep it warm; long before we’re living there, the crew will benefit.

The Living Building Challenge requires that we achieve net zero energy: Over the course of a year, we need to produce on site all the energy needed for comfortable living. Our house design tackles this directive in several ways.

First, the thermal envelope is radically airtight and thickly insulated. Walls, down to the footings, and roof are heavily insulated. Every single joint is caulked and taped against air leakage, and thermal bridging between interior and exterior has been studiously avoided.

Second, the building design incorporates passive heating and cooling elements. The long south-facing wall of windows plus concrete floors will absorb heat from the sun. That long south wall of glass is interrupted in three places with Trombe walls, thick concrete floor-to-ceiling slabs, designed with a special surface film to absorb the heat of the sun through the day, and release that heat to the interior late in the day and overnight. We thereby avoid overheating during the day, and prolong the usefulness of that passively-gained heat.

The 10′ x 10′ x 31′ tall entry tower provides passive ventilation, especially in spring and fall. Our expert wind dynamics consultant Devki Desai modeled for us the optimum positioning, footprint and height of the tower, and positioning and size of the windows. The stack effect and Venturi effect will draw air through the house, up through the tower and out the tower windows.

A ground source heat pump uses the earth as a heat source in winter and heat sink in summer to deliver active heating and cooling, via an hydronic radiant floor heating system and forced air cooling.

Last but not least, 60 photovoltaic panels on the south-facing roof of the barn will generate 16 kilowatts of electricity. The solar array will be tied to the public utility grid. On gray days and at night, we’ll likely draw electricity from the grid, while on sunny days, our solar panels will produce more electricity than we need, sending the excess to the grid. Over the course of a year, we’ll be well set to produce more electricity than we use, or at a minimum, break even on the electric bill.

HERS rating

HERS rating

In fact, the current measure of energy efficiency of our house, based on the construction drawings, yields a HERS score of –11, boding well for our aim for net zero energy.

Be sure to check our photo gallery, as we’ve posted some new new photos.

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