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Thinking Inside a Box

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This house is on the 2012 Solar & Green Building Tour. See Tour Details...

 Actually it is a high thermal mass, insulated box. 

In 1993 I moved into a passive solar, post and beam, wooden home.  I knew nothing about solar energy and for ten years I found out what I liked and what I didn’t.  That home had four feet of sand, the thermal mass, under the ground level floor; the sand had insulation under it and on all sides..  The sun would shine through the clearstory windows and heat the air.   A fan would move the hot air from the top of the house down through ductwork contained in the chimney and through ductwork in the sand and then through registers in the floor.  The sand would give back its heat at night with or without help from a fan.  Shortly after I moved in my cats had a pissing war with the resident cat – down a duct.  I no longer wanted to use the fans.  I also fear mold in the duct work.  The house did NOT smell fresh.  The cats also loved to climb the interior beams, the squirrels took up residency in the siding, the carpenter ants and bees co-habited with us and the woodpeckers were constantly knocking.  There had to be a better way to use the sun’s energy to heat a home.

In 2006 when I began my research, another way to use thermal mass began to surface.  In fact it rose up and hit me on the head.  Let the entire house be the thermal mass.  It made so much sense to just let the sun hit the surfaces in the house, - those surfaces would heat up and gently give back the heat at night.  Not only could the mass store heat for warmth in the winter, it could soak it up to keep you cooler in the summer.  It just was being itself.  Doing what it does.  I didn’t need to push the air around.  It was truly passive.  Also very important – insulate well, everywhere.  Here are the details of what I decided to do, from the bottom up:

Build slab on grade, so all the dirt within the foundation, the slab and the porcelain tile provide thermal mass.  Two inches of blue board went down on the sand before the 6 inch slab was poured.

Pour in place concrete walls and foundation, 14 inches thick, with 9 inches of concrete on the inside and five inches of solid cell foam on the outside.  The foam was held in the forms so that it was permanently and automatically attached to the concrete.

Install a 47 foot solarium to allow sun to enter,. thus heating the interior concrete walls (6 inches thick), the floor, the pillars and the concrete wall that contains the 8 cubic yards of soil for plants.  An awning prevents the summer sun from heating the surfaces.

Build a cool roof so that the ventilation is in the roof (two layers of decking with a two inch airspace between) and then blow 10 inches of foam insulation on the bottom of the inner decking.

Finally add another half inch of stucco to all interior walls to make them pretty and to provide a rougher surface to increase air turbulence and absorption of heat.

Img 1988

Here is how it looked when it was finished on the outside:

And on the inside (notice that all rooms are open to the solarium and sunshine).


We have lived here for almost three years and have been pleasantly surprised by many things.  The house is always comfortable (cool in the summer and warm in the winter), with wonderful air quality and perfect humidity (more on that in a later blog).  It is wonderfully quiet except when snow slips off the roof onto the solarium glass.  There are no creaking floors. The house never sways, even when winds are huffing and puffing.  Our greatest surprise was that the screened in porch was not necessary.  Even though we are on an island in a wetland, we have no mosquitoes, but that probably doesn’t have anything to do with the high thermal mass.

Nancy is a retired secondary teacher.  She built and lives in an active and passive solar, high thermal mass home.  She is an avid gardener and helps others to learn how to garden.

Comments on "Thinking Inside a Box"

  1. Dan Gibson's avatar Dan Gibson October 27, 2011 at 6:42 pm

    Hi Nancy,
    Would you mind sharing how much gas and wood you use to heat your home? Also, if you can let me know how much of the gas is used for heating water. I’d like to calculate your Home Heating Index (HHI), a very interesting and useful number that allows homeowners to get a sense for how energy effective their homes are for heating. Thanks, Dan

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  1. nancy white's avatar mike_member October 17, 2011 at 12:50 pm


    test rating