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In my cabin this thermal mass is built into the tile floor. The cement backer board beneath the tile adds about 800 pounds of "mass", the tile and adhesives, grout etc. add another 600 pounds or so. Other things in the cabin add a bit as well, like the granite countertop, the cast iron of the wood stove and the wall tile behind the wood stove. This tile rests over an R-30 insulated floor, which also "works" by keeping the storage within the thermal envelope. In many ideal designs, poured concrete floors or slabs, trombe walls, water walls and other massive internal components can make my tile floor's thermal mass seem wimpy.

Earthships are a type of passive solar structure than use tons and tons of soil and concrete as their thermal mass and are common in the area of my cabin. (Stay tuned for a future article on this type of building.) Before the temperature in my cabin can rise too quickly, a lot of the Sun's energy is "used up" warming the floor rather than just the air inside. When the Sun goes down, the warmth and energy stored in the floor then radiates back out keeping the room warm.

Here's a measurement of how effectively this system can work: We took a trip to our cabin over the Thanksgiving break. It was a sunny but cool day for our trip. When we arrived an hour or so after dark, I took surface temperature readings both inside and out. The deck outdoors was sitting at a chilly 26°, while inside the cabin the tile floor and walls were reading 65 degrees. I repeated this same experiment on our New Years' trip. On that trip it was not quite as sunny and much colder; those numbers were 4 degrees and 41 degrees respectively. 30 degrees to 40 degrees warmer seems to be a pretty average spread (inside vs. outside) when the sun is shining. This added warmth eventually dissipates during the night. Late at night and during very cloudy or snowy weather, we fire up the small wood stove to stay warm and cozy. In late fall and early spring we can often get by with out needing to make a fire at all.

Seasonal Shading

The key to successful seasonal shading has to do with the relationship of the Sun's angle above the horizon and the roof overhang above the windows. This concept was understood and utilized in prehistoric times. Ancient Anasazi people ago used the same sun angle/overhang principle. They built entire clusters of dwellings in these "winter sun pockets". Mesa Verde National Park's "Cliff Palace" is a fine example.

My cabin in northern New Mexico sits at about 36 degrees of latitude. During the peak of the summer, the Sun is about 42 degrees above the southern horizon. During this time of the year the roof's overhang prevents the midday sun's rays from entering the windows. This is demonstrated in the following photo. Notice that the shadow of the eave can be seen near the bottom of the picture window/sliding doors.

During the winter months, the Sun's angle is only about 12 degrees above the horizon. (This seasonal difference has to do with the Earth's tilt on its axis and our annual trip around the sun.) The low solar angle allows the sunshine to enter the cabin and shine on the tile floor.

We now have a number of cabin winter visits under our belts, and I am pleased to say that my design seems to be working quite well. Before we got the inside of the cabin fully insulated we would need to stoke the wood stove quite a bit to stay comfortable. Now, that job is mostly done by our nearest and dearest star...the Sun. In a way you could say my cabin in heated by nuclear fusion ... after all, it's what keeps the sun burning away.