One of the benefits of living in the Wild West is our abundant sunshine. An advantage for home builders in Denver is that Colorado averages over 300 sunny days per year; Arizona and New Mexico have even more. My home here in Colorado is located on the north facing side of a mountain. We have some pretty sweet views of more mountains and the Continental Divide to the west and north of us, but not much potential for passive solar heating. It is not uncommon to have snow in my yard well into May. My neighbors across the valley, however, bask on the Sunny side of the mountain. The snow in their yards often melts only a few days after it falls. When I designed and built our small cabin in New Mexico, one of my primary design considerations was to provide some passive solar heating.
What is Passive Solar?
Passive solar is pretty simple: The solar part is obvious…it refers to the Sun. The passive part means it works without any mechanical devices, added energy inputs or efforts from the occupants. It basically heats itself by smart design. Anyone who has ever left his car sitting in the hot summer sun knows all too well that windows allow sunshine to enter and warm the inside. In a vehicle, these temperatures can reach 150° F or more very quickly. Concentrated versions of this same phenomenon allow solar ovens to bake and cook foods at 350° F to 400° F. When it comes to using this system to heat a home some considerations need to be addressed.
- Site location: For starters, you’re going to need some sun. Having a home located deep in a dense pine forest is not going to work to well. Similarly with deep canyons and hillsides, the quantity of sun that "lands" on the property will have a direct effect as to how effectively it can be harnessed. In urban areas, large buildings may provide shade that can restrict or prevent good passive solar designs.
- Orientation: Another design constraint is to point your solar collectors (a.k.a. "windows") towards the Sun. In the northern hemisphere this direction is south. A home with a large glazed southern exposure and minimal northern glazing is the basis for practical design.
- Thermal Mass: All of this sun energy needs to be stored or moderated; this is best done with heat-and energy-absorbing components.
- Seasonal Shading: This is the trickiest part. You want the Sun to work for you in the cool months when if provides needed warmth, but not in summer when you do not need it.
When solar designs were first being developed, there was a common idea that the more windows you had the better off you were. This design led to homes that would get smoking hot when the Sun was shining but then get wickedly cool once the Sun went down. Others would just be baking hot all summer long. These homes lacked the critical thermal mass component to moderate day to night swings and often lacked the seasonal shading part to prevent overheating in the summer.
In a passive solar design, the thermal mass element is what keeps things from swinging widely from hot to cold, during a typical day and night cycle. An example of how it works: Thermal mass is like cooking a very large pot of water. When you start, the water is cold and the stove adds heat. The pot of water "collects" the heat until it reaches a boil, and then when the heat is turned off the pot of water will remain hot for a long time. The thermal mass acts a bit like a mechanical flywheel to even out the highs and lows.
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°. 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° and 41° respectively. 30° to 40° 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.
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° of latitude. During the peak of the summer, the Sun is about 42°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° 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. In this late October photo you can see the Sun’s shadow of the eave up at the tops of the picture window and sliding door locations. (Author's Note: This photo shows the cabin in its more rustic early build state. Keep in mind that this is a "pay as you go" remote project for us.)
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 its what keeps the Sun burning away.