Common Sense Home

Our home uses solar architecture to provide for some of our heating and cooling. This is typically referred to as passive heating and cooling (or natural conditioning), because it does not require mechanical heating or cooling. We have found this to be something of a misnomer. The house does the bulk of the work through the virtue of thermal mass, glazing and overhang placement, but human intervention is required to open and close the insulating shades and open and close windows for ventilation at appropriate times. Without someone thoughtfully being "non-passive" in the house, performance suffers significantly. Windows large enough to contribute significant solar gain suffer significant losses at night or during cloudy periods, and should be covered during this time. Natural cooling is best accomplished by flushing the house with cooler air overnight or early in the morning and closing the house during the heat of the day. The typical American mindset is to push a button or set a dial and let the mechanical systems do the work. A "passive" home means an active homeowner.

It may be extra work, but I think this is a valuable tool for raising awareness of our solar resource and our climate. I never realized exactly how far north the sun sets at summer solstice until I lived in a solar home. Years of tending the world's largest solar water heating plant taught me how to coax the most energy out of a 5000+ panel flat plate collector array, now I watch the sun with an eye to maximizing our home energy performance. Electric lights stay off when daylighting will due. Shades stay closed when the winter winds are bitter and the skies are dark. All the shades are wide open and the heat recovery ventilator is running at solar noon in the middle of February with single digit outside temperatures and a toasty inside temperature due to solar gain from the cloudless blue skies. If you are not home during the day to monitor conditions, or at the very least be willing to adjust things morning and night, this will limit the performance of the home.

High thermal mass is probably the most important feature of a naturally conditioned home. The more mass your home has, the less affected it will be by fluctuations in outside temperatures. Well-insulated exterior walls (or alternatively earth berming or earth sheltering) are also critical. We've utilized Insulated Concrete Forms for our exterior walls, which provide both (high thermal mass and excellent insulation). There is also got over a ton of mass sitting in the heart of the home in the form of the masonry stove, and tile with Durarock underlayment through most of the main floor. The basement slab has a sand bed underneath it about 12 inches deep with insulation below. Other alternatives include trombe walls, water walls, waterbeds, planter boxes or maybe an enormous Buddha statue.

While a spherical shape maximizes your surface area to internal volume ratio, they are not the easiest to build and maintain, and I'd have to imagine that they'd be a real challenge to furnish or put any cabinets into. I don't think anyone manufactures "off-the-shelf" items for curved walls, so you either need to be handy or independently wealthy. In lieu of living in a Buckyball, aim for a boxy rectangle and add visual/architectural interest through exterior/interior surface treatments. There's a reason New England is littered with salt box houses - they work. All those decorative protrusions and indents add surface area, increase heat loss and are likely increase the amount of waste generated in construction of the home. Any time you are using standard materials and can fit the dimensions of your project to the dimensions of these materials, that means less cutting and less waste. If you're hand building with more flexible or salvaged materials, this may be less (or more) or an issue.

The recommended amount of windows on the south wall of a passive solar home is 7% to 12% (glass on south wall as a percentage of square footage of home). We ended up closer to 12% and wish we were closer to 7%. Even with insulating cellular shades, when it's nasty cold and the sun has come out for a month, we cringe at the heat loss from the windows. When we've got a brightly sunny day, I'm sure we'd get enough gain to get the job done with less windows. Ideally your south facing windows should have a high SHGC (Solar Heat Gain Coefficient) as well as being low-E (low emittance). A high SHGC means more sun gets into the house, low-E means that the heat inside stays inside. In our experience, this is easier said than done. We could not find a source for high SHGC windows when we were building. Hopefully this has changed. I did find an article from a Canadian source that said that when faced with a choice between a high SHGC and no low-E coating and a low SHGC and a low-E coating that it was better to choose the window with the low-E coating in a cold climate because your reduced losses would more than make up for your reduced gains.

Along with the windows, you need to plan an overhang to prevent overheating in the summer. In our case, the deck is a long narrow strip that runs the entire length of the south side of the house. The roof extends over this deck area. The deck functions as an overhang for the basement windows to prevent excess solar gain and overheating while the roof does the same for the main floor. Online software to help you model overhang shading can be found at www.susdesign.com/overhang/index.html. Our deck is slightly deeper than would probably considered optimal for solar gain (just under four feet), as is the roof overhang above it (just over four feet with the gutter), but we felt this depth would provide a functional walkway. It also allows some room for seating against the house (probably a built in bench at some future date).

An ICF home gives 10 inch deep sills, which are nice for displaying things, sitting and so on, but consider the materials for the sill. Temps and moisture can be a problem. For our computer room windows the outside temp was 25°, the sill temp was 55° and inside was 75° (people and computers really heat up the room). Between the cellular shade and the window a microclimate is created, with humidity levels that are significantly higher than the balance of the home. This means you get condensation. If you've got wood windows or window sills, this means you run the risk of rot. I don't know how many times I've heard people say that they tried using insulating blinds but had to stop using them because they were damaging the window frame. I know vinyl is frowned upon by some in the "green scene", but it really limits the heat loss from the window frame and it won't rot when you get condensation. I don't care what kind of house you have, if you have any level of humidity in your home that's comfortable for human habitation and the house doesn't leak like a sieve, when you get daytime highs of zero F you will get condensation on your windows.

A passive solar house can be built in a city, but watch for trees or other obstructions blocking your solar gain. If you would also like the home to be earth sheltered, you really need to find a south sloping lot. It’s very challenging to force an earth berm or walkout, and it may not fit in with the neighborhood. You’ll also have to pay to bring in dirt, taking money away from your other needs. Envelope houses are another option, but I am uncomfortable with the potential pathogen breeding ground created in the envelope. It's altogether to easy for me to visualize mold, mildew and other unpleasant beasties breeding in that environment. I've only been in one, and it was older and rather cave-like. I suspect it could be done much better, and I know some people claim they are the preferred choice for homebuilding, but I am still a skeptic at this point.

Since we are around 15 miles from Lake Michigan, in summer we are sometimes able to use the lake breezes for natural cooling. Between four and five pm the winds generally switch and come off the lake. At this point we open up windows on the east and west end of the house and let the cooler air blow through. We placed windows at the east and west peaks of the interior space so there is a natural ventilation corridor running the length of the home. In retrospect, from an energy perspective, it probably costs more to heat this additional volume of interior space (there is a high ceiling running the length of the great room) than we save by using natural cooling, since we are in a heating dominated climate, but it does work. It also allowed us to add a loft above the closets in the boys' bedroom, which is a favorite spot in the house for the boys and their playmates and a point of architectural interest. The in floor radiant heating, passive solar and woodstove tends to keep the heat concentrated down where it's needed, so it all works reasonably well even if it's not as efficient as is possible.

Update: I've received my first series of questions on how the passive solar actually works, so I thought I'd share both the questions and my response.

Questions: Does the sun warm the floors & mass? Or is it that it doesn't raise it too many degrees to make it 'feel warm'? What does the mass do? With your reference to 'reflecting the solar heat deeper into the house. Is that more a temporary thing in that it heats the air deeper in the house? How are you 'harvesting' the sun's heat coming through your windows? How can solar carry your load when your floor is so cold you need slippers?

Answers:

The sun does warm the floors, but not to the extent that you think it would. When I started researching passive solar, what I was reading gave me the impression that the floors would be toasty and radiate long into the night, and that the direct gain (that is, where the sun shined directly on the floor) is what heated the house from that spot on the floor. We have not found this to be the case in our home. The semi-infinite slab model basically shows that heat penetrates a volume quite well within, say, an inch of the surface of that item. After that, heat penetration drops off exponentially. A little heat will trickle down through the mass, slowly warming the “bulk” of the house, but the most dramatic temperature change will happen near the surface. Therefore, by reflecting that energy deeper into the house, you impact more surfaces, heating them up and creating that slow seep into the mass of the house.

The air temperature within the house does fluctuate during the course of the day, as does the depth of sunlight penetration into the house. At winter solstice, the sun angle is such that we get direct gain to about 2/3 of the floor area. Now, the sunlight hits the window ledges and only creeps beyond them when it’s at an angle. So, if we’ve got a nice sunny day around solstice, the floors where the sunlight hits and the area around the masonry stove (in the basement and the main floor) are quite toasty. The bulk of the house air temp may be in the low to mid 70’s. In addition to the masonry stove and the passive solar, we have radiant heat provided by a Combi-Cor water heat, which is a 55 gallon propane water heater with a heat exchanger built into it that provides heat to the radiant in floor heating system. I usually keep this set at around 67 air temp during the day, 64 at night. On the day described above, even if it was quite cold (highs in the 20’s, lows around zero), the auxiliary heat would likely kick in until sometime in the middle of the night (possibly 4:30 am, when I start bringing the temp back up to 67).

The biggest influence on passive conditioning of the house is using thermal mass to hold your temperature where you want it, hot or cold. That’s why earth shelter houses can work so well if designed correctly. Maybe we would see more influence from solar gain if we had windows with a higher solar heat gain coefficient, but right now I’d have to say on a really sunny day I don’t think I’d want any more heat gain. Last winter, our btus/sqft/HDD from propane ran from between 2.3 to 1.7. The solar input was responsible for the difference in the two numbers, so you can see it does have a significant influence. It really varies wildly how much of the load the sun and the stove can carry, because our weather and our sunlight vary so much. By the way, just because a surface is uncomfortably cool on bare skin doesn’t mean it’s “cold”. 70 degrees is a pleasant day outside, but a 70 degree shower is not something I’d like to step into. Even a floor that’s in the 80’s can feel cool on your feet. The HVAC guy was ready to set our floor thermostat in the basement up to around 84, “because that’s where everybody sets them”. If we did that we’d roast. I usually keep it set at around 74 in winter, although I did turn it up to 78 when the temps were well below zero, just in case we had an extended power outage and no sun.

I suspect the house would hold above freezing indefinitely, even if all the heating went out, just because of the mass, earth sheltering and solar gain, but I wouldn’t want to live in it like that to find out. I know for sure the masonry stove is still warm over 24 hours after a burn.

The History of Solar Power - "Passive Solar - it's so easy a cave man can do it." Seriously, although it's not mentioned on this particular timeline, the Neanderthals, in some instances, did choose south facing caves with overhangs, which is an application of passive solar architecture. This timeline mentions Egyptians, Greeks, Romans and an assortment of others who have walked the solar path.

Passive Solar Design Guide from the US DOE - pdf file that gives a good overview of passive solar design principles.