The house faces nearly due south (about 15° east of due south), with most windows and sliders doors on the south side to capture energy from the sun when it is low in the sky in winter. Getting the right coatings on the windows was an important and nontrivial issue which is discussed on the Passive Solar Design page. During summer, with the sun high in the sky, overhangs keep the interior totally shaded. The goal of passive solar heating is to slowly heat the large thermal mass of the concrete floors (downstairs), tile floors (upstairs), and the bale walls, then let that captured energy keep the house warm overnight.

We do have supplemental heat for when we get several continuous cold, rainy days with no solar input: a natural-gas fireplace downstairs and two electric baseboard heaters upstairs.

To test how well the passive-solar heating works, we conveniently were out of town for 3 weeks one year in late November and early December. Other than a pet sitter coming in once a day, the house was just sitting there doing it's thing. The supplemental heat was turned off downstairs. Upstairs, baseboard heater thermostats were left at 65°. Three data loggers – one upstairs, one downstairs, and one outdoors – recorded the temperature every 30 minutes for 23 days. The daily electric production of the PV panels is used as a proxy for the amount of solar heating.

It was mostly sunny for the 23 days of data. During a sunny stretch, the indoor temperatures more-or-less tracks the daily outdoor high temperature. For example, Nov 23-26 were 4 days of steadily increasing temperatures, and the indoor temps similarly increased day by day over this period. The downstairs temperature oscillates 4 to 5 degrees a day on sunny days, peaking at 1:00 p.m. The upstairs temperature has a larger oscillation, more like 7 degrees, since it has heat loss through the roof as well as less wall insulation than the bales provide downstairs. The later peak, at 3:00 p.m., is likely due to a small west window getting reasonably good afternoon sun.

Temperatures were running above seasonal norms for the first 12 days, but the 3 days marked as "Average," Dec 2-4, were sunny days at very close to seasonal temperature norms of mid-30s to mid-50s. During these average winter conditions, the downstairs temperature oscillated 65-70 degrees while upstairs was 67-74. Had we been home, we probably would have turned on the downstairs gas fireplace for an hour or two in the morning, but basically the house is maintaining a comfortable temperature range from solar heating alone in normal conditions. Because of the lack of drafts, with the house tightly sealed, it's quite comfortable at 68 or 69 degrees.

Dec 8-9 were a more stringent test – below freezing mornings and with highs only in the low 40s – that followed a cloudy day on Dec 7 with no solar heating. With the outdoor temperature at 28, the downstairs dropped to 61 - but bounced back to 67 on both days when the sun came out. Upstairs, it looks like the baseboard heater came on - notice how the minima are rounded rather than the much sharper minima earlier in the record when the baseboard heater was not coming on - but the upstairs recovered nicely to 73 on both days. So even on days about as cold as it gets here, only a small amount of supplement heat is needed as long as it's sunny.

But the sun doesn't always shine. The last three days of the record were totally cloudy and rainy. Even though the outdoor temperature wasn't too bad - averaging about 50 - downstairs settled to a fairly steady 64 degrees. Upstairs had more variation but averaged about 67. It probably would have gone lower without the baseboard heater kicking in a couple of times. We do need the supplemental heat when we get extended cloudy/rainy weather – but vastly less heating need than a conventional house.

The thermal performance of the house in summer depends on being able to open windows, sliders, and two upstairs skylights at night to cool the house. Most of the time this works. Summer temperatures typically range from the low 60s to highs in the mid 90s. With no sunlight entering and with the thick bale walls, downstairs temperatures typically peak at 74 or 75, then drop back to 70 overnight with windows open. The upstairs gets warmer during the day, typically 77-78, but also cools nicely overnight. With a few fans to keep air moving it is extremely comfortable. Summer humidity here is extremely low, often 10% on summer days, and that helps with the comfort factor.

However, there are usually a couple of hot spells lasting 5 or 6 days each summer when the high temperatures reach 105-110 and, more significantly, it doesn't cool off at night. The outdoor temperature can still be 85 at 11 at night, after when we would normally have opened up, and it may only reach a "low" of only 75 or so at dawn. The higher midday temperatures wouldn't be a problem if we were able to cool off at night, but our cooling fails during these hot spells. Even so, the downstairs temperature settles in at 78 to 80. With fans and the low humidity, that is not a problem. The upstairs, however, can reach 85-87 during these spells. With fans, one of us finds that annoying but not terrible and continues to work upstairs. The other simply stays downstairs for a few days.

There are two other situations where the upstairs can be problematic in the summer. One is if we're away and the upstairs doesn't get opened up overnight. We return to a toasty upstairs, but it takes only one night to cool off. The second is California's notorious summer wildfires. It doesn't happen every summer, but there have been a few summers when we couldn't open the house at night because of the smoke.

Supplemental heat in winter means that the house will never get unpleasantly cold even during a long, cold, rainy period when we have no solar heat. But we don't have any supplemental cooling in summer, so the occasional spells with no nighttime cooling are a bit more challenging. Even so, overall we do quite well with no furnace and no air conditioner in a climate where yearly temperatures range from 25 to 110.