NetZero – Solar gain on a sunny day.

By Matt,

We incorporated passive house design elements in the building of our net-zero home. The house is positioned with its long axis in the East/West direction allowing us to capture the winter sun by putting the majority of the windows on the extensive south facing wall of the house.  The windows have excellent thermal properties, letting the sun in (high Solar Heat Gain Coefficient-SHGC) and keeping the cold out through Low U-values (thermal transmittance) and low air leakage.  The first and second floor roof overhangs are extended so that the high in the sky summer sun is blocked from the south facing windows. Finally, a large concrete slab ( 17 feet by 40 feet by 4 inches thick) on the main floor captures the solar radiation during the day and emits it back to the space at night. 

We used sketch-up to design the size and location of the windows and the size of the roof overhang. Sketch-up has a feature which allows you to model the summer and winter sun to see how much sunlight will enter the building at any time of the year. Here is the sketch-up model and the final house build. 

Sketchup and Final Build Showing sun angle on the same October 18 morning.

Here is when the home was half finished and we could check the solar transmittance into the living room through the large living room windows.

Sun almost hits the entire 4″ concrete slab to the back wall on a Jan 8 full sun day. Shown as a model and prior to insertion of Wasco Windows.

So when the house was finally built it was interesting to see how much solar gain vs thermal losses to the extensive south facing windows we would achieve. Much of October and parts of November were cloudy so we were not able to measure our solar gain, but then in late November we had a string of sunny days where we could turn off the ductless mini-splits and see if we could operate with no external heat source other than the sun. Using our Arduino temperature sensors in the main room and an isolated upstairs bedroom we could monitor the temperature over the three days. (See Figures Below) The outdoor temperature fluctuated between 20 and 40 degrees F over the same time period. The main room started at 68 degrees and than warmed after 9 AM as the sun entered the main living space heating that area up to 77 degrees by 3 PM. The main space than started to cool and returned to 68 degrees over the next 17 hours, where the main room again heated up to 77 degrees during the next sunny day. The upstairs bedroom fluctuated between 64 and 71 degrees with the peaks and valleys less pronounced and delayed an hour to three hours as the heat from the main floor worked its way to the upper spaces.

Out utility use dropped from 30-45 kWH with the mini-split on to approximately 20 kWH with the mini-split off. So with moderate outdoor temperatures the house can be completely heated with the sun (as long as it makes an appearance) and on colder days will reduce the use of the minisplit.

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