Wednesday, April 25, 2018

Building a net-zero house


A couple of days ago, I saw my brother grinning broadly and occasionally giggling. The occasion for this mirth was a recent encounter with his meter-reader for the local utility company�Duke Energy. The news was that last month his photo-voltaic cells had generated roughly 300 kw of electrical power more than he had consumed. While much of this joy is due to the fact that he was billed the $9.86 minimum for being connected to the grid while most of his neighbors were getting $350 bills for homes of similar sizes, the giggling was probably due to the idea that he is "sticking it to the man."

Tony heard this story yesterday and insisted I explain what was involved in producing such a house. So this morning I asked and while his story was hardly complete, it highlighted the parts he thought important. So in no particular order, here they are.

1) His property in the country was large enough so the house's site orientation was pretty much anything he wanted. So the house's ridge line runs east-west, the roof pitch was selected to maximize solar gain, and the overhangs are large enough so the south wall is completely shaded between the equinoxes.

2) The holding tank for the solar hot water heater was located close to the showers so the water used to get hot water on start-up is minimized.

3) The air handler for the air conditioning system was located smack dab in the middle of the house which does wonders for efficiently distributing the cool air.

4) Room for the insulation was designed in from the beginning. The exterior walls were framed with 2 x 6 studs and the roof trusses were scissor-type which made for higher ceilings while provided plenty of space for 16" of insulation.

5) Because excessive heat was the primary problem in Florida, aluminized Mylar was chosen for the moisture barrier.

6) The steel roof was put on sleepers which allowed the underside of the roof to stay cooler and dry.

7) The original air conditioning compressor was a SEER 13, which in 1992 was the most efficient unit one could buy. It was replaced in 2010 with a SEER 18.

And so on. They key to understanding this house is that VERY important decisions that significantly affected long-term performance were made in the planning stage. None of the features mentioned above cost much to implement but have saved thousands in energy costs over the years. As I have said since the 1980s, pollution is a function of design.

Then in 2008, the decision was made that photo-voltaic cells were now inexpensive enough so adding them to the mix should be cost effective. This was pretty easy to do because the roof was already pointed in the right direction. PV cells were still pretty spendy in 2008 so only 5 kw were purchased. By 2011, the costs had dropped significantly so another 5 kw were added which leads to the monthly grinning when the meter is read.

So while the costs of building a solar home from scratch are not much more than conventional construction, retrofitting an existing house is difficult, time-consuming, AND quite expensive�even though the price for PV cells is now lower than $1 per watt.

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