The round house is built and we have lived in it over the winter while finishing the interior. During this time the drywall, window and door trim was installed and the PerformWall interior surfaces were plastered. We relied primarily on solar heat with a propane stove for backup. Cooking and hot water are also provided by propane.

The Passive House Standard

In order to have something to measure performance, I have co-opted the passive house standards published by The Passive House Institute. This is advertised as the most stringent energy efficiency standard around, so it’s a natural to use it as a reference point. The basic standard is this:

Performance Characteristics

1. • Airtight building shell ≤ 0.6 ACH @ 50 pascal pressure, measured by blower-door test.
2. • Annual heat requirement ≤ 15 kWh/m²/year
3. • Primary Energy ≤ 120 kWh/m²/year

In addition, the following are recommendations, varying with climate:

1. • Window U-value ≤ 0.8 Watt/m²/K
2. • Ventilation system with heat recovery with ≥ 75% efficiency with low electric consumption @ 0.45 Wh/m³
3. • Thermal Bridge Free Construction ≤ 0.01 W/mK

I do not have a way to measure the pressure, so I’m going to ignore it. I do have a good air/heat exchanger, but I have yet to install or use it, so I’ll ignore that. I bought the windows because they had greater solar heat coefficient (.59) than most new windows. The U value is .50 and the visible light transmittance is .63 . I thought it was a good compromise for solar heat gain and U value, but they seem to disagree. Maybe thick curtains or movable insulation over the windows will help there. There are no thermal bridges in the walls other than windows and doors.

For the standard, the annual heat requirement is a calculated figure and includes heat and cooling energy. This is done by using R (or U) values of all the building materials and using a baseline of 68 deg. F.. I am more interested in actual performance, so I’ll use actual (paid for) energy use for this comparison.

How the round house performs

It comes down to the actual performance. How much in kWh/m²/year did we use in the last year? To start out with let’s figure out what the standard calls for in this house. The house is 1256 square feet, which translates to 166.7 square meters. Figuring this into the above standard we have;

We will assume that we can ignore heat supplied by sunlight and just look at heat from external (purchased) sources. In this case that is propane. 1 gallon of propane is equivalent to 27 kWh and we used 300 gallons of propane for cooking,hot water and backup heat. Primary heating was passive solar gain. I will estimate 150 gallons was used for heating and add the rest into the primary energy use figure.

I don’t have a firm electrical figure since I’m not monitoring power drawn from the batteries and panels. I’m estimating 3kWh/day because it takes roughly two hours of full sunlight to recharge the batteries each day. Given the inefficiencies involved in charging the batteries and variable sun conditions, I’m guessing that 1.5kWh are being put into the batteries for evening, when we use the most, and night use. Double that and we have a conservative estimate for daily electrical use. In the summer there is additional use for a small window air conditioner that draws 800 watts for a maximum of 6 hours a day (4.8kWh) and could run for 60 days in the summer. The total energy use rundown comes to this:

If we convert this to the original kWh/m²/yr units of the standard, we get

In spite of using almost twice the allowed energy for heating than the standard, the overall primary energy use is roughly 1/2 that of the standard. It shows what efficient appliances can achieve. Does this mean we meet the standard ? I don’t know. My guess would be that we’re in some sort of gray zone.

In order to do better in efficiently heating the house, there are two things we plan on doing:

1. Install insulating shades on the windows.

2. Get the installed,but unused radiant floor heat going.

We believe that with these two projects completed, propane use could be reduced to the point that it might meet the heating requirement figures.

If we only consider purchased energy in our calculations, we see a different picture. Since we are off-grid with solar electric, the electric components, including air conditioning drop off and we only have 8100 kWh/yr or 48.5 kWh/m²/yr. If we succeed in getting the propane use down, we could see 38 kWh/m²/yr or less. Is this cheating ? I don’t know. They don’t appear to include solar heat as a contributor to the annual heat requirement, so shouldn’t solar generated electricity get the same treatment ?