Extreme Efficiency: Page 2 of 2

Intermediate

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House framing photo
Double-stud wall framing and a gable roof with scissor trusses create ample space for blown-in cellulose insulation.
Interior home view.
The small house was built for about $100 per square foot, including the PV system that covers all of the home’s energy needs.
House vapor barrier installed photo
Meticulous installation of high-quality vapor barrier prevents air exchanges and moisture transfer.
Exterior photo of complete home, with PV array.
A 4.9 kW PV system provides 100% of the home’s energy needs, including space heating.
House framing photo
Interior home view.
House vapor barrier installed photo
Exterior photo of complete home, with PV array.

The bottom edge of the wall membrane was attached to the vapor barrier inside the crawlspace with a transitional piece. This had to be pre-planned. After the foundation was finished, but before the sill plate was attached, I laid a strip of shield down over the top of the foundation.  This gave me something to tie the wall membrane to on the outside, and something to attach the vapor barrier to on the inside. This means the air barrier is unbroken, from inside the crawlspace to outside the house and up and over the roof. Air leakage will transport moisture, so if you eliminate air leakage, you also eliminate potential structural water damage.

I control the humidity and ensure adequate fresh air in this extremely tight house with a Venmar heat recovery ventilator, which creates nice cross-flow in the house. The home has a 0.1 ACH50—compared to Passive House’s 0.6 ACH50 requirement. ACH50 measures air changes per hour at 50 pascals of pressure—the lower the number, the more airtight a house is.

The entire heating system is a single 6-foot, 240-volt standard electric baseboard heater mounted in the crawlspace. It cost only $80, including the thermostat. This setup works in my situation because the crawlspace lies inside the thermal envelope, which means that the crawlspace and living space are close to the same temperature. I set it to keep the crawlspace about 75°F, and that warms the underside of the first floor and keeps the home at 68°F. In the future, I am planning to install a ductless minisplit air-source heat pump, which will use even less electricity to do the same job.

Two windows in my home face south and have a solar heat gain coefficient of 0.65, so I get some free heating from passive solar (an estimated 20%). About 70% of my heating load is handled by internal gains (anything that gives off heat as a byproduct of doing something else, including refrigerators, TVs, computers, cooking appliances, lights, and people). The remaining 10% is provided by the baseboard heat.

Electricity is the home’s only energy source besides the passive solar. My grid-tied PV system with net billing offsets all of my energy needs. After doing some initial heat-loss calculations, I found that adding a PV array was less expensive than buying windows with lower U-factors. Finding the least-expensive way to arrive at a NZE home is not always the most energy-efficient way.

Comments (2)

Kevin Dickson's picture

This house gets my vote for most ingenious and inexpensive radiant floor heating system. I hope there is some waterproof insulation on the floor of the crawlspace, rather than only cellulose.
Upgrading the heating source to a minisplit does triple the efficiency, but with all this insulation, it still may take too long to pay off. The radiant floor would need to be abandoned if the air conditioning feature of the minisplit will be used.

At another website, I found that apparently he did install a minisplit for $2000, which is definitely below the average installed cost for these: http://www.dolphin-insulation.com/b...

SolarManJD@DCemail.com's picture

Wow... you must of stole my Idea... I have been building these homes for over 5yrs now and got the price down to $85.00per sq ft

Environmental Prototype Homes of Tomorrow Inc
EPHOT

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