The 10kW solar array provides enough electricity for the demands of the house. (Photos: Wes Golomb)
Wes Golomb and Bob Irving
This is the first in a series of articles based on Wes Golomb’s newly published book and video series. Warm and Cool Homes, Building a Comfy, Healthy, Net-Zero Home You’ll Want to Live in Forever. The book is a look at five high performance homes, four of them net-zero, and the techniques and technologies used to build them.
A net-zero home is built to be airtight and efficient with enough electrical generating power to supply the home’s energy needs. This requires an overall strategy for our society’s transition from a fossil fuel to a sustainable energy economy. I call it the three-legged-stool approach.
The first leg of the stool is efficiency. We use as little energy as absolutely needed to accomplish a task. A net-zero home achieves efficiency through a series of air, moisture, and thermal barriers built to keep heat and moisture where we want them and to eliminate infiltration of outside air.
The joints of sheathing on the walls were air-sealed with 3M tape.
The second leg is electrification. We use only electricity instead of fossil fuels. This approach generally saves at least a third of the energy needed. High efficiency homes replace the combustion of fossil fuels with heat pumps, which use a technology similar to a refrigerator to heat or cool efficiently, as needed. Gas cook stoves can be replaced by equally fast induction stoves.
The third leg of the stool is to produce all the electricity we need to with sustainable energy. Thanks to their precipitous drop in costs, this means we are able to use solar photovoltaics (PVs) to supply our electricity.
Let’s look at the general strategies used by net-zero home builders, and how they were applied to Mike Marion’s home in Newmarket, New Hampshire.
We start with efficiency. Designers of high-efficiency homes take a holistic view of the project. This means dividing the job into tasks.
The first task is to define the building “envelope” which includes the surfaces that provided the air, moisture, and pressure boundaries between conditioned inside space, and the unconditioned outside. The keys to a highly efficient home are air-sealing, effective insulation, and controlled ventilation.
Minimizing infiltration is critical so it is important to seal every penetration into the house including the sill. To do this, we use a blower door test to measure how air-tight a house is, and we test prior to insulating so problems can be fixed. (THIS IS CRITICAL!!!)
To do the blower door test, we put a large fan into a door, air-sealing it to the frame. All the doors and windows are closed, and the fan is turned on blowing air out of the house. This lowers the air pressure inside, relative to the outside pressure. When the house has been depressurized to -50 Pascals (Pascals are a measure of air pressure), the amount of air being pulled through the fan is measured. From this information, the number of air changes per hour (ACH50) is determined.
Mike Marion’s home tested at 0.6 ACH50. This is less than a tenth of the NH standard of 7 ACH50. It is good for a net zero home, as they generally get less than 1 ACH50.
The basement walls of the Marion’s home were poured and then rigid foam insulation was installed on the inside to minimize moisture infiltration.
Double wall framing allows the space for the 10 inches of the blown-in cellulose insulation.
Above grade, to avoid the problem of thermal bridging (heat loss through the studs and framing.), a second independent wall was built to allow ten inches of cellulose to be blown in. No wood contacts both inside and outside walls, so heat cannot be conducted from warm to cold through wood. The seams between sheathing are taped to air-seal any leaks and form the air barrier. A house wrap was applied to the outside, to help move water away from the siding. LP Smartside claps were installed over the rain screen.
The roof and floors of the Marion home are trussed. Manufactured for strength, trusses avoid the need for load-bearing interior walls, allowing more flexibility in designing the floor plan. The roof of the attic is insulated to R-62 with cellulose.
Triple-pane windows are used throughout.
Now for electrification. The home is heated and cooled with Mitsubishi mini-split air-source heat pumps. It has LED lighting and an induction cook stove.
And finally, we look at the source of electricity. The Marion house has a 10kW PV system, which provides for its electricity needs.
Mike Marion’s family moved in in 2017. That year, they spent $1542.16 to heat, cool and supply their electricity. They then installed 10kW of solar PVs. In the winter of 2020-2021 they spent $347.00 to heat, cool and supply electricity. Most of that is the cost of the connection for net-metering with their electric utility.
The Marion’s report this is the most comfortable house they have ever lived in.
Wes Golomb, is a long-time clean energy and climate advocate from Deerfield, NH. Bob Irving is the owner of RH Irving Home Builders, specializing in high performance building practices since 1972.
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