Definition: A Net-Zero House is a house means that this home produces as much energy through renewable resources, such as solar panels, as it consumes.
Definition: A Zero Energy Ready House is a house that is built above the current building code to be very energy efficient and will become a Net-Zero house once a renewable energy source is added.
I my research I came across a builder who was building what he was calling a Net-Zero house fro himself, so I had to check his methods out. He is one of the guys I ran into who was spending $100 to save $10. He used 2 x 6 standard framing with R-19 fiberglass bats for insulation, standard double hung double pane windows, and did very little to seal the house beyond normal practices. He was putting a huge of geothermal system, I estimate at a cost or around $40-50K, then covering his energy usage by a very large solar system, another $40-50K. He might have gotten to Net-Zero but because of the extra costs it would have never had a brake even point. If he’d put in $10K of insulation, double studded the walls, $1K he wouldn’t have needed the geothermal and could have use a solar system 20% of what he was putting in. Thus a $60K savings.
The easiest way to describe a Net-Zero house is that is built like a ‘Thermos Bottle” with windows. It’s a house the is so well insulated and so tight it takes very little energy to heat or cool it, There are a number of reported instants of Net-Zero house loosing their heat over night, 1 in New England when it was 7 degrees outside and the house only dropped 4 degrees overnight. Another in Ohio where it was 20 degrees outside and the home only dropped 2 degrees. And heating bills going from thousands of dollars a year to low hundreds. Then what little energy it takes to run the house, that gets covered by a small solar photovoltaic (PV) system. Here is a link to an Architect Dennis Wedlick speaking about his transition from a Passive Solar design to the “Thermos Bottle” concept. The Hudson House. Lesson closely to the performance of the Hudson House.
The start is the insulation: from the basement floor and walls, to the above ground walls and the roof. The idea is a continuous flow of insulation surrounding the house. The basement R-10, under the floor; R-20 in the walls; the exterior walls (in zone 4) R-40/45 and the roof R-50/60. There are many ways to do this, my approach was KISS.
Thermal Bridging is a four letter word in house construction. A traditional exterior wall the studding will “thermal bridge” cold into the house, so will traditional windows, sill plates, and on and on. So fighting bridging is a high goal.
The house must be sealed, very, very tight. Measured by a “blower door” test. The house is closed up and a sealed fan is installed in a door. The fan sucks the air out of the house. The measurement is the number of Air Changes per hour at a pressure of 50 pascals or XXACH50. The houses we grew up in would be 10ACH50 or more, houses in the past 10 years 7/7ACH50; today’s (2015) code built homes are 3ACH50. The Passive House standard 0.6ACH.
Then comes the windows: triple glazed are a must. The European windows set the standard. Window rated fromR-6 to 9. With a double glazed window the inter pane will be many degrees below the room temperature, thus cooling the air next to it. Causing that chilled air to drop and be replaced with warm air to be chilled. If you had a house that had a 0.6ACH50 with these window there would be a draft.
See the chart below, with a high performance triple pane window the inside pane is near room temperature, thus a minimal cooling and no drafts. So the house just feel more comfortable at the same temps.
Now that the house is tight, it must be ventilated. The best approach is a positive ventilation system; an ERV or HRV.
Houses are rated on a Home Energy Rating System (HERS) Index The index goes from 0 to 150. A HERS index of 0 means that this home produces as much energy through renewable resources, such as solar panels, as it consumes. Only a Net Zero Energy Home can score 0 on the RESNET HERS Index. A HERS index of 50 This home is 50% more energy-efficient than a standard new home and 80% more efficient that the average resale home, which already puts it in a better bracket than a standard new home.
The DoE Zero Energy Ready Home Program target is a HERS 50. My target is a HERS 30 or below.
How to get to a Zero Energy House Ready house and thus a Net-Zero house? There are many, many ways, I’ve tried what I call the KISS method (Keep It Simple Stupid). I tried to pick solutions that regular trades men would be used to doing. i.e. a double 2 x 4 wall to create a 10″ cavity for insulation.