Carter Scott of Transformations, Inc has built numerous Zero Net Energy Homes in Massachusetts, ranging from custom homes, to development homes and to very affordable low income Zero Energy Homes. He shares some of what he has learned about building Zero Energy Homes in this interview. For articles written by Carter on his Zero Energy Homes go to Zero Energy Home Case Studies under the “Professionals” button.

What year did you build your first Zero Net Energy Home?

Carter Scott (CS): We built our first Zero Net Energy Home in 2008

How many Zero Net Energy Homes have you and your company, Transformations, Inc., been involved in building?

CS: We have built 11 highly insulated airtight homes with solar photovoltaic systems that have Home Energy Rating System (HERS) Index ratings ranging from -9 to +8 (which were Zero Net Energy Homes or Zero Net Energy Attainable).  We have another ten, which should be in the same range that we are currently building now. There are a total of an additional 90 Zero Energy Homes or Zero Energy Attainable Homes in this range that are in various stages in the development pipeline.

Do the homes with a HERS rating above 0, function as Zero Net Energy Homes if the occupants are energy efficiency oriented?

CS: I have had a home with a HERS 2 come in with a surplus of 1,574 kWh of energy over a yearlong time frame.  There is some thought that all homes under HERS 10 are Zero Energy Attainable and that the homeowner’s habits and the building’s specific energy efficient attributes will dictate whether they are net zero energy homes over time.

How much higher have the building costs been for the average Zero Energy Homes you have built compared to the costs of a similar homes built to code?

CS: We have gotten the cost down to about $5 per square foot over the code home.  This is with a leased photovoltaic (PV) system. The average solar photovoltaic system costs about $25 per square foot when you do the math. In Massachusetts, the Solar Renewable Energy Credits, the 30% Federal tax credit, the $1000.00 state tax credit, and the electric energy generated, all translate into a payback of about 6 to 7 years for the solar PV system.

Which of your homes has had the least cost premium for becoming a Zero Energy Home?

CS: I would say the smallest homes in an absolute dollar basis and the larger 3 and 4 bedroom colonials in a marginal cost per square foot basis have the lowest cost premium of all our Zero Energy Homes.

Has the relative cost premium for Zero Energy Homes gone down as you have gained experience since 2008?

CS: Yes

What have you learned to do differently that has brought the cost down?

CS: The Navien instantaneous tankless hot water heaters have been very cost effective.  We have also learned to use the low density (open cell) foam and cellulose as much as possible while minimizing the more expensive high density (closed cell) foam.

In your experience in New England, which insulation material and R-value have you found to be most cost effective for the walls, floors and ceilings of your Zero Energy Homes?

CS: For under the slab in the basement, 2 inches of the blue rigid foam (R-5 per inch) have worked well.  For the basement walls, 3.5 inches of high density foam (R-6 per inch) with a fire coating paint applied.  For the above grade walls, either 12″ of cellulose or 12″ of low density foam.  We have gotten a great price from our insulating contractor, where the price of the low density foam (R-3.8 per inch) is the same as we would pay for cellulose (R-3.5).  For the attic we use 18″ of cellulose for a total R-Value of 63.

What are the average Air Changes per Hour  (ACH@50 Pascals) in your Zero Energy Homes?

CS: Our average blower door test is about 1.0 air changes per hour at 50 Pascals (ACH50).  Our lowest was 0.50 ACH50.

Which air sealing techniques have you found to be most cost effective?

CS: We have found that a triple system works best: 1.The Zip System (sheathing with an air barrier built into it) on the exterior walls;  2.The low density spray foam, and  3.The air sealing of the dry wall on the inside.

What Window to Floor Area (WFA) are you using in your Zero Energy Homes?

CS: The Window to Floor Area ranges from about 8 to about 12 percent.

Which insulated windows have you found to be most cost effective for Zero Energy Homes?

CS: We are now using the Harvey Tribute line, which is a triple pane window with Krypton gas and has a U-Value of around 0.20 depending on the size.

Since Zero Energy Homes are so airtight and ventilation systems are needed which heat the incoming fresh air with the warm stale air that is being exhausted from the building; what experience have you had with Energy Recovery Ventilation (ERV) or with Heat Recovery Ventilation (HRV) systems that are designed to supply this pre-heated fresh air?

CS: We have tried many different ways to ventilate our homes.  They range from simple exhaust only Panasonic fans in the bathrooms to a full Heat Recovery Ventilation system.  In high radon areas of the country, one must be careful not to pull in radon from under the slab in a bathroom exhaust only system.  A passive radon mitigation system (a LEED requirement in high radon areas) including sealing a layer of plastic under the slab is critical in this regard.  The more expensive balanced ventilation is achieved with the HRV and ERV systems.  We have installed the LifeBreath system and both the Fantech 704 and the Fantech 1504. The exhaust-only Panasonic fan model FV08VKS2 has been the most cost effective. It runs on 30 CFM with a boost to 70 CFM. Two of these exhaust-only fans usually meet the Energy Star ventilation requirements for a home.

How have you used passive solar heating or heat tempering in your Zero Energy Homes and how significant has it been?

CS: Yes, we have used passive solar heating in many of these homes.  We did a study a few years back and found that putting the garage on the North side of the home and having a couple extra windows on the South side saved about 5% of the home heating and cooling loads.  Some of the models have additional glass in the south, minimal glazing in the North, East and West, and overhangs on the Southern glass to minimize the summer heat gain.  This increases the solar heating and provides additional savings.

Have you found that using an interior thermal mass to enhance passive solar heating is cost effective.

CS: Yes, but only if homebuyers want to have slab on grade with a colored polished concrete as their floor surface.

What heating system have you found to be most cost effective?

CS: The Mitsubishi mini-splits have been fantastic.  The newer dual stage, inverter driven heat pumps cost us about $2850 installed, as part of a volume deal arrangement.  The model number of the indoor unit is MSZ-FE12NA and the model of the outdoor unit is MUZ-FE12NA.  We have used anywhere from 1 to 3 of the systems to heat and cool a home.

From your experience building Zero Energy Homes, which energy saving measure or measures have you found to be the most critical to making a Zero Net Home affordable?

CS: The super insulation and air sealing of the shell combined with a leased PV system.

What has the selling price range been for your Zero Energy Homes?

CS: Our Zero Energy Homes range from a very cost effective $125,000 to custom homes that are worth about $1.5 million. In Massachusetts, we have an affordable housing law that helps developers build more affordable homes.  Prices of these homes have been $125,000 to $145,000 in Easthampton, MA and $168,000 to $195,200 in Townsend, MA.  They are deed restricted and available only to buyers making less that 80% of the median area income.  We are building “Workforce Housing” for MassDevelopment in Devens, MA.  The homes there start at under $350,000.  There are no income restrictions, just a base price restriction.

The market rate Zero Energy Homes in the Massachusetts developments in Townsend, Princeton, and Easthampton tend to be in the $299,900 to 429,900 range depending on the model size and customizations. The custom Zero Energy Homes can range from $140 to $250 per square foot to build plus the cost of the land.

What kind of premium are buyers willing to pay, if any, for a Zero Energy Home compared to a similar home built to code?

CS: The premiums a buyer is willing to pay for a zero energy home vary from just a few percent to 30% or more.  At the lower end of the spectrum the photovoltaic (PV) systems are leased.  At the higher end of the spectrum the homes tend to be custom, and the PV systems are purchased.

Thank you so much Carter. You are a true leader in the Zero Energy Home movement. What you are doing is a wonderful inspiration for both homebuilders and homebuyers who are committed to creating a sustainable world.

CS: You’re Welcome – it has been a pleasure Joe. Thank you for helping get this information on Zero Energy Homes out to the public