Green Building 101 Traverse City Record-Eagle

Green Building 101

Traverse City Record-Eagle
Home and Garden Section April 14 2013

Net Zero Energy Homes

Recently the media has been talking a lot about “net zero energy housing” and I thought it would be worthwhile to explain what that means.  For a local reference point I will be using the Depot Neighborhood Development which will start construction in the next couple of months.  This development is a joint effort by Habitat for Humanity GTR and HomeStretch, two local low income housing providers.  My explanation will be for the average home owner and not a comprehensive treatment of the subject.

Why are Net Zero Energy homes important?

As a nation for lots of reasons we have to find ways to use less energy and our homes and buildings are the low hanging fruit.  It’s just common sense on lots of levels. We have the technology and we know it’s cost effective; it’s just a matter of making the decision to move forward with it. The State of California has passed legislation that requires all new homes to be net zero energy by 2020 and all commercial buildings by 2030. It’s a win-win for the home and building owner, the community and the nation and planet.

The simple explanation is a net zero energy home produces as much energy as it uses on a site energy basis.  The main ingredients are: a highly efficient thermal envelope (insulation, air sealing, windows and framing system), a highly efficient mechanical system (heating, air conditioning and domestic water heating) and some form of onsite energy production (in our area that will almost always be a solar PV system).  The goal will always be to build the most energy efficient thermal envelope and install the most energy efficient mechanical equipment possible, and to make up the additional energy requirements with onsite energy production (solar PV system).

The Depot Neighborhood Development Habitat homes have been energy modeled using REM/Rate software which produces what’s called a HERS Index.  This software analyzes the home and all of its systems then gives it a score on the HERS Index.  A home built to the 2004 International Energy Conservation Code would have a score of 100 and a net zero energy home would have a score of 0.  Most new homes score at about 80 to 90.  For each 1 point reduction of the HERS Score the home will use 1% less energy. The Habitat homes have a score of 10 or so depending on the final choices of the thermal envelope and mechanical system. The final 10-12% of its energy requirements will be supplied by the solar PV system.

If the home is built as currently modeled the approximate energy cost would be as follows per year: 

Heating $202.00, cooling $43.00, water heating $240.00, lights and appliances $559.00, utility service charges $54.00, minus photovoltaic (solar PV system) production ($775.00). The total energy cost for the Habitat home is predicted to be $323.00 per year. Habitat is continuing to work on further reductions of the energy requirements for the proposed homes in an effort to reach the net zero energy goals.

Some of the major features planned for the Habitat homes are as follows:

Foundation: Insulated Concrete Forms (ICF’s),  R23;  Floor: Slab on grade concrete with 6” of rigid foam insulation, R30; Above grade walls: 2X6 frame with 5.5” fibrous cavity insulation and 4” rigid foam on the exterior, R43; Windows: U.25; Doors: U4.4; Ceiling: Combination of spray foam and cellulose insulation, R80; Heating System: Mini-Split Heat Pump HSPF, 9.3; Cooling System: Mini-Split Heat Pump SEER, 18; Water Heater: Heat Pump water heater, EF 2.35; Energy Recovery Ventilator: total recovery efficiency, 97%; Appliances: Very High Efficiency, Lighting fixtures: LED with motion sensors, Solar PV System:  6.2 KW;  Projected Building Leakage Test: (Blower Door Test) .05 Air Changes Per Hour (ACH); Total Carbon Dioxide:  (CO2) emissions 1.1 tons per year (same home built to code would be 15.1 tons).

Notes:

Site Energy: Energy produced on the site where the energy is consumed.

Source Energy: Energy consumed on the site where the home is located but produced in another location. Source energy must include the cost of the raw fuel plus production and transmission cost. Approximately 70 % of the energy embodied in coal is lost by the time that energy reaches the home site.

Air Sealing is the most important element in a highly efficient thermal envelope.

Mini-split heat pumps are about 3 times more energy efficient than a conventional 95 % efficient warm air furnace.

Heat pump water heaters are about 2.4 times more efficient than a conventional electric water heater.

Studies show that occupant behavior can affect energy cost by as much as 50%.               

Max Strickland has been involved in the construction industry for 45 years. He is currently a principal with Strickland Ewing & Associates providing building performance consulting, green building program verification, Energy Star/HERS ratings, infrared imaging, energy auditing, energy code inspections and building forensic investigations.  Questions and comments can be directed to max@stricklandewing.com.