Spotlight: Net Zero Lexington House

A3 Architects 6.26.18-5.jpg

These clients in Lexington MA came to us with an idea of creating a new net-energy zero home for their family of five.  The husband is self-employed and wanted a large office that could accommodate clients.  The wife makes jewelry and wanted a place to be able to work on her craft. The three kids are in high school and college (when they aren’t travelling the world!) but this new home was to be their family’s hub and home base.

We worked on an existing site with a rundown 1950s ranch that was removed.   We sited the new house, connector and garage to maximize passive and active solar.  The garage is located closer to the busy street while the house overlooks the gorgeous backyard oasis with lots of glass overlooking the owners own landscape design.

The house is designed with superinsulation strategies including a double wall and roof insulation values. This superinsulation, and near airtight envelope with ventilation, reduces the heating and cooling load so that the entire house is heated and cooled with several mini split heat pumps.  Everything in the house is electric, by design to match the electric production of the photovoltaic panels on the garage roof facing south. Over the course of the year, the solar panels provide enough energy for the entire home!

The interiors of the home are modern and open.  The IKEA kitchen opens to a dining area and open vaulted living room with balcony above. There’s a quieter study/bedroom to the east, and several bedrooms on the second floor.

Project Details:

  • Location: Lexington, Massachusetts

  • House Type: Single Family Residence

  • Conditioned Area: 4212 sqft

  • Conditioned Volume: 52458 cubic ft

  • HERS Index:  42 (calculated w/o renewable, w/ renewable net zero ready)

  • ACH:  0.72 Air Changes per Hours


  • Heating/Cooling: Wall Mounted MSHP

  • Ventilation System: ERV

  • Appliances:  All Electric

  • Water:  Heat Pump Hot Water Heater

  • Lighting:  Energy Efficient LED Lights through Mass Save Program


  • Ceiling:                        R:  60

  • Exposed Floor:            R:  40

  • Above Grade Walls:  R:  40

  • Foundations Walls:     R: 19

  • Slab:                            R: 20

Construction Details:

Builder: AEDI Construction

HERS Rater:  Home Energy Raters LLC

Photographer:  Kyle Caldwell


A3 Architects 6.26.18-15.jpg
A3 Architects 6.26.jpg

Net Zero Lexington Construction Slideshow

Brewster Flats Blower Door Test

 Blower Door Test at Brewster Flats House by  Home Energy Raters

Blower Door Test at Brewster Flats House by Home Energy Raters

After insulation is complete we typically test houses for air tightness.  This is done by a blower door test.  The required tightness by code is 3 ACH (or air changes per hour).  We are typically targeting around 1 ACH.  During this test, we typically find several “weak links” in the envelope. Often one can feel the draft around windows, at the plate or around masonry fireplaces. 

Our HERs rater is locally based in Sagamore Beach Home Energy Raters.

They are a 3rd party meaning they aren’t connected to the builder or architect and therefore have an independent role in evaluating the tightness of the house. They provide us with a final HERs certificate required by building code, effectively proving the home is in compliance with the current energy code.

In addition to testing air tightness, the HERs rater typically tests duct tightness.  They also provide an independent visual inspection of insulation and mechanical systems.  After their feedback we typically tighten up a few areas and have them test again at the final inspection.

  • Area of Conditioned Space:                      3,437 ft2
  • Volume of Conditioned Space:                 35,233 ft3
  • Loss:                                                           CFM at 50pascals
  • ACH                                                            1 1/2*

*This test was done after insulation, but before drywall was installed. We found some areas that needed additional insulation and air sealing.  The contractor will address these areas and another blower door test will be performed when the house is near completion.  We expect the final test to be around 1 ACH. 

 Street View of Brewster Flats House

Street View of Brewster Flats House