IMG_5217Our Deep Energy Retrofit project in Lexington managed to beat the air sealing target prescribed by the National Grid utility pilot program; thereby qualifying the clients for an additional $2000 in incentives. We achieved an enclosure air leakage rate of less than 0.1 CFM per square foot of building enclosure.

This project included adding dormers to the second floor in addition to super-insulation. The ability to re-frame and re-roof the building made a tremendous difference to eliminate thermal bridging and wrap the building tightly.


The existing sunroom was not remodeled, so we utilized a technique called the chainsaw retrofit, which entails cutting the overhangs off to be flush with the wall framing; continuous exterior insulation to eliminate thermal bridging and then reconstruction of the overhangs outside the insulation.

Keen attention to detail and a liberal use of membranes / tape to seal all joints and transitions contributed to a tight building.

Another interesting aspect of this project was the judicious use of spray foam and the generous use of cellulose.To achieve the R-20 walls, we filled the existing 4″ cavity with dense-pack cellulose and 4″ polyiso foam board on the exterior of the sheathing. To achieve an R-60 in the roof, we applied a 3″ flash coat of spray foam on the underside of the roof deck and filled all the rafters (12″) with cellulose.  While some spray-can foam was used to fill voids and gaps, the real show stopper was the Pro-Clima joint sealing tape. Lesson learned: in the battle for a tight envelope, air superiority is won with the use of high performance tape.

6 thoughts on “Lexington DER update

  1. Proclima tape + judicious use of spray foam and dense packed cellulose insulation. Sounds like a winning recipe. Well done!

      • Building Performance question: What guided BgB’s choice to use two different insulating materials? For example, wouldn’t the same end be achieved by using all foam which would provide air sealing and expand to fill gaps where leaks might occur. Would Pro Clima tape still be required?

      • Hi Zeke, the utility pilot program has Building Science Corporation Inc as their consultants and consequently the walls, roof and other assemblies follow a prescriptive path that ensures both the long term integrity of the structure and the air quality of the occupants as the primary considerations before energy reductions. If you search the Building Science Corp site for “The Perfect Wall” you can read about the control layers and the roles they play in management of precipitation, air infiltration, vapor, and thermal control. While can foam is often used to seal air leaks, it is better suited to ensuring gaps in the insulation are closed up for thermal control. The Pro-Clima tape has proven to be a much more effective long term part of the air control layer than can foam. Of particular interest is the control of vapor transmittal that might be caused by air leakage that could lead to elevated moisture in part of the shell and subsequent mold, rot and structural damage, particularly now that the building’s ability to dry itself through energy loss is nearly zero. This really brings all the details into sharper focus…

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