Aiming Low: Postgreen Homes HERS Scores
The HERS (Home Energy Rating) score is one of the key ways in which we judge our homes. Affordable energy efficiency is our highest priority when designing and building a home, and the HERS system of rating is the most widely accepted model for understanding success in that pursuit. The rating system is part of LEED, but we like to pull it out and look at it on it’s own as we feel it says more about a home’s actual performance than the occasionally murky waters of LEED certification.
HERS rates homes on a point scale with lower scores showing better performance (like golf). The scale reflects a percentage difference from the baseline which is set at a score of 100 for a code built new construction home. An Energy Star home needs to be 15% more efficient than a code built home which means a score of 85. Existing homes get an assumed score of 130 though we would guess that most Philadelphia row homes are actually significantly higher (150-180?). We are obviously aiming for the extreme low end of this scale.
Our first two homes in the 100k House project hit HERS scores of 49 and 46. This means they use half the energy of a code built home and our energy bills seem to support that claim. The Skinny Homes improved on this through additional insulation, greater attention to detail and the addition of a 3 kW solar PV array. They came in with scores of 23, 26 and 29. According to MaGrann, our HERS rater and LEED provider, these are some of the lowest scores in the state and particularly impressive due to the relatively small solar system.
To illustrate the relative energy use we created the following collection of energy spheres. I considered a simple bar graph, but spheres seemed cooler. I hope you agree . . .
As you can see, we could build five of the Skinny Homes, and they would collectively use less energy than one existing Philadelphia row home. More importantly, an average code built new home uses about four times the energy of the Skinny Home. This is simply irresponsible. The existing home has the excuse of age, but the new built home has simply taken the easiest, cheapest path without regard for the environment or it’s future owners.
To put this energy use in context we created a somewhat ridiculous looking map. This map assigns one of our spheres to all the houses surrounding the Skinny and 100k projects. Most of the homes are existing row homes that should likely be sporting a much higher number than the 130 we have assigned them. There is also one new construction home in the area (the slightly smaller sphere).
We plan to continue to pursue lower and lower HERS ratings with Postgreen Homes. Much of this continued improvement is made possible by our new construction company, Hybrid Construction. Hopefully, we will end up contributing to a significant improvement to the overall “sphere” map of Philadelphia.
Questions? Concerns? Rambling responses? Let us have them in the comments.