One of the first steps in aspiring to a LEED certification is to project the number of points that could be accomplished during the building so that choices can be made to provide the best energy efficiency possible.
During the preliminary rating we targeted a HERS index score of 40. That is considered a “great’ rating, however a zero is a net zero energy use house. 100 is a typical house built to 2006 energy code standards and 150 is the highest score for a house that uses 50% more energy than energy code standards for its size.
The Energy Logic folks just completed the preliminary modeling for our HERS index and they are projecting a score in the 60’s. That was a huge disappointment as it means far fewer points towards our LEED rating than a 40. So I wondered if I had set up an unrealistic expectation for the work we are doing to the house.
The HERS score is based on the energy that a house will use compared to a “reference house”. The reference house that the building is compared to is the same size and configuration as the house being evaluated–as built to the 2006 Energy Code which gives a rating to the house of 100. Then the BTU’s of energy projected to be used in the house based on the energy efficient improvements are figured and divided by the BTU’s that the reference house would use. The result is the percentage improvement over the reference house–or the HERS score. As noted in the diagram, an Energy Star home would be expected to score about 85.
So where did we get a target of 40? The HERS rating diagram calls 60 “GOOD” and 40 “GREAT”–so we thought a house specifically designed and upgraded to be very air tight and energy efficient, supplemented by passive solar heat and high efficiency appliances would be “GREAT”! That may have been unrealistic.
Energy Logic’s Adam Jonash told me that the lowest HERS they see without adding produced energy to the equation is in the 50’s. That seems to be confirmed from other sources. Energy Star 3 would score about 70-75. And 45-50 is about as efficient as possible without adding renewable energy.
On the other hand, this is what a Passive House score might look like–they generally don’t add photovoltaics and/or wind but rely on heavy walls and tight construction.
And a typical LEED certified home has a score between 40 and 70. But some of those might have produced energy added to the equation but they may have a lower certification level than our goal of Platinum too.
Our bills have averaged $70 for the last year–between about $30 and $200. According to this chart our HERS would be about 50 without even finishing the house. This is based on a 2250 square foot house and ours is 2213.
Since our house is passive solar, perhaps the added heat from the sun could be quantified to offset our HERS score. RESNET doesn’t count passive solar as renewables though. Otherwise we could start thinking about adding solar, photovoltaic and/or hydrothermal to lower the score. The difference between 40 and 60 HERS rating is 7 LEED points. We are trying to break 100, but 94 will give us Platinum, the highest rating, so 7 points is a big part of the total.