Why doesn’t RESNET put more weight on air sealing in their proprietary formula for determining the HERS Index of a new home?
JoeUSAF
| Posted in Energy Efficiency and Durability on
I recently was granted a HERS rating of 40 for my new ENERGY STAR Vers 3.1 home without renewables. The blower door test was CFM@50 Pascals = 95 and ACH50 = 0.27. My HERS Rater told me that the blower door test data has a minimal affect on the overall RESNET HERS rating. In my opinion and that of many professionals, air infiltration is the single most important factor affecting the energy efficiency of a new home. A new home with high R values for the walls and ceiling may be very energy inefficient if its air sealing is poor.
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Replies
Your HERS Rater is wrong. The blower door test result has a fairly "significant" impact on the HERS Rating*.
Having said that, I would disagree in saying that "air infiltration is the SINGLE (my emphasis added) most important factor affecting the energy efficiency of a new home". There is no silver bullet in energy efficiency, which is one of the reasons why I think it gets short shrift in the mainstream. Excellent energy efficiency in homes requires significant improvements over standard construction across several aspects of construction: air tightness, insulation, window performance, heating/cooling equipment + distribution, water heating equipment + distribution, lighting, major appliances, and electronics.
Though I don't know the details of your house, a HERS score of 40 shows to me that you or your designer and/or builder have done your homework on these details. You don't get to HERS 40 on air-tightness alone.
*The only time this is not the case is in the case of a fairly tight house that does not have mechanical ventilation. The RESNET HERS rating rules do not credit improved air-tightness beyond a certain limit (roughly <5-6ACH50, I think) at the expense of indoor air quality.
Thank You for your comments. Greatly appreciated.
From: Joe Poland
My rater said: " A reduction of the ACH50 from 7 to 4 yields a 4 point drop in the HERS score".
I would guess that going from 4ACH50 to 1ACH50 will yield much less than a 4 point drop because of diminishing returns.
So I think your rater is correct down at low values of ACH50. You could make the house four times tighter by going from 2ACH50 to 0.5ACH50, but you won't save much energy because at 2ACH50 the house is already losing very little energy due to infiltration.
Kevin,
Within the energy models that I work in (including Rem/Rate) there are no diminishing returns on decreasing air leakage. Each cfm of improvement is as good as the one before it.
Assuming mechanical ventilation is present, going from 4ACH50 to 1ACH50 will give you the same improvement in the HERS score as going from 7ACH50 to 4 ACH50.