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Community and Q&A

A Simple Building Energy Index

Karl Overn | Posted in Energy Efficiency and Durability on

Here goes a late night post, but this has been bothering me. One of the big issues with PassivHaus seems to be the way energy usage is measured.

1. The building measurement side is strange since interior partition walls don’t count as floorspace and basement floorspace is multiplied by 0.8 (if I understand everything correctly).

2. The somewhat arbitrary space heating energy usage limitation designed around German conditions, which have apparently been modified by the Swedes and Austrians to match their more severe climates, becomes a problem in other areas.

Perhaps a simpler, climate adjusted measurement would work better such as:

Building Annual Energy Index = (Energy Used) / (Degree Days) / (Building Conditioned Volume) x 1000

Energy Used is the energy required to maintain the conditioned volume within the set temperature range for the entire year.

Set Temperature Range would be the acceptable temperature range within the conditioned volume.

Degree Days would be the heating degree days plus the cooling degree days corresponding to the lowest and highest temperatures within the set temperature range.

Building Conditioned Volume is just what it says.

Lets see an example. A simple rectangular house with inside (drywall to drywall) measurements of 30 ft x 50 ft and a height of 18 ft (basement slab to insulated main floor ceiling). Building conditioned volume = 27,000 cubic feet. The set temperature range is 68F to 78F. For the year that the building performance is measured or simulated the HDD (base 68F) is 8,200. The CDD (base 78F) is 500. The energy used to maintain 68F to 78F interior temperatures for that year is 40,000,000 BTU. The Building Annual Energy Index is then:

BAEI = (40,000,000 BTU) / (8200 HDD + 500 CDD) / (27,000 Cu. Ft.) x 1000 = 170

Just like golf the lower the BAEI the better the building performance. What are the advantages? The BAEI makes comparing building performance between various years, climates, and configurations an apples to apples affair because the result is adjusted for climate and interior volume. This also eliminates the effects of entirely interior configuration on the calculated performance. I’ve wondered if PassivHaus’s often use lofted second floors to increase the building’s volume and south facing window area without increasing the floor square footage. The BAEI formula makes no distinction between lofted and standard second floors.

BTW, the 1000 multiplier is just to move the decimal point to a convenient place. A metric version would have a different multiplier to make the result the same in either units.

Well, there it is. What do you think? I am sure that someone else has used or proposed this type of formula before – I’m not smart enough to have thunk it up first. It just seems like a better, fairer measurement to analyze energy performance.

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  1. Aj Builder, Upstate NY Zone 6a | | #1

    BEI... Go with three letters...

    SEI... Simple Energy Index

    KOE... Karl Overn Index

    Great formula Karl.

  2. Karl Overn | | #2

    BEI works for me.

    KOI is wholly owned and operated by my wife. I usually don't want to know that number.

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