Sunday, February 17, 2008

In response to "Energy Efficient Homes via Passive Houses"

In regards to Friday’s post, “Energy Efficient Homes via Passive Houses,” I would like to investigate a couple of things. First, I think the 30 percent energy-saving targets provided in the DOE’s new voluntary program may be more substantial than they appear and, second, I think the German Passive House (Passivhaus) estimates provided are unrealistically skewed.

The DOE Challenge calls for homes that consume no more than 70 percent of the energy of a typical new home built to the 2006 International Energy Conservation Code standards. The IECC standard only addresses sealing and insulating the building envelope and ductwork, with little reference to HVAC systems and no reference to lighting or appliance energy efficiency. I’m not sure if that fact will help or hurt a builder in aiming for the DOE Challenge. It is unclear to me whether the Challenge targets have to be met by addressing only the building envelope and ductwork measures laid out in the IECC or if systems, lighting, and appliance efficiency improvements will count in the reduction estimate as well. If homes are rated in the first manner, a 30 percent reduction will prove quite tough to match from a builder’s point of view – much more work will be done on the actual structure of the house. If homes are rated the second way, a builder could almost achieve the entire goal by upgrading equipment only: 90+ AFUE heating equipment, 20+ SEER cooling equipment, and EnergyStar refrigerators, washers, dryers, and CFLs. Either way, let’s take a look at the feasibility of various energy consumption reduction measures and think about a sort of maximum reduction value.

Let’s look back at the pie chart of U.S. Residential Electricity Consumption by Application in 2001, from the first day of lecture, in conjunction with the Energy Ratings of Homes listed on the Passivhaus website:

Obviously, the Current Average on the Passivhaus picture on the right represents some sort of weird extreme cold climate where space heating measures make up about 75-80% of a home’s electricity usage. Hot water and household electricity make up the remaining 20 percent or so, split almost evenly. Compare that with averages from all U.S. households in 2001, where space heating makes up a mere 10 percent and air conditioning another 16 percent. It is important to note the huge chunk of electricity Americans use to simply preserve our food, light our rooms, clean and dry our clothes, and run the rest of the appliances that make our life comfortable, easy, and entertaining. If you follow the primary standard in Passivhuas (“a building in which a comfortable interior climate can be maintained without active heating and cooling systems”) by theoretically negating all heating and cooling energy but leaving the rest of electricity use as-is, home energy consumption would then be reduced to only about 75 percent from 2001. This is a far cry from the proposed 90% reduction. And even if a builder decides to rely completely on passive heating and cooling – which is rare on the average – the DOE target hasn’t been met yet.

If you then consider applying stringent appliance efficiency standards and counting towards the home energy rating – standards that aren’t even addressed by the 2006 IECC – by upgrading to extremely high efficient refrigerators, freezers, clothes dryers, and compact fluorescent light bulbs, home energy use drops another 20 percent or so, down to about 50-55 percent of the 2001 average. This trims energy consumption down to a theoretical bare minimum U.S. average. Basically, half the electricity Americans use in our homes can be considered baseline and nearly necessary for our current standard of living. Thus, the 90 percent reduction that Passivhaus speaks of is unrealistic. I think the DOE’s new challenge to homebuilders of a 30 percent may therefore be a better target than at first look. I do, however, wholly agree that the number of homes targeted should be multiplied many times over. In the last few years in the U.S., about 1.6 million new homes have been built each year. The target of 220,000 new energy efficient homes over the next 5 years will then affect only about 2-3 percent of the new home market. I think this number should be increased by a factor of 10 or more.


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