Monday, October 18, 2010

The balancing act

Since completing the first half of Consultant Training, I worked on 3 new prototypes in an attempt to understand how to optimize Passive House design on what I now realize is a most challenging building type and site.  First, the issue of compactness: a 20' house on 2 stories is relatively long, and therefore its surface-to-volume ratio is high, resulting in big heat losses.  If you try to overcome those losses with big south windows, you soon overheat the house or cause an overly large cooling load: hence the balancing act.  So you increase the insulation as high as possible and simplify the volume as much as possible.  It's interesting to note that more insulation and simple volume means a less expensive building to build than a more complex one with lots of pricey windows!

After some wrangling with heroic amounts of insulation I managed to get the first new prototype to work, barely hitting the targets for heating, cooling, and primary energy use.  This was a tiny house-- 3 bedrooms, treated floor area (TFA) 1284 s.f. (gross area about 1,600 s.f.), compactness of 3.4. It didn't have or need the big clerestory as in this blog's previous design.

For the next one, I tried an even simpler shape and 4 bedrooms, TFA 1,414, gross area 1,800 s.f., compactness of 3.2.  This was slightly easier to bring to the energy targets.  Then I tried a 3-story 4-bedroom version, TFA 1,781, gross area 2,400, compactness 3.1; this was easier still.

But "easier"-- it still means 16" of rigid insulation under the slab, 18" thick walls filled with insulation, R-100 roof, and the finest windows made.  (There are many ways to get to compliance, but these are the ones I used.) It's evident that compactness really matters.  That point came clear in training: our first exercise was a single family detached house, which we didn't quite get to certification in the Chicago climate.  The next one was a 4-unit multi-family house, and it was, by comparison, a breeze. 

As we look ahead to renewed development, particularly in transit-oriented districts, I think we're going to see many Passive House apartments, condos, duplexes, and townhomes.  For minimal upcharge, fully offset by low operational costs, new homeowners will be able to live well, comfortably, and green.  Looks like it's time for a multifamily prototype exercise!