Monday, March 29, 2010

PHPP as a design tool - part 2

This is the follow-up to my first post.  In PHPP I decided my first objective would be to analyze the thermal shell: overall building orientation, window, roof, and wall R-values, surface-to-volume ratio, window areas, window shading, etc., which all feed into a calculation of Specific Space Heat Demand.  We're aiming for 4.75 kBTU/(ft2yr), the maximum SSHD to make passive house certification.  I've been working on the 21st c. version of the bungalow, an urban house that is right-sized, reasonably affordable, and delightful.  I wanted to know if I could also make it a Passive House.

Before I get into details, though, let me state that I haven't been through the PH Consultant training class; they haven't been offered in the Midwest since I became aware of them.  The software, while complex, is fairly intuitive, at least to the extent I'm into it. At this point I'm looking mostly at space heating and not overall energy use (electrical loads haven't been considered, for instance), but it's enough to turn up some interesting insights.

I started with a few assumptions: approximately R-48 walls, R-60 roof, R-40 foundation, and triple-pane fiberglass windows and doors.  Really high performance stuff by current US standards, but typical for Passive House.  It's assumed that the house would have less than the maximum permissible .6 air changes per hour when pressurized to 50 Pascals (equivalent of having a 20 mph wind blowing on all sides of the house).  Through earlier planning and energy analysis I arrived at a scheme with bedrooms in the lower level (floor 3' below grade), and living spaces above (about 6' above grade).  It has a full-width front living room with side entry like many bungalows.  Here's an overview of the volume from the northwest (remember the visual design elements will emerge from or alongside the analysis--not starting with visual preconceptions):
 Note the "green garage" in the back!

Then I loaded up the south side with windows, put in generous east and west sliding doors for access to porches and yards, and ran the calculations.  Here's the basic box from the west (like most Oak Park lots, ours runs E-W):
Strangely, when I ran the calculations on this without context or shading, I came up with 4.74 kBTU/(ft2yr), just barely making the Specific Space Heating Demand (SSHD).  So I added context (which blocked out much of the solar gain)...
...and it jumped to 6.66 kBTU/(ft2yr).  These images are from Oct. 15, about the beginning of heating season.  That shadow on the south wall climbs for 2 months!  I tried pumping up the insulation to crazy amounts, and couldn't get the SSHD to come close.  That sunlight is just plain necessary.  So what do you do on an urban lot with a neighbor to your south?  You need a different shape.  I was worried that adding a clerestory for sunlight would throw off my surface-to-volume ratio, but it was about my only option with this footprint:
Just loaded that south clerestory face entirely with windows.  To my surprise, the SSHD came down to 4.68 kBTU/(ft2yr), even better than the single story house without neighbors.  It was evident that adding the "right" volume could be a good thing.  Before I added the clerestory, the house was 3 bedrooms, 2 baths.  In the east portion of the clerestory I put a family/guest room, which gives some flexibility to the function of the house.  The clerestory also provides a dynamic spatial experience at the entry/stair volume, in which you approach from the shaded north, then enter into space, light, and view to the sky, an underrated and guaranteed view.

The following image shows a general outline of porch and basic overhangs (to provide shelter from summer overheating and rain/snow, and to provide spatial continuity of interior to exterior):
When I look at the basic mass, I see continuity of scale with the gable-roof neighbors.  This house can't be gable-roofed and perform as it needs to.  The low-slope (almost flat) roof allows green roof, though, which has a host of benefits.  In the next post I'll show development of the basic mass into something of architectural merit.