Passive House Retrofit Characteristics: Brooklyn Cohousing
I recently have undertaken a building project with Brooklyn Cohousing following Passive House standards. My clients decided to pursue this course after carefully examining the affordability and predictability of energy usage with the Passive House option.
Predictability: USGBC’s own commissioned report, Energy Performance of LEED for New Construction Buildings (pdf), March 4, 2008 concluded “…measured performance displays a large degree of scatter…” While this is true of other standards, it is not true of Passive House. Passive House has an unparalleled track record of predicting performance reliably. CEPHEUS, an E.U. sponsored program, verified Passive House's reliability by developing hundreds of units of housing in multiple countries with different architects and contractors. I'd argue that the reliability of the methodology makes Passive House the least risky approach to super-efficient building. That reliability makes Passive House convincing - and was one of the factors, I believe, that convinced Brooklyn Cohousing to go for it.
Affordability: We needed to consider two aspects of affordability: operating affordability and upfront cost affordability. If a typical home has a $300 heating or cooling bill, a Passive House of the same size and in the same geographic area would have a bill of approximately $30. That’s dramatic savings in operating affordability - savings, even without taking into account the certainty that the disparity in those costs will increase as energy costs rise in the future. This operating affordability should be considered in light of the modest upfront cost increase, of typically between 5% and 10%, of conventional construction. The key to the modest increase in construction cost is multifaceted: concentration on building envelope, omitting costly renewable energy systems and substantial reduction of heating and cooling systems. In Brooklyn Couhousing's case this means a reduction from 64 tons of AC typically needed in such a development, to just 18 tons. With the PHPP, material and component selection can be optimized regarding cost and performance while insuring the energy goal is met. So while we've dramatically decreased the heating and cooling systems, Brooklyn Cohousing is also seriously considering investing in the really great windows available from Germany.
Finally, affordability goes hand-in-hand with the drive toward simplicity in construction.
For Brooklyn Cohousing, this will mean an annual energy bill savings of approximately $66,000.00. At the same time, the increase in building cost as compared to "typical construction" will be approximately $400,000. (That comes out to $13,500 in additional cost per household on average and approx $2,200 per household in yearly savings). This makes payback possible in just 6 years.
Typical Passive House Characteristics:
1) Virtually thermal bridge free construction. Great care is given to the detailing of the building envelope to ensure that the insulation layer is continuous. Particular scrutiny is given to the intersections of floor and wall and roof, and where windows and doors are installed. Where thermal bridges cannot be eliminated, they are calculated in the PHPP. (A thermal bridge is a material or assembly of poor insulating value that allows heat to easily escape through the building envelope.
2) Superinsulation. Invariably, in optimizing the elements, slab, wall and roof insulation become much thicker than in typical construction – they become superinsulated. A slab value of R20, walls with R40 and roofs of R60 or greater are not uncommon in Passive House construction.
3) Triple-pane windows. Windows often have a U value < .2, with insulated frames and multiple seals. The windows are often casement or fixed, as double-hung windows cannot provide the air-tightness required. In heating climates, windows will also have a relatively high solar transmission ratio, becoming a major component of the passive heating calculation in the PHPP.
4) Air-Tightness. Air-tightness testing occurs during construction or renovation. Air-tightness is verified with blower-door tests. Like the insulation, the air-tight layer must be continuous. This air-tight layer must be accessible at the time of the first blower-door test, because the house is likely to fail the first time. (Note that you can open the windows in an air-tight Passive House. Common sense says just don’t open them when you have the air-conditioning or heating on).
5) High efficiency heat recovery ventilation. A Passive House has continuous ventilation with heat recovery efficiency greater than 75% in an air-tight house. A heat recovery ventilator has a heat exchange core through which the continuous exhaust air passes, transferring the interior heat to the continuous incoming fresh air without intermingling the air-streams. If the heat recovery ventilator is not of adequate efficiency it will undermine all the advatages of the Passive House thermal envelope. This low volume ventilation is constant and provides 100% filtered fresh air to the house – providing superior indoor air quality. Air is supplied to bedrooms and living rooms and continuously exhausted from bathroom and kitchens, in a mechanically balanced air-flow.
6) Thermal Comfort. The combination of air-tightness, superinsulation and high performance windows eliminates typical air temperature stratification – so that the temperature at the floor is the same as that at the ceiling, and is virtually the same at the exterior wall as it is at the interior wall. Because the fresh filtered airflow is constant in all occupied rooms, there is much less dust in the space. All these improvements not only lead to greater occupant comfort and health, but occupant comfort at higher temperatures in the summer and lower temperatures in the winter than would be typical.
Finally, a word about PHPP.
Fundamental to understanding the Passive House methodology is the role of the Passive House Planning Package (PHPP). The PHPP is the most sophisticated static energy model in use today. It is continually being refined and updated from field measurements and dynamic modeling study. With the PHPP, one enters a straight-forward yet exhaustive array of information from basics of climate and treatable floor area to shading elements, window frame profiles, wall assemblies, and the motor efficiency of the ventilation systems, to name just a few. The resulting verification sheet clearly and precisely predicts the energy usage of the proposed building – indicating whether or not the building will meet Passive House standards.
Occupant load is one of the PHPP inputs for both energy gains and loads and ventilation requirements. For typical usages, the PH approach of constant fresh air provides exceptionally good air-quality. The ventilation systems typically have very simple controls where one can put on a high fan speed if there is a party and greater air-changes are needed. For special gathering spaces, one might have a dedicated ventilation system, and in such cases it's really a mechanical engineering issue how to meet the precise requirements of that space.
One can then manipulate all the element inputs based on cost effectiveness, construct ability, design priorities, etc… and arrive at the appropriate combination of elements to meet the standards - providing a truly integrated design.
* Ken Levenson is a partner in Levenson McDavid Architects P.C., in Brooklyn, NY. Ken also maintains the blog Checklist Toward Zero Carbon.