There has been no shortage of discussion lately about modifying the Passivhaus standard to make it more adaptable to, and address more precisely, regional climate conditions.
Passive House Institute U.S. is exploring ways to fine tune the Passivhaus heating and cooling requirement, and possibly impose a load requirement for dehumidification, to enable Passivhaus builders to cost-effectively address the variety of climate conditions in North America. That initiative prompted architect and Passivhaus consultant Hayden Robinson to suggest in an online petition that PHIUS use a name other than Passive House for its certifications should the group’s requirements deviate from those of the existing Passivhaus standard.
While the debate over the appropriate use of the name Passive House has been lively, it doesn’t seem to have curtailed suggestions from those who believe the standard is extremely valuable but also in need of improvement. A case in point is a document titled “Proposed New England Passivehouse Amendment,” which was posted online on March 31 by energy efficiency specialist Marc Rosenbaum.
A focus on primary energy
Rosenbaum identifies what he believes are five principal deficiencies of the Passivhaus standard:
â— The standard’s annual heating demand and annual cooling demand (AHD and ACD) limits are the same for all regions, leading to extreme solutions in severe climates.
â— Meeting the AHD requirement in severe climates diverts focus from the overall primary energy consumption of the building and its occupants.
â— The Passivhaus standard limits are set per unit of usable floor area, which means large houses meet the standard more easily than small ones. That creates a perverse incentive to build big houses. Accordingly, the standard sidesteps the core issue of resource-use equity.
â— The standard counts solar input from direct-gain solar heating and solar thermal hot water, but it does not allow solar electricity to be used in meeting the standard.
â— The standard does not require performance reporting beyond a blower-door test that shows a maximum of 0.6 air changes at 50 Pascals.
Addressing these issues, Rosenbaum points out, means focusing more on primary energy consumption (PEC) per person and less on the building’s annual heating demand (AHD). A focus on AHD – the most challenging criterion of the Passivhaus standard – leads to “extreme solutions” that are not only costly but diminish the attention that should be paid to PEC.
An AHD-centric approach, he says, “can lead to buildings with excess south glazing, with increased heating season temperature swings (and higher cost). It can lead to quantities of insulation that likely exceed any defensible rationale when compared to investments in renewable energy.”
The Passivhaus standard’s AHD, he adds, is based on the idea of lowering the design heating load enough to deliver heat via the home’s ventilation air system – 10W per square meter, which in the climate of central Germany, where Passivhaus concepts were developed, yields 15 kWh per square meter per year. In New England’s climate, it is “extremely challenging” to push the annual design heating load down to 10W/m2, Rosenbaum says.
Key features of the amendment
He contends that building comfort and durability can be achieved with less aggressive levels of insulation than those that would be required to meet the existing Passivhaus AHD criterion. The shift in focus should be to a design heating demand (DHD) limit of 30W/m2 (or 9.5 btu/hr/ft2) based on treated floor area — which, he says, better reflects the fact that primary energy consumption attributable to heating in New England is 25% to 30% of total PEC.
The standard’s annual cooling demand criterion would be eliminated for New England because cooling in the region accounts for too small an amount of overall energy usage. Rosenbaum adds, though, that a separate dehumidification calculation (as yet to be developed) should be incorporated into the PEC calculation.
As mentioned, the amendment makes PEC the principal focus of the New England amendment, with the PEC limit set according to the number of bedrooms — a surrogate for the number of occupants — in the building rather than setting energy-use limits according to total usable floor area. This per-person budget for energy consumption is intended to help address the fact that, under the current criteria, it is more difficult for smaller houses to meet the Passivhaus standard than it is for larger homes.
With the AHD and ACD limits eliminated, the design heating demand limit, based on treated floor area, is 30W/m2. (Rosenbaum notes that this is the limit before solar and internal heat gains are calculated, and so is not directly comparable to the 10W/m2 limit shown in the Passive House Planning Package worksheet, which shows net heating load after those gains are added.)
Other key features of the amendment: the domestic hot water limit per occupant is raised to 10 gallons per day from the German limit of 6.6 gpd; photovoltaic power can count for as much as 20% of the primary energy consumption limit (each kilowatt of PV offsets 2kW of primary energy); the dehumidification load is added; and certification is renewed annually based on energy use data.
Calculations and expectations
Rosenbaum developed an algorithm to calculate the PEC limits — in kWh per year and million btu (MMbtu) per year (see table above) — based on the number of bedrooms in the project. He also describes the processes for calculating design heating demand, annual cooling demand, domestic hot water, and auxiliary electricity and electricity for appliances, lighting, and plug loads – all as they would apply to the Passive House Planning Package program. The data, added together, show total primary energy usage, which can then be compared to the PEC limit for that particular project.
Rosenbaum, who founded Energysmiths consultancy, in West Tisbury, Massachusetts, to advance design and construction strategies that make buildings more energy efficient, notes that the Passivhaus standard has been modified to both suit climate conditions in other countries and focus more tightly on primary energy consumption.
But he also observes that his proposed amendment is unlikely to be adopted in eager, speedy fashion by existing Passivhaus organizations (including Passive House New England). Nor does he delve into concerns about whether modified Passivhaus criteria should carry the name Passive House. He does hope, however, that his ideas precipitate useful discussions about the standard’s benefits and weaknesses.
“At the least,” he writes, “the aspiration is that this proposal may guide a well-focused and thoughtful exploration of how to live ethically and effectively on this planet for practitioners and clients, rather than aiming blindly at what are ultimately arbitrary numbers despite our best intentions.”
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