How Do Passive Houses Hold Up In Cold Weather?

It is of great interest to our Western Pennsylvania Passive House Alliance Chapter to know how well Passive Homes are performing during the cold spells that have engulfed much of the U.S. this winter.

We’ve received several reports on this and wanted to share our findings with the Green Building Alliance community.  As many may know, the Passive House standard limits building energy use to 4.75 kBtu/sf/year for heating and cooling.  In the context of residential homes, a heating system  is approximately 12 kBtu/hour instead of  a more typical 75 – 100 kBtu/hour in traditional construction practices.  We often joke that heating a residential  Passive House could be accomplished by running a hair dryer instead of a furnace!  But how is this level of efficiency possible and how do Passive homes really fare in very cold weather?

The main principles of Passive House construction can be summarized as follows:

  • Start with a simple shape for the building to avoid nooks, crannies, and architectural features that may contribute to thermal bridging.
  • Super-insulate the home, but more importantly, provide an air-tight envelope for the building.  How air-tight, you ask?  The standard calls for 0.6 air changes per hour at 50 Pascal pressure (verified by an independent rater).  This makes a Passive House’s envelope approximately 10 times tighter than what’s achieved in a home that’s built to traditional code.
  • Put a lot of care into the design and execution of air sealing (avoiding open vents, etc.), as air infiltration is often the biggest contributor to poor energy performance.
  • Rely on some solar gain and internal gains for heating.
  • Finally, since footers and floor-to-wall junctions are wrapped in insulation, isotherm lines will need to follow the outline of the interior space rather than sneak to the outside.

As for performance, we’ve found that – Polar Vortex or not – it appears that Passive Houses still did not need to run their heating equipment much at all.  Some examples include the following:

As temperatures plunged into the single digits, a Brooklyn row house retrofitted to Passive House standards hovered in the mid-sixties to low seventies, using heat for just a single half-hour period in mid-week (as shown in the graph below).

Credit: Cramer Silkworth
Credit: Cramer Silkworth, www.baukraft.com

Another example is that of the Belfast Cohousing and Ecovillage in Belfast, Maine, where residents were left without power for five days.  “The week that the power went off, we had a morning that was 15 degrees below zero here,” says Mike Shannon (of Belfast).  “But the remarkable thing was that all of these houses maintained a temperature from the mid-50s, close to 60, for those five days.  We felt pretty good about that.  These insulated houses performed terrifically.”  Much more on this topic can be found in the report prepared by Ted Cushman for JLC Online.

This is all happy news for Passive Houses, it seems!  Although we don’t have recent data, the same design principles and likely the same outcomes would also apply to commercial Passive buildings.  We’ll share that data if it becomes available.

Are you interested in learning more about the Passive House standard?  You can join the Passive House Alliance of the U.S., Pittsburgh chapter!  Please contact Chapter Secretary Lucyna de Barbaro to learn more or become a member.  Or attend our monthly presentations, which are hosted at Green Building Alliance on the second Tuesday of each month.

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