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A Passive House Reality Check

passive house certification stampMy previous post, “A Post-Passive House Paradigm for Energy Efficient Design” was recently re-published in The Green Building Advisor. ¬†This generated some lively follow-up; you can check them out here.

As it turns out in these dialogues, they lead you to reconsider your own assumptions, and things that you have been taught. Here are two notions I came away with from that experience:

This dialog brings to bear the nagging issue that underlies its topic–that is, the inherent fragility of what we build due to its vulnerability to moisture.

In my original blog post, I omitted the discussion that preceded the “epiphany.” What I said to Bill Updike that evening–spoken arguably in desperation–was that perhaps we are veering off into the weeds with our pursuit of these immaculate levels of airtightness and wall thickness, and that maybe the best approach for affordable, mass produced housing–a resource that is substantially denied to upcoming generations–maybe the best approach is some good prescriptive formulas based on regional climate, and most importantly, with a focus on vapor management.

Steve’s observation of the kid’s running in and out of the house all day reinforces that notion. And just imagine if the kids have a dog they need to coax along with them.

More fundamentally, we need to take a reality check on our own, personal requirements for thermal comfort. My mother grew up in an affordable mass produced home built at the brink of the Great Depression. The house included a state of the art device for mitigating latent loads during the swampy summers of Washington, DC: a sleeping porch. It would not provide ASHRAE levels of thermal comfort, but it damn sure was net zero.

You can see where this is going. Maybe the problem is not so much in calibrating the astonishing technology of ultra efficient minisplits, but in recalibrating our expectations, and our exaggerated sense of entitlement…


dropping back to illustrate my point that our expectations matter–for study purposes, I did some tweaking of my PHPP file for the Grant residence, changing the summertime overheating value to 80dF from 77, and the winter interior design temperature to 66dF from 68. I keep the T-stat in my own uninsulated home (it’s a high rise condo–I have no access to the wall cavities) set to 66, and I’m comfy with a fleece sweater on . In a modern house, with modest insulation in the walls, and low e- glazing, radiant heat loss is significantly defeated, further raising comfort levels.

The effect of these changes on the PHPP verification page is significant. Specific space heating went down to 3.95 kBtu(ft^2yr) from 4.73. Specific cooling demand was cut in half, to 1 kBtu(ft^2yr) down from 2.

Specific primary energy load (energy consumed at the generating station, and including line losses) went down to 33.8 kBtu(ft^2yr), down from 35.0. That is the equivalent of 610 kWh (or 226 kWh at the meter of the house)

looked at another way–it would have allowed the house to achieve PH certification with 2 1/2″ of subslab insulation, instead of 9″.

None of what is suggested here is a hardship. I think it is reasonable for PHIUS to consider giving people the option of defining their own comfort standard. We are all grownups, I presume.