PHIUS+ 2018

December 10, 2018

AirtightnessPassive House Architecture

The Passive House Institute US reviews the metrics and rationale of their standard on a three-year cycle. PHIUS+ 2018 was announced at the Passive House Conference in Boston this September, and I find it a brilliant and positive advancement.

Keep in mind that the Passive House Standard prioritizes energy conservation, so its main measure is the annual and peak consumption of space conditioning (how long and hard the mechanical system needs to work). When PHIUS+ 2015 was announced, it was noted that the space conditioning metrics were set so that the amount of insulation required to meet the standard would be beyond the cost-optimized amount; the 2015 standard was pushing hard on passive conservation.

Critics have been saying for years, however, that too much insulation means diminishing returns—and that instead of spending on added insulation, it would make more sense to put that money into solar PV. Indeed, on several of our projects aimed at net zero energy, we insulated a bit below the Passive House Standard (though we used all the Passive techniques like thermal bridge-free connections, triple glazed windows, efficient ERVs, etc.).

PHIUS seemed to take this (and the urgency of climate change mitigation) into consideration: the standard is now designed to be at local climate-determined cost-optimized insulation levels. While that sounds like the standard is relaxed—and the insulation amounts are, slightly—the Primary Energy metric (total energy consumption) got much more stringent. This means that building compactness and electrical and mechanical systems either need to be super-efficient, or that solar PV will be required. Given the clarity of the metrics, PHIUS+ 2018 really does seem like the best route to net zero and cost-optimized.

Here’s a comparison table of the 2015 and 2018 standard I put together:

PHIUS+ 2015 to 2018 Comparison: Chicago
(for a 4 person house of 1,617 iCFA)
Heat Demand5.89.9kBTU/sf/yr
Cool’g Demand4.37.8
Heat Load56.8BTU/ft2hr
Cool’g Load4.44.5BTU/ft2hr
Source Energy6,2003,840kWh/person

Note that these metrics are location- and building size-specific. Check out PHIUS website: at the bottom of this page is a calculator you can use to see how it would apply to your project. The PHIUS+ 2018 pilot program launched, and by March 2019, it will be the required protocol. I think everyone should be happy about the direction!