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Green Building Curmudgeon

Passive House 2021+ Now Offers a Streamlined Prescriptive Path to Certification

A quick-and-dirty outline of alternative measures for reaching PH standards

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Although not Passive House certified this home meets many of the criteria of the 2021+ Prescriptive Path PHIUS Prescriptive Calculator for Climate Zone 3

The latest Passive House Institute U.S. (PHIUS) standard is now available for review, with a public comment period open through December 7, and an expected release early next year. Although I have been a Passive House rater for about 10 years, the lack of interest in my area (Atlanta, Climate Zone 3) has kept me from completing any projects.

Cost has been an issue with single-family homes. In addition to the high cost of custom construction, there’s the required services of a Certified Passive House Consultant (CPHC) and the necessary WUFI modeling, which can put the cost of PHIUS certification out of the range of many interested homeowners. Apparently PHIUS has recognized this, and in response, has issued a new Prescriptive Path for single-family homes, duplexes, and town houses. This path eliminates the requirement for a CPHC and the attendant modeling, although it is still suggests a consultant can help a project meet certification requirements.

The new Prescriptive Path is a two-part process. First, an online calculator requires a few entries to determine minimum R-values and maximum window U factor. Looking at a house in the Atlanta area, some of the requirements might be considered extreme, but this is Passive House, so you have to expect a lot of insulation. They include a window U factor maximum of .25, R-31 for cantilevered floors, R-61 for roof or ceiling insulation, and R-20 for slabs and floors over unconditioned space. I can’t find clarification on the slab insulation. If it is required to be continuous underneath the slab, it seems like overkill for CZ3, but if it’s only slab edge, I guess I can live with it. Optionally, providing a calculation that meets the same overall UA as the prescriptive requirements allows for tradeoffs. Ventilation systems require minimum 66% sensible and 60% total recovery; and heat pumps must achieve an HSPF of 9.6 and SEER of 18.

The Prescriptive Design Review checklist is four pages, plus some footnotes. It will look familiar to anyone who has used ENERGY STAR rater checklists. Some key requirements are essentially just good practices including a total allowable home floor area of 900 sq. ft. per bedroom (not unreasonable), continuous exterior insulation or equivalent through SIPs, ICFs, or double-wall framing; ENERGY STAR, Zero Energy Ready, and Indoor Air Plus certification; and maximum leakage of 0.04CFM50 per sq. ft. of envelope area. Restrictions include no fossil-fuel-fired equipment, no jetted tubs, and no natural draft fireplaces.

There is a “Compactness” requirement that I find appealing because it pushes designers toward simpler shapes without being overly restrictive, although I expect many architects would bristle at the limitations. Compactness options include a “Simple Form” or a “Unique Form.” Simple Forms require that all floors align with no overhangs or upper level insets—simple, straightforward, and efficient. Unique Forms require a maximum variation of 35% of the floor area between levels, and a limit on “reentrant corners.” Honestly, I don’t understand the details of this calculation but conceptually, it makes sense to avoid unnecessary corners for overall material and energy efficiency. In my house I followed these basic concepts. I guess great minds think alike.

Under the new Prescriptive Path, the window to wall ratio must be 18% or less, and skylights can be no more than 3% of the roof area. Regarding north and south glazing: Each must be no more than 40% of total glazing; and east and west glazing can be no more than 20% of total each. Alternatively, a Manual J report showing adequate exposure diversity is acceptable. Overhangs are required on south-facing windows in Climate Zones 0-2.

Condensation resistance is about the only place where I get completely lost. All opaque building-envelope components must meet requirements found in Appendix B, where I see pages of impenetrable charts and calculations. (Some better guidance and a calculator tool would be very useful.) Window condensation resistance should be available from any good manufacturer.

Ventilation calls for the required recovery efficiencies, insulation on ducts connected to the exterior, and limited duct lengths—all good practices. HVAC must not use electric resistance for heating, and fans and dehumidifiers must be ENERGY STAR labeled. Lighting must meet a maximum lumens/watt, and appliances must meet specific efficiency levels. Water heaters must be installed in conditioned space, and efficiency requirements imply that they have to be heat pumps. Alternatively, a performance tradeoff calculation can be used in lieu of individual requirements. Finally, homes must be EV-ready.

I find the Prescriptive Path checklist fairly straightforward (with the noted exceptions), although some of the requirements could be confusing to anyone unfamiliar with high-performance construction. Passive House U.S. seems to be following the path of other green programs, such as NGBS 2020 and EarthCraft House, in moving to include a Prescriptive Path. I am a strong believer in keeping things simple, and PHIUS has made a big step forward with this new program. I expect that it will bring more homes into the program as it streamlines the process and reduces consulting fees. I encourage anyone who is interested to review the PHIUS 2021 documentation and submit comments before the December 7 deadline.

Now that I have been complimentary, it’s time for the curmudgeon in me to come out. I think the required insulation values for Passive House are pretty extreme, particularly in moderate climates such as mine. I have been tracking the energy use in my house for over three years, and I have an EUI of around 14.5, which is as good or better performance than most PHIUS-certified homes, and I have a lot less insulation than PHIUS requires.

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-Carl Seville is a green builder, educator, and consultant on sustainability to the residential construction industry. After a 25-year career in the remodeling industry, he and a partner founded SK Collaborative. Image courtesy of the author.

4 Comments

  1. Antonio Oliver | | #1

    Did I miss the Climate Zone 3 wall insulation requirement in the article?

  2. Rick Evans | | #2

    Great Article- Thank you.

    There is much to love about this. I really like the R-value calculator and checklist approach. Framing fractions are all there in the footnotes. Really, really good stuff.

    But no jetted tubs? Looks like you are sitting this one out, Jevons.

  3. User avater Expert Member
    Carl Seville | | #3

    Antonio - Walls are R-31, same as cantilevered floors, look at the image of the calculator in the gallery.

  4. User avater Expert Member
    Armando Cobo | | #4

    Great summary Carl, thank you. I believe the bottom line comes down to ROI for most of us. If any excessive requirements do not support a good investment, it may not be worth doing a PHIUS project. I for one have admired folks that want to do these projects,, its awesome, but not for the mainstream.
    I do applaud the PHIUS for coming up with a Prescriptive Guideline, and an easy to understand Design Review Checklist. It'll save costs and headaches to many.
    See: https://www.phius.org/PHIUS+2021/PHIUS+%202021%20Prescriptive%20Design%20Checklist.pdf

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