The architectural portion of the plans for the new Passive House in Derwood, MD has been approved by the Montgomery County building authorities.   Site plan review is on going.

Concurrently, PHIUS‘s review of the project is nearly complete.   Ryan Abendroth and Lisa White have scrutinized nearly two dozen, densly detailed Excel worksheets that comprise the Passive House Planning Package (PHPP), and have identified several items that needed refinement.   Final revisions should be complete later today.

The long and short of the process is, our initial design was extremely close to meeting the Passive House Standards .  To make the nut, we will have to improve the thermal performance of the building envelope–so for simplicity and economy, we have chosen to add 1/2″ of insulation under the floor slab.

If you are ever suffering from insomnia, call me, and I will tell you all I learned about WUFI-Passive.  It is the blessing (or curse?) of geekdom that led me enroll in a three day long WUFI-Passive training class (sponsored by PHIUS—the Passive House Institute of the United States) in downtown Chicago last week.
passive house geeks

A room full of Passive House geeks — left brains are throbbing  PHOTOS BY DAVID RITTER

Don’t get me wrong—I love Chicago, and I got there a day early to wander the fabulous architectural canyons, and visit a few old friends at the Art Institute, like El Greco, Rodin, and Picasso.  And the class itself was conducted at the nexus of the universe of architecture, in the exquisite High Modern Equitable Building.  One can stand in its plaza at Michigan and Wacker Avenues, and from that single vantage point scan the history the skyscraper—from its inception in the days of Burnham, Jenney, and Sullivan, through its exuberant celebrations of the Wrigley and Tribune Buildings, to the megalomaniac culmination of the Trump Tower.  Fine art and architecture—and Passive House—the perfect concordance of right brain and left.

What is WUFI-Passive?  Simply put, it is the next generation of energy modeling, on track to supersede PHPP (the Passive House Planning Package), which only 3 years ago I had imagined was its apotheosis.  Previous posts tell more about Passive House, so I will leave it to the non-geeks to come up to speed here.  What WUFI-Passive adds to the mix is a comprehensive analysis of the behavior of water vapor as it passes through walls, ceilings, and floors, over a period of years, and modeling of the energy consumed in removing water vapor from a given space.  It then combines this information with another interface that includes all the inputs required for PHPP.  The result is a real-time, dynamic energy modeling system that factors in the full costs of dealing with cooling loads.  This is a marked improvement over PHPP, which could only summarize thermal loads by month or year, and dealt with latent loads in a cursory fashion.


This all goes to the heart of the split between PHIUS and its mother organization, the Passive House Institute.  As Passive Houses began to come on line in the United States, it became clear that PHPP—which was developed in the milder climates of Germany—was not ideally suited for the range of extreme climates here at home.  In class, it was noted that a recent Passive House project in New Orleans disappointed its planners—and occupants—because it failed to provide adequate comfort for its extremely humid climate.  It was telling to hear from Katrin Klingenberg, the founder and guiding light of PHIUS, question some of the fundamental propositions of Passive House.   WUFI-Passive will be the tool that gets us over that considerable hump.


WUFI-Passive—in its present state— is still a bit of a kluge.  Our instructor, Prudence Ferreira, who is likely to be the smartest-person-in-the-room wherever she goes, struggled with the idiosyncrasies of the program, and with class input, came up with a significant list of proposed refinements.  I stumbled through the fast paced class, often losing track of the lecture while I tried to correct some entry on our practice exercises.  Moreover, WUFI-Passive is an expensive piece of software, costing 460 Euros for a one year license.

For the near future, we can still use PHPP to certify our projects—but the future of Passive House is headed down WUFI lane.

In my recent blog “Makin’ WUFI,” I misspoke:  PHPP, the Passive House Planning Package is not free, not even to those who payed to enroll in Passive House training.  When the New York Times or Washington Post makes a mistake, it is published on page two in fine print.  When I make a mistake–particularly about a sensitive issue like intellectual property rights–and even moreso, when the misstatement is reposted in other resources–I want to let people know.

Hence this posting.  Mea Culpa.

It was a gorgeous day to bike out and visit this Passive House project, located in Derwood, MD.  The site is surrounded by forest, and abuts Lake Bernard Frank.   The bike trail passes over the earthen dam, where I saw great blue herons sunning on the berm, and a banded kingfisher perched in a branch near the water.  When I arrived at the site, a red tailed hawk was wheeling right above the house.

passive house rough framing

Here’s a view of the west elevation.  What has been erected so far is the first layer of the wall and roof assemblies, which will function as the structural shell of this Passive House.  The yellow membrane–which extends under the slab–will be lapped over the green sheathing, and taped together with an aggressive, waterproof tape.  Similarly, the reddish brown sheathing on the roof will be sealed to the walls with additional membrane and tape.

Over all that will nail base–a sandwich of foam and plywood.  The walls will get a 3 1/2″ layer, and the roof will get a 6″ layer.  The roof layer will extend about 30″ beyond the outside walls to form the eaves.  This projection will not only protect the walls from the weather, but will also shade the windows from the summer sun, while admitting the winter sun.

Then, a second set of framed walls will be built inside the walls shown in the photo.  they will form a 10″ deep cavity that will be filled with cellulose insulation.  According to PHPP, the walls will achieve R-52.1, and the roof will achieve R-66.1

passive house framing the structural shell


This is a view from the south west.  When complete, the low sloped roof on the left will continue across the porch (defined by the block piers), and wrap around to the right and join the roof where the carpenters are working.


Here are views of the southeast (above) and northeast (below).  A small screened porch will fill in the recess on the right.





Passive House certification by Elysian Energy

Andy Corral of Elysian Energy, getting acquainted with the project. Andy will be certifying that the work complies with Passive House requirements

Now that the rough carpentry is nearing completion, it was a perfect moment to bring out our Passive House Rater, Andy Corral of Elysian Energy, to see things in the flesh.  I spent a good part of yesterday morning compiling a set of plans and documents for Andy to review, including a revised Passive House Planning Package (PHPP) that reflects the field changes and revisions that have taken place.

Some of the changes are minor tweaks–for example, substituting 4″ of polyisocyanurate insulation for 6″ of expanded polystyrene over the primary roof structure.  The former has more than a 50% higher R-value than the latter; even as the total thickness is reduced, the net insulation value of the roof is increased by R-2.

This gain is offset by the substitution of a custom wood front door for the Intus E-Forte UPVC door.  Although the wood door has a smaller window, it has a lower net R-value, because–counterintuitively–the three layers of glass is more efficient than 2″ of wood.  It was almost jaw-dropping, how much this one change added to the Specific Space Heat Demand.

Passive House Rough In Phase

The massing of the Grant Residence is now apparent

Another major change is to enclose what had originally been designed as a screened porch.  This space is intended for three season use, and will not have a heat source, but it will still reduce heat loss through the adjacent spaces.  I’ve asked Elysian to help quantify the energy savings.

An area of some controversy remains.  During the Passive House precertification process, I initially modified some of the PHPP defaults for ground conditions.  These were rejected by PHIUS, and I had to beef up other areas of the building envelope to compensate.  But during construction, we actually improved conditions beyond the initial input.  The entire building subgrade was backfilled with an average of 3′ of crushed stone, and then topped with 4″ of sand to support the subslab insulation.  This is a very effective thermal break with the underlying soil.  Including these values in PHPP makes a significant positive impact.

On the other hand, the local climate seems to be slightly cooler than the PHPP default climate for Washington, DC.  Passing by on my bike this morning, Lake Frank–a few hundred yards southeast of the Grant house–was completely frozen over.

If you’d like to read more about the background of this project, click here.

photo 2

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.