Air-tight architecture: Milos Jovanovic on shifting to Passive House principles

Shift House in Hood River (image courtesy Root Design-Build)

BY JULIETTE BEALE

On the second Wednesday of the month, the AIA Committee on the Environment (COTE) invites the design community to bring their lunch to the AIA for an informal “greenbag” lecture. The talks are generally themed around green building and planning with past topics focusing on Net Zero Apartments and community energy production. Milos Jovanovic, of Root Design-Build, presented May’s talk on “Passive House Principles.”

Many Americans may find these airtight houses that are powered on the amount of electricity it takes to run a hairdryer to be cutting edge. Yet, the Passivhaus concept and subsequent Passive House Standards were developed more than twenty years ago in Germany.  The homes rely mainly on solar gain for heating and are passively cooled during warmer months. These passive strategies draw roots from vernacular building. Jovanovic showed a slide of a cluster of traditional homes built into an Icelandic hillside.  Like these homes with their heavy sod roofs, Passive Houses use high thermal mass and insulation to regulate indoor temperatures. Both types of dwellings are oriented to capture solar gain from winter sunlight.

There are only three main guidelines for achieving Passive House certification. First, primary energy use from electricity, hot water, and heating/ cooling must be capped to120 kWh.  Basically this requires that there be no mechanical HVAC unit. Instead,  many of these homes have solar hot water heaters and the spaces are heated passively by occupants, equipment, and solar gain. Second, and most controversial, the home must be airtight with no more than 0.6 air changes per hour. To achieve this, all penetrations must be carefully sealed and a Heat Recovery Ventilation unit (HRV) is installed to strictly control air exchanges. Third, space heating and cooling must each not exceed 15 kWh.  The HRV doubles to provide the extra heating and cooling needs.

Unlike LEED and its prescriptive system of credits, there are no rules, strategies or technologies specifically stipulated by the guidelines. To achieve the low energy use standards, however, typically demands the use of super-insulation, high thermal mass, triple paned glazing, insulated doors, an HRV, low energy appliances, and elements that help regulate the exterior climate such as vegetation and reflective surfaces. The Passive House Institute has developed its own energy modeling software where designers input the values from wall systems, windows, and foundations to determine whether the house is meeting the standards and which areas need to be improved.  This is followed up with in-field testing to insure the model’s accuracy.

Shift House in Hood River (image courtesy Root Design-Build)

In these days where climate change, peak oil, and fears of nuclear fall-out are growing concerns, Passive Houses sound like a great panacea to the energy crisis.  The typical Passive House uses a mere 15 kW per year.  When you compare this number to the average US home, which uses 130 kW per year, it quickly becomes apparent why Jovanovic sees this as the next revolution in the field of green building.

The economics of building and operating a passive house are equally intriguing.  A major goal for Jovanovic is to achieve Passive House Standards while on the budget of a typical home.  He is in the midst of doing this with Shift House, a 2,200 square foot Hood River residence with a budget of just $330,000.  The added costs associated with the efficient walls, windows, and doors are made up for by omitting the mechanical system.  Shift House is designed using quality products that last longer and need to be replaced less often to help reduce maintenance costs.  Lastly, the long term energy savings attributed to minimizing heat loss through the envelope help offset the more expensive cost of the wall systems.

Based on these points, I am a little surprised I haven’t seen more Portlanders rushing out to get triple paned windows or demanding architects use structural insulated panels (SIPs).  Yet as a practitioner in an office that has a great deal of respect for traditional details, I can see why we are skeptical of this technology.  Like the members in the audience, I found myself asking, “Where is the dew point in the twelve inch wall? How does the rainscreen get attached?  If the house must be airtight, how do you ventilate the roof? Can you really limit all thermal bridges? Are the windows operable?  Is the bathroom, at least, ventilated?”   

For many folks, the technologies are still a little unsettling.  Most of the products Jovanovic has used in his own projects, from windows to building tape, must be ordered from Europe. The construction practices and materials are new to contractors and require a learning curve. Jovanovic showed an image of a crane hoisting the large roof SIPs into place, a construction technique that appeared costly and cumbersome for a single-family residence.  After so much attention has been focused on Sick Building Syndrome, the concept of an airtight house, even with an HRV, still sounds like an unhealthy environment.  What will it take for us to shift our paradigm and be comfortable with these diagrams and figures?

Jovanovic patiently answered each of the questions from the audience with assured confidence. He described the architectural details and new systems he used in Shift House, that are frequently employed in other Passive Houses, in response to people’s skepticism.

In detailing Shift House, Jovanovic solved the problem of thermal bridges by separating the structural system from the insulation.  An eight-inch-thick structural insulated panel (SIP) forms the basis of the wall system by acting as both insulation and structure.  (SIPs are composed of two OSB panels with rigid insulation sandwiched between them.) A four-inch layer of rigid insulation is attached to the outside of the SIP to limit any thermal bridges at framing joints, openings, and additional structure. The combined layer of rigid and SIP creates, in essence, two cavities which can house mechanical, plumbing, electrical, and other structural elements. Siding and battens, making up the rainscreen, are attached by long screws that tie through the outer layer of rigid insulation and into the OSB panel of the SIP.  The overall wall system is rated at R-40.

The walls are not all that is super-insulated. A great deal of heat loss in the typical home is attributed to exposed foundation walls.  In Shift House, the concrete rat slab is topped with a vapor barrier and then a nine-inch layer of rigid insulation. A final skim coat of concrete forms the finish floor and adds more thermal mass to the structure. For the roof, twelve-inch thick SIPs are positioned into a gable form and a six-inch layer of rigid insulation is added to the outside bringing it to R-72, nearly double what is required by Code. Jovanovic noted that the roof is ventilated by an air gap between the outer layer of rigid and the standing seam metal roof, allowing the interior to remain airtight.

Many audience members expressed concern about the consequences of an airtight envelope. Jovanovic asserted that passive houses like Shift House have as much clean air entering as regular houses.  The only difference is that air is let in at a controlled rate by the HRV. Moreover, the house functions like the average American Home. There may be subtle differences in how the systems operate, but the outcome is the same. The kitchen range hoods must be vented with special systems, but they still vent to the outside. The windows may be triple-paned and thermally broken, but they are operable. In many cases, the materials cost the same as traditional products of the same quality. Jovanovic noted that Shift House’s European windows cost the same as Marvin’s triple paned line.  

While everyone in the green building community can be excited about Passive Houses’ energy efficiency, I can’t help but getting an image of a huge Styrofoam container with a house inside. How much embodied energy does it take to produce these walls in comparison to the energy they save? How many petrochemicals? Passive House Standards force designers to consider the environment and climate in which they are building in order to determine the best passive strategies. They allow for functional homes with beautiful spaces. I agree that in the near future it will be a necessity for homes to operate on the amount of energy Passive Houses use. Yet, it seems like an oxymoron that these homes are often built using materials derived from the same resources the homes are attempting to reduce demand on through their energy efficiency. I applaud Passive House, but hope that it evolves to include a standard that addresses embodied energy.

Juliette Beale is a designer with Portland's Emerick Architects and a member of the American Institute of Architects/Portland Committee on the Environment. COTE Greenbag talks are held the 2nd Wednesday of the month from noon-1pm at the AIA Center for Architecture, 403 NW 11th Ave.  The next talk, scheduled for June 16, is entitled “Urbanize and Insulate: The Development of a Fossil Fuel Exit Strategy ” and presented by Dylan Lamar of Green Hammer Construction.

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Comments

[Zacblodget]

It still bugs me that people make the argument that a passive house is too airtight, and its better to rely on sloppy construction and bathroom fans instead. If it gets too stuffy, open a window like you would on any other house!
And to be clear, the SIP panel is only one way of framing a passive house. A double stud wall with dense packed cellulose is another approach that works well, and is both common and easy to build.
And lastly, I think our reliance on "traditional details" has more to do with laziness than their actual effectiveness. It takes time to run a model in WUFI to determine where the dew point occurs in a wall design and whether it will dry properly, but that doesn't mean it shouldn't - or can't - be done. A little bit of extra thought and care by the designer isn't that big of an expense relative to the payoff in energy savings over time.
sorry for the rant!

[Bruteforceblog]

Yikes...

A few misconceptions worth noting:

Not all Passivhaeuser rely on solar gain for heating – in cooling-dominated climates battling internal heat gains is hard enough, solar gain just increases cooling demand. Additionally, not all Passivhaeuser in heating-dominated climates are able to utilize passive solar gains, and have to rely on minimizing losses (usu. better glazing/more insulation) and maximizing process energy (from internal sources) and supplemental heating.

Not all Passivhaeuser rely on high thermal mass – in fact many don’t have much mass at all, unless you count 5/8” GWB to be high thermal mass. There are some PH designers who do utilize high thermal mass, but those are few and far between.

You can build a Passivhaus with products here in the US, although the window manufacturers are light years behind Europe and don’t seem to be in a hurry to catch up.

The embodied energy and operational energy of a Passivhaus is far below that of the embodied energy + operational energy of a code minimum house (and even most LEED projects). This is because operational energy of a typical building is about a factor 10 greater than embodied energy (and this is before operational energy source factors are calculated).

While there are several Passivhaeuser surrounded w/ petroleum-based foams, it’s hardly an oxymoron. To me, the biggest ‘green’ irony is spending hundreds of hours to reduce the embodied energy of projects by maybe 15-20%, while doing very little to fix the larger issue - curbing operational energy and their subsequent CO2 emissions. Or utilizing HCFC-based spray SPF and calling it ‘green’.

Lastly, there are oodles of Passivhaeuser built of straw, rammed earth, cross laminated timber, clay brick, and wood stud with cellulose – all having significantly lower embodied energy than one with petroleum-based insulation. All of these projects significantly outperform ~95% of LEED projects.