Kicking the Tires on a Passivhaus Project
The first in a series of articles on planning and building a single-family Passivhaus in Maine
[Editor's note: Roger and Lynn Normand are building a Passivhaus in Maine. Their goals are modest: “Passivhaus, LEEDLeadership in Energy and Environmental Design. LEED for Homes is the residential green building program from the United States Green Building Council (USGBC). While this program is primarily designed for and applicable to new home projects, major gut rehabs can qualify. Platinum, net zeroProducing as much energy on an annual basis as one consumes on site, usually with renewable energy sources such as photovoltaics or small-scale wind turbines. Calculating net-zero energy can be difficult, particularly in grid-tied renewable energy systems, because of transmission losses in power lines and other considerations., universal access, and sustainable.” This is the first article in a series that will follow their project from planning through construction.]
There is small but broad-based cadre of building professionals in southern Maine who are very interested in pursuing green building initiatives. Each month, Maine Green Building Supply brings together interested architects, engineers, builders, energy auditors, insulators, solar installers, and other building trades at its store in Portland to discuss a broad spectrum of green building topics. One month last year, the topic up for discussion was our Passivhaus plans.
The April 5, 2011 meeting began with an informal social of beer, hot dogs, chips and other nutritious delights (not a canapé in sight). Our architect, Chris Briley, came prepared with architectural scale (1/4″ = 1′) drawings of our project. For some 2 1/2 hours, the 50 participants questioned, poked and prodded Chris on the whats and whys of the German Passivhaus building standard in general, and the particulars of our project.
Energy modeling shows that the heating load is high
While there was a good-natured banter back and forth, the questions were both perceptive and probing. No softball pitches here! It was sobering to have heard earlier in the day from Marc Rosenbaum, our Passivhaus consultant, that the initial heating load calculation was 7.7 KBTU1,000 Btus/sq. ft./yr. versus the Passivhaus standard of 4.8 KBTU/sq. ft./yr. The design is reasonably close, but not up to standard yet.
Here’s a sampling of the discussion:
- Why do most Passivhaus buildings look so ugly (homes designed as rectangular boxes are more economical to insulate to the very high Passive HouseA residential building construction standard requiring very low levels of air leakage, very high levels of insulation, and windows with a very low U-factor. Developed in the early 1990s by Bo Adamson and Wolfgang Feist, the standard is now promoted by the Passivhaus Institut in Darmstadt, Germany. To meet the standard, a home must have an infiltration rate no greater than 0.60 AC/H @ 50 pascals, a maximum annual heating energy use of 15 kWh per square meter (4,755 Btu per square foot), a maximum annual cooling energy use of 15 kWh per square meter (1.39 kWh per square foot), and maximum source energy use for all purposes of 120 kWh per square meter (11.1 kWh per square foot). The standard recommends, but does not require, a maximum design heating load of 10 W per square meter and windows with a maximum U-factor of 0.14. The Passivhaus standard was developed for buildings in central and northern Europe; efforts are underway to clarify the best techniques to achieve the standard for buildings in hot climates. standards)?
- How do you translate the Passivhaus heating/cooling and total source energy standards to units that are easier to understand? (For the former, a 2,000-square-foot home can expect to use only 100 gallons of oil per year for heating, which is dramatically less than other Maine homes; the latter standard for source energy is harder to compare.)
- Are there follow-on tests that confirm the continued air tightness of Passivhaus homes in the years after they are certified and occupied? (Post-occupancy testing has just started; some early results indicate that performance drops slightly in the years following certification, perhaps as a result of occupant behavior, drying of building components.)
- Does it make sense/”cents” to achieve the full Passivhaus standard if it is cheaper to install some active alternative source (e.g., solar) to achieve that same level of energy efficiency? (Hmmm....)
How durable is Zip System tape?
There were numerous comments regarding the design of our home:
Suggest changing the perimeter drain in the room designated as the study so that it is at the same elevation as the footer (yep – drawing showed it just below grade)
Do you plan to include fly ashFine particulates consisting primarily of silica, alumina, and iron that are collected from flue gases during coal combustion. Flyash is employed as a substitute for some of the portland cement used in the making of concrete, producing a denser, stronger, and slower-setting material while eliminating a portion of the energy-intensive cement required. More info into the concrete to reduce the carbon penalty for concrete production and increase strength (will investigate it’s availability – expect that will contribute to LEED points)
Consider an alternate means for subslab insulation that runs the EPSExpanded polystyrene. Type of rigid foam insulation that, unlike extruded polystyrene (XPS), does not contain ozone-depleting HCFCs. EPS frequently has a high recycled content. Its vapor permeability is higher and its R-value lower than XPS insulation. EPS insulation is classified by type: Type I is lowest in density and strength and Type X is highest. insulation continuously below the slab and between the footer and the ICFInsulated concrete form. Hollow insulated forms, usually made from expanded polystyrene (EPS), used for building walls (foundation and above-ground); after stacking and stabilizing the forms, the aligned cores are filled with concrete, which provides the wall structure. basement walls (will consider)
Consider using Grade D building paperTypically referring to Grade D building paper, this product is an asphalt-impregnated kraft paper that looks a lot like a lightweight asphalt felt. The Grade D designation has come to mean that the building paper passes ASTM D779 (minimum 10-minute rating with the “boat test”) and different products are called out as “30-minute” or even “60-minute” based on D779 results. At times confused with roofing felt, roofing felts and building paper differ in two ways: felts are made of recycled-content paper, building papers of virgin paper; felts are made of a heavier stock paper; building papers a lighter stock. See also roofing felt. instead of Tyvek or Typar. It’s much cheaper and just as effective (hmmm...)
Consider adding fibermesh to strengthen concrete for floors and reduce chances of cracking (will consider)
Consider alternate means of attaching the floor joists to the ICF walls to achieve greater insulation at that weak point in the building assembly. Options discussed included using Simpson strong ties, using an ICF course with an interior brick ledge to support the floor joist (will consider)
Consider wrapping peel-and-stick membrane around the top of the main floor top wall plate as a means on providing a continuous air barrierBuilding assembly components that work as a system to restrict air flow through the building envelope. Air barriers may or may not act as a vapor barrier. The air barrier can be on the exterior, the interior of the assembly, or both. from the top of the sheathingMaterial, usually plywood or oriented strand board (OSB), but sometimes wooden boards, installed on the exterior of wall studs, rafters, or roof trusses; siding or roofing installed on the sheathing—sometimes over strapping to create a rainscreen. to attic (will consider)
Adding Tyvek/Typar is cheap insurance against long term tape failure at the joints of the Zip wall sheathing (agreed — that’s why it’s included in the wall cross-section)
Liked the high-compression insulation as a means of creating a thermal break between the concrete interior of the ICF wall and the concrete footer (engineer participants confirmed that the rebar ties between the ICF and footer will transfer the weight between these two sections of concrete – be sure to specify the proper size rebar!)
Probing questions yield a better design
I thought Chris did a masterful job presenting the details of our project and fielding comments. I am thankful for the insights and suggestions offered by participants in helping make our project even better.
Kudos to Maine Green Building Supply for initiating and sponsoring these monthly sessions. I look forward to attending future sessions. Knowledge is a powerful force!
- Roger Normand
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