Breaking News: TerraHaus Passes Passive House Blower Door Test!

Blower Door (photo courtesy of Minneapolis Blower Door)

The TerraHaus passed its initial Passive House blower door test today! A blower door depressurizes a building and measures the rate of air leakage into the building at a standard test pressure of 50 Pascals. Whereas we might expect a building this size to have 10 air changes per hour (ACH) at this pressure, the TerraHaus has met the standard at less than 0.6 ACH.

How much is 50 Pascals? It simulates a  20 mile per hour wind blowing directly on on all sides of the house simultaneously. To convert this to natural air changes per hour energy auditors would divide this by about 20 in wind-protected sites or 15 for exposed homes in Maine.

For certification, G O Logic will submit the results of a final blower door test when construction is completed  and details such as insulation levels, solar heat gain, and thermal bridging  to the Passive House Institute of the US (PHIUS). PHIUS uses a sophisticated modeling program known as the PHPP to calculate the building’s energy requirement to maintain a 70 degree F temperature.

The successful blower door test is huge hurdle to overcome on the way to certification. It is a testament to both design (kudos to Alan Gibson and Matt O’Malia of G O Logic) and craftsmanship (kudos to the G O Logic construction crew). Congratulations to all involved on this milestone!

Stats: CFM50 = 132     ACH50 = 0.53


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Unity House and TerraHaus

Unity House

This fall, Unity College will boast two cutting edge buildings on campus, Unity House and TerraHaus. How do they compare, and how do they both help Unity promote green building?

The first Unity College building to garner national attention was Unity House, built in 2008. This net zero building was the first college president’s house to earn LEED’s Platinum rating. It was designed and built by Bensonwood Homes. Tedd Benson believes in building for the long term and his Open-Built Concept of “disentangling” systems—plumbing, electrical, framing, siding, etc.—makes for easy upgrades and adaptation over time. Unity House will house its second president and family this July, but the building is laid out in a way that it could someday be converted to multiple classrooms or offices.

Unlike most Bensonwood homes, Unity House’s style has a post-modern ironic feel to it. Rather than confining AdvanTech OSB to its usual hidden use as subflooring or sheathing, Bensonwood’s Hilary Harris, working with environmental educator and presidential spouse Cindy Thomashow, used it as interior trim. The siding includes corrugated metal more often seen on an industrial building than on a $400,000 campus residence. Like several other artistic statements found in the design, though, the trim and siding convey an environmental message, generally about waste re-use or recycling. Advantech is a high quality product made from low quality trees, and the corrugated siding is from recycled metal. Do they work? This innovative building pushes the envelope and invites viewers to answer that question for themselves.

Net Zero

The key sustainable attribute of Unity House, however, is not its recycled content but its net zero performance. Passive solar windows for space heating and a solar thermal system for hot water captures on site much of the energy needed by the home. Its heat pump and all appliances are powered electrically, and its active solar system, a 5.4 kV photovoltaic array, generates more electricity than the building uses.

Grid-Tied Solar

The PV system we use in Unity House is grid-tied. Solar PV systems generate electricity only when the sun is shining. For continuous access to electricity PV systems store energy by generating more than is needed by the building while the sun is shining, and draw from that storage when the sun is not. Off grid systems use a bank of batteries for storage, and these batteries have a number of environmental concerns as well as limited storage capacity. Grid tied systems are connected to the conventional energy transmission system, the grid, which acts as a huge battery for home solar systems. While the sun is shining, the PV system may generate 4-5 times what the house requires and the excess is sent to the grid where others can use it—commonly referred to as “running the meter backwards.” When the solar system is not generating electricity, the house pulls energy from the grid the way any other home does. Through an accounting system called net metering, the electricity supplier credits the electricity that we generate and subtracts it from our electric use before calculating our bill. At present, the utility companies in Maine do not pay small generators of electricity so we don’t benefit financially from the net annual excess we generate (monthly excess is credited to the next month until the last month of the year). Our excess generation during hot, sunny, summer days, however, helps reduce Maine’s peak usage due to air conditioning because prime air conditioning usage and prime solar collection often coincides.

A Different  Design and Message for TerraHaus

The central message of Unity House is that net zero is achievable. To reach net zero, however, superinsulation, superior air sealing and passive solar gain are just as important as the showy PV panels. Having given numerous tours of Unity House, I realized that this part of the message was getting lost. Our visitors tend to focus on the solar panels and the recycled content rather than on Unity House’s low energy requirement. While Unity House does not achieve the energy conservation levels of TerraHaus, it is close. (At less than 5,000 Btu/square foot, TerraHaus will use only 10% of the energy used by code-compliant homes in our region.)

Someday we may add solar photovoltaics to make TerraHaus net zero, but for now TerraHaus unambiguously sets a new standard for energy performance in a residence that will look and feel like it grew out of its rural New England site. Designing to Passive House certification standards for insulation, air sealing and passive heating reinforces the message that before people think about renewable energy they should think about how to reduce their energy consumption. Through careful design, this low energy consumption can be achieved without sacrificing quality of life. We hope that this beautiful residence reinforces that message.


Doug Fox, Director, Center for Sustainability and Global Change, Unity College


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Air Sealing TerraHaus

June 24, 2011


Last year energy audits in Unity revealed that many homes were losing every hour over 1/2 of the air they paid so dearly to heat.  When it is zero degrees outside and 70 degrees inside, the air pressure of the warm air rising equals that of a hot air balloon. Under that pressure, warm air finds every possible opening in the top half of a home to leak to the outside. All of this escaping warm air is replaced by cold air from the outside, leaking in from openings in the bottom half of the home. In addition to this “stack effect,” winds blow in cold air from one side and draw heat out the other.

Energy audit calculations show that weatherization in the form of “air sealing” is the most cost effective retrofit available. Even $200 worth of caulk, foam, and weatherstripping (and a few weekends of work) can cut 10%-25% of a household energy bill. The best way to find these openings is with a blower door and an IR scanner.

The best time to air seal, however, is during construction. You may have seen builders putting down a gasket between the concrete foundation and the 2×4 or 2×6 sill plate. On house the size of TerraHaus this measure alone is equivalent to blocking an eight-inch diameter hole in the wall. If you don’t seal around your windows during installation, you may as well buy cheap single pane windows because you have nearly eliminated any measureable advantage to double-pane low E glass. Just a few years ago, contractors thought they had taken care of the air sealing issue with house wrap. Since then studies have clearly shown that house wrap alone misses almost all of the primary leaks and must be supplemented with caulk, foam and gaskets.

The Passive House standard for air sealing is very rigorous* but cost effective. Alan Gibson of G O Logic estimates that the extra air sealing measures used to achieve the Passive House standard can add $2200 to the cost of a home (over standard, code-compliant construction). This investment, however, leads to about a 27% reduction in fuel use according to his model so the payback period is quite short.

Taped Seams of SIPs

What are some of the air sealing measures used in TerraHaus? Thick polyethylene sheeting was placed under the foundation and extends up the wall where it will be sealed to the sheathing. Each of the seams between the SIPs is sealed. Scissors trusses sheathed on the interior with coated (Zip) OSB create a space between the bottom of the truss and the drop ceiling. This creates a chase for lighting, ventilation ducts and other utilities, eliminating every penetration of the sealed truss except for the bathroom stack. In fact, one of the biggest gaps for air leakage in most homes is the chase along chimneys, air space to keep combustibles from contact with the chimney. TerraHaus needs no chimney because it requires so little heat that a traditional heating plant is unnecessary.  The goal is to achieve a completely air-sealed envelope.

Foundation Poly taped to wall (6/30)

Coated OSB for Sealing the Bottom of Roof TrussesZip Sheathing under Scissors Truss (6/30)

Taped Zip Sheathing (6/30/2011)

All Gaps Carefully Sealed

In another post, I’ll look at ventilation, the flip side of air sealing, and how heat recovery ventilation (HRV) will be used to meet the ventilation standard of 35% air exchange per hour.

*The Passive House standard for air sealing: At the standard test pressure (50 pascals) measured with a blower door a Passive House must allow no more than 0.6 Air Changes per Hour (ACH). This translates to about 0.03 ACH under natural conditions, 17 times less air exchange than found in typical Unity homes. Mechanical ventilation that exchanges fresh air for stale air while transferring the heat energy from the stale to the fresh is necessary in passive house construction.

9/26/2011 update on the TerraHaus Stats: CFM50 = 132; ACH50 = 0.53. Using the energy auditors’ rule of thumb that your CFM50 divided by 10 is roughly equivalent to the total square inches of opening, the total opening would be roughly equivalent to a circular hole with a two-inch radius. Compare that to my home (pre-weatherization) with a CFM50 of 3180, a hole 318 square inches, or a circular hole with a radius of 10 inches!



An inch or so of spray foam in stud cavities  is commonly used as an air sealing measure in stick frame construction. For best results, a third party blower door test and IR scan should be used before drywall is installed but while the spray foam contractor is there to fill the gaps.  The photo below comes from an auditor who works with spray foam contractors. He finds the leaks while the contractor is still there and points them out with a laser pointer. The contractor can then fill the leaks. The dark areas in the photo indicate leakage. To the naked eye, no gaps existed, but the blower door and IR scanner indicated otherwise. We didn’t use spray foam for sealing on TerraHaus (other than a small amount of the foam you can buy in cans at the hardware store) because we wanted to avoid the high global warming potential of the blowing agents in spray foam.


Air Leaks after Spray Foam

Doug Fox, Director, Center for Sustainability and Global Change, Unity College

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Avoiding Thermal Bridging with Structural Insulated Panels

“This is not how my dad taught me to frame a house!”

Then again, dad heated with oil at 25 cents a gallon.

SIP Ready for Installation

The use of Structural Insulated Panels (SIP) is on the rise but it is still fairly uncommon. So why was this type of construction chosen for TerraHaus?

Short answer? To avoid thermal bridging—conductive materials that allow heat to bypass insulation. In standard construction, wood in the form of studs, top plates, headers, etc. rather than insulation may make up over 10% of a wall, and wood is not a very good insulator.

Structural Insulated Panels (SIPs) are one of the key pieces of technology that makes it possible for TerraHaus to achieve its remarkably low space heating fuel requirement, the equivalent of under 80 gallons of fuel oil or ½ cord of wood for 2000 square feet per heating season, less than 10% of the fuel used in a standard code-compliant home constructed in our area.

SIPs are sandwiches of oriented-strand board (OSB) sheathing and some type of foam core insulation. TerraHaus uses a relatively new foam insulation, graphite-coated expanded polystyrene (EPS) by Neopor.  EPS was chosen for its low global warming and ozone potential (see previous blog post). The graphite coating is an innovation that creates a radiant barrier which boosts the R-value of our 8.25-inch panels from R-33 to R-36. After cellulose is added to our cavities, we’ll have an R-value of 50.

Large SIPs Installed with Crane

A SIP is strong enough to be used as a load bearing wall but it is often used over timberframe or stick built construction. TerraHaus is framed with 2 x 4s and, around the south windows, some heavy posts. This framing creates a chase for electric utilities, provides vertical strength to support the second floor and roof, and will be filled with cellulose insulation to add to the total wall insulation.

Again, a thermal bridge is a gap in the insulation that forms the thermal envelope of the house. The problem with standard framing is that wood has an R value of only about 1.4 per inch. In standard 2 x 4 construction this means that every 16” on center you have a thermal bridge of 1 ½” (the actual width of 2” nominal lumber) where heat can bypass the insulation and conduct through the wood rapidly. The real R value of a wall ends up being only 80% or so of the R value of your cavity insulation.

SIPs construction on TerraHaus forms a continuous barrier without thermal bridging from studs. In addition, the seams are minimized because, unlike 4 x 8 sheating, the OSB-covered SIPS are up to 8 feet wide and they run the entire height of the wall. The window and door openings are precision cut at the factory based on the architect’s specifications. After installation, each seam is filled with spray foam and sealed with tape.

Detail of Window Cut Out in SIP

Can you minimize thermal bridging in standard framing? Yes. Many green builders have switched to Advanced Framing. Advanced framing is too complex to go into in detail in this post, but I’ll mention the highlights: 2 x 6 construction 24-inches on center (rather than 2 x 4s at 16 inches), two-stud corners (rather than three), rigid insulation included in the center of headers, minimization of unnecessary cripples and jacks, and single top plates. These changes necessitate some other modifications such as metal bracing and drywall clips on corners, rafters or trusses set directly over studs, and  5/8” drywall rather than ½“. As architect Matt O’Malia points out though, Advanced Framing still results in serious thermal bridging. Rigid insulation on the outside of the sheathing can help, especially when installed over new types of sheathing that can be taped for air sealing.

Doug Fox, Center for Sustainability and Global Change, Unity College


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Not all Insulation Choices are Created Equal…

…if you care about climate change.

Building insulation reduces greenhouse gas (GHG) emissions associated with space heating and therefore has long been a strategy in climate change mitigation. Unity College students learn about life cycle analysis in their Environmental Sustainability course so they and other environmentally-literate citizens know that there is some GHG cost in the production and eventual disposal of insulation that will subtract from these savings.

As it turns out, the GHG emissions from insulation are much higher for some forms of insulation than most of us have probably assumed, and the differences in global warming potential (GWP) for different insulation choices is huge. Environmental Building News (June 2010, Volume 19, No. 6) documents the findings of several researchers on this issue.

The foundation insulation and the SIPs panels of TerraHaus will use variations on one common form of insulation known as expanded polystyrene (EPS), also known as “beadboard.” EPS was chosen because the typical GHG “payback period” (the fuel oil energy savings needed to offset its manufacture) is about 4 years for R-50, the insulation value of the TerraHaus walls. Compare this to other common forms of insulation:

EPS (beadboard)                                                                                 4 years

XPS (such as Corning Pink Board*)                                           98 years

ccSPF (Closed cell spray foam)                                                   80 years

Cellulose                                                                                             ¼ years

Polyisocyanurate (Tuff R or Celotex; foil backed)            3.5 years

*I wrote to Dow about their blue board which historically has had the same payback as Corning’s pink board. I got a detailed and clear answer about the new ozone-friendly blowing agent that they have switched to using, but the answer with regard to the GWP of its blowing agent was much less transparent. I’m still not sure where their product stands. Yes, the energy used in manufacturing the product is quickly paid back in space heating savings, but what about the potency of the blowing agent as a GHG?

 These estimates were modeled for Boston. In warmer climates, the payback will obviously be longer.

EPS for Slab

The problem with XPS and closed cell spray foam (high density spray foam) is in the blowing agents used to expand the products to size. These blowing agents are gases with many times (1000 to 1500 times) the potency of carbon dioxide as greenhouse gases. The estimates used in the studies assume that 50% of the blowing agents will escape over time. Others feel that this estimate is too conservative.

Should we avoid closed cell spray foam completely? Note that the above analysis looks only at the conductive heat loss values. In other places in this blog I have emphasized that both insulation for conductive losses and air sealing for convective losses are important. The GWP in this analysis is probably too hard on spray foam, as spray foam can be used effectively for air sealing; if air sealing was taken into account, spray foam should pay back in less than 80 years. Also, the whole insulation and sealing system should be analyzed because combinations of insulation choices are often used. Spray foam for air sealing should be used in combination with a blower door to find invisible missed spots because spray foam doesn’t air seal quite as well as its users usually assume.

One lesson from this article: if you are concerned about climate change, make sure your architect and builder are green building experts who follow journals such as Building and Environment, Environmental Building News, and Building Science.

TerraHaus Stats:

Foundation:           R=32          8″ EPS @ R4/inch

Wall:                       R= 55      3.5″ BIB@ R4.3/inch; 8.25 EPS SIPs @ R 4.8/inch

Roof                         R=84          24″ DPS@ R3.7/inch

Doug Fox, Director, Center for Sustainability and Global Change, Unity College


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Frost-Protected Shallow Foundations

See the Progress Log page for images of yesterday’s framing work. It’s exciting to see the TerraHaus enter the third dimension.

The TerraHaus design calls for an insulated slab foundation called a frost-protected shallow foundation (FPSF). Standard foundations use a frost wall that extends below the frost line, about four feet below grade. The idea is to prevent frost heaving which would damage the structure. The floor is then poured near the base for a basement design or the space is filled with gravel and poured at the top of the frost wall if the poured floor is to form the bottom floor of the building.

The FPSF uses insulation below and extending a couple feet out from the foundation to prevent frost formation under the building. This not only prevents frost heaves, but it also insulates the floor. Four feet of soil has an R value of about 1, so 6” of foam insulation can easily take care of the frost issue while keeping the floor toasty.

Two basic designs can be used for FPSFs: the Monolithic Slab (also known as the “Thickened Perimeter Slab”) and the Perimeter Grade Beam used for the TerraHaus. The monolithic slab, formed in one pour, has a 16-inch deep x 12-inch wide thickened slab edge poured over and a foot beyond tapered gravel edges. The perimeter grade beam uses insulated concrete forms for the thickened perimeter. Both use a five-inch slab on top of compacted gravel fill.

The diagram below shows a monolithic slab detail, and the two photos show the TerraHaus Perimeter Grade Beam in progress.


Due to the ease of installation and the reduced excavation and concrete costs, a FPSF may save $1500, $3000 or up to $20,000 over a standard full foundation in a residential setting. A standard carpentry crew can do most of the work rather than relying on foundation specialists. Builders like avoiding the deep trench adjacent to the frost wall that they constantly have to bridge over. In many cases, the result is a more thoroughly insulated foundation and floor.

FPSFs are recognized by the American Society of Civil Engineers (ASCE), the International Building Code, the National Association of Home Builders (NAHB), and the Council of American Building Officials (CABO). NAHB has been enthusiastically promoting the FPSF since about 1995.

For more information on Frost-protected Shallow Foundations:



Gibson, Alan. 2010. Super-insulated Slab Foundations. Journal of Light Construction. April 2010. 8 pp.

Doug Fox, Director, Center for Sustainability and Global Change, Unity College


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