Moving Heat in TerraHaus with a Heat Pump and HRV

October 2012 (Photo credit: Jonah Gula)

Transition time. This weekend our first ten students will move into TerraHaus. Other than a post I’m still writing comparing the construction cost of TerraHaus to the cost of other residence construction on college campuses, I have completed posts on the major features of TerraHaus. All but a couple of these posts are listed in the box to the right. The others can be seen by hitting “older posts” at the bottom of this page.

I don’t have cost figures ready for public consumption yet, but preliminary data indicate that this type of construction is very cost effective for college campuses–much lower cost per square foot and per student than reported for other new residence hall construction in the Northeast, even before we consider the savings due to energy bills in the neighborhood of $30-$50 per year per student.

The last two green building components of TerraHaus I want to add to the blog are the Heat Recovery Ventilator and the Heat Pump.

Our Daikon Heat Pump

The name of the Passive House standard is derived from the emphasis on passive heating. Passive heating means that the space heating requirements are met largely through passive means—the natural sunlight coming through the windows, the heat generated by appliances in the course of routine daily activities, and even the warmth given off by the residents.

The rest of the heat needed to keep the building at 70 degrees is generated by small electric baseboards and by a heat pump. Heat pumps are more common in the southern US than in the north. They have the advantage that they can be used to cool air in the summer and heat it in the winter. They are more efficient than electric heat or combustion heat because they are not creating heat but rather are moving it from one body of air to another.

A heat pump is, in a sense, a type of solar energy because it is the heat from air warmed by the sun that is moved into the home for space heating:

…outside air is heated by the sun (even what we consider cold, winter air contains heat energy given to it by the sun).

…the air is drawn through an evaporator where it warms a refrigerant like freon into a gas even at low temperatures. The refrigerant in a gaseous state is compressible.

…as a compressor reduces the volume of gas, the temperature goes up.

…the gas is transferred to a unit in the house where it condenses and releases its heat.

The heat pump circulation can be reversed to cool the house in the summer.

Given that the heat pump is not creating heat but simply moving the heat energy around, it is very efficient. One kWh of electricity for the fan and compressor results in 2.74 kWh of heat. (As our Sustainable Energy students on campus can explain, COP is the Coefficient of Performance and is calculated as the Btu of output produced divided by the Btus of electricity used. Our COP = 2.74)

In addition to the heat pump, individual bedrooms have small sections of electric baseboard which the occupants can set themselves.

HRV—The Magic Box

Once the heat is captured in the house, from passive means or from the heat pump, we want to hold onto that heat. Air infiltration is one of the major forms of heat loss in most homes, but we benefit from air infiltration too because it ventilates our homes. TerraHaus is so tight that we rely on mechanical ventilation. In order to hold onto the heat though we want to extract the heat and use it to warm the incoming air.

To do this we use heat recovery ventilation (HRV). The stale air passes through an aluminum plenum which absorbs its heat. Air entering from the outside passes through separate passages in the plenum and picks up heat.

The Magic Box

Our HRV unit, a Zehnder CA 550, is rated as over 88% efficient meaning that if it is zero degrees outside and 70 degrees inside, the incoming air is warmed to a temperature of 62 degrees before it enters our rooms. No wonder Alan Gibson of GO Logic estimates that 10% of the 90% energy savings of TerraHaus comes from the HRV! (This also explains why he refers to the HRV ventilator as “the magic box.”) The ventilator and flexible tubing ductwork is housed in a space between the ceiling and the bottom of the scissors truss system. The trusses are sealed on the bottom with Zip sheathing. This puts the HRV within the thermal envelope with just two perforations of that envelope for intake and exhaust.

The whole house is ventilated by this system, but the duct work and vents are set up to pull stale exhaust air from bathrooms and the kitchen while supplying fresh air through the bedrooms and living areas. The system is designed to assure that TerraHaus meets the ASHRAE standard of 35% air exchange per hour or 15 cubic feet per minute per occupant. Most homes in Maine, regardless of their age, meet this requirement, but most do so through natural leakage of heated air through the upper portions of our homes and intake of cold outside air from leaks in the lower portions of the building.

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

Solar Hot Water (aka Solar Thermal)

Solar Evaculated Tubes on Roof (Jesse Pyles Photo)

The domestic hot water needs of TerraHaus residents—showering, washing—will largely be met through a solar hot water system designed and installed by ReVision Energy, a local leader in solar energy employing several Unity College alums.

TerraHaus will use a closed loop system consisting of four 30-tube collector arrays that circulate food-grade propylene glycol from the collectors to coils in two water storage tanks and back up to the collectors. As the glycol passes through the coils located in the bottom half of the tanks, the heat is transferred to the water. An electric resistance coil in the top of each of the tanks serves as a back-up that kicks in automatically if the water in the tank drops below 110 degrees F. Circulation of glycol stops if the temperature in the collector is less than 20 degrees higher than the temperature in the bottom of the tanks to prevent the circulating glycol from cooling the tanks.

Hot Water Storage Tanks

Circulation Pump

I have the same system on my home, except that I have only 60 tubes and one tank. I pick up some shade to the east and west in the fall, winter, and spring. Still, I have saved as much as 270 gallons of oil per year  (oil is my backup rather than electric). I did a net present value calculation on my system, using very conservative estimates for the future cost of oil (I used a ridiculously conservative  assumption that oil would never get above $4/gallon) and 20-year treasury bonds as my alternative investment, and I estimated that I’ll easily make $8,000 over the next 20 years, with a 7-8 year payback. (To be clear, that is $8,000 in profit over what I would have made from treasury bonds, and after paying off the system.)

In an era of global change and uncertainty about future oil costs, thinking about resilience in our home and work systems is prudent. The cost of heating water with solar is independent of the price of fossil fuels, hence adding resiliency to our residential systems while also mitigating climate change. Leaving aside the alternate investment calculations that a financial advisor might want me to make, it gives me comfort to think that in my home I have, in a sense, pre-paid for 250 gallons or so of heating oil per year for the next 20 years or more at $1.40/gallon.

(250 gallons per year savings x 20 years = 5000 gallons @ $7,000 installed cost (after incentives and rebates) = $7000/5000gallons = $1.40 per gallon.)

I’m proud to have several Unity alums involved in this project: Matt Wagner on the installation, John Luft as General Manager in the Liberty branch which did the design and installation, and Brett Irving back at the garage as support. And I look forward to alum Brian Byrne’s contribution when we contract with ReVision to install a PV system to make TerraHaus net zero!

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

If I Could Modify the Passive House Certification Standards…

…I would give credit for efficient use of space.

8/18/2011

8/19/2011 The GO Logic crew has left, the Certificate of Occupancy has been authorized, and Unity College staff are nearly finished setting up the furniture. Ready for our students on time!

The Passive House standard is the highest international standard for building energy efficiency. While it does not address some of the sustainability issues covered by the LEED standards, I hope that it influences LEED to adopt higher standards for energy efficiency than the USGBC currently promotes.

The Passive House certification requirements speak to minimizing thermal bridging, set a very high standard for air infiltration, and of course, require buildings to use 90% less energy for space heating than similar buildings.

From an energy conservation standpoint, though, the Passive House standard misses one key energy issue: efficient use of space.  The standard uses an energy per floor area measurement (<5,000 Btu/square foot) rather than an energy use per person measurement. While it may not be an issue in Germany, the home of Passivhaus, homes in the United States are getting larger all the time. Ironically, some of the most boasted about energy efficient homes in America may be 3000 square feet or more and occupied by only two people. On a square foot basis, the energy use is low, but it is much larger than it needs to be for comfortable living and hence the homes use far more energy than is needed.

Unity College and GO Logic went beyond the Passive House standard by seeking to provide comfortable housing for 10 students in 2186 square feet. After the first year, we will evaluate whether we have hit on the optimal number, but certainly we have achieved high density occupancy. Some of the design features used to achieve high occupancy, and, therefore, low energy use per student, include:

• Careful attention to acoustical separation to provide privacy in bedrooms and bathrooms.
• Open design for the kitchen, dining area and living area
• Generous mudroom space with “cubbies” for outdoor gear
• Use of white paint and large windows to increase the feeling of spaciousness
• Separated shower and toilet facilities for efficient privacy
• Individual thermostats in each bedroom
• Good connection to outdoor spaces

I wouldn’t ask the Passive House folks to shift away from an energy per unit area requirement, but perhaps a provision could be added to modify the energy requirement for apartments and college residence halls designed to accommodate more than one person in 500 square feet. The effect on energy conservation would not be reduced because building footprints would shrink.

Not only would this accommodation send an important message about right-sizing buildings, but it could make the passive house standards more attractive for builders of apartments and college residences by increasing the options for appliances such as dryers and range hoods that carry the commercial ratings necessary to meet apartment fire codes. Unity College was happy to make the extra effort to overcome these hurdles so that TerraHaus can meet the Passive House certification standards. Once we are certified, though, we hope we will have earned a place at the table where these standards are discussed, reviewed and promoted.

Douglas Fox, Director, Center for Sustainability and Global Change

Solar Orientation and Thermal Mass

8/11/2011

As I have discussed in previous posts, windows with low U-value (or high R-value, the inverse of U-value) and high solar heat gain coefficient are necessary for passive solar gains. Two other features of good solar design—solar orientation and thermal mass—are the topics of this post.

Solar Orientation

If we pay attention to the sun’s movement, we can readily see why our TerraHaus designers placed windows where they did. The goal is to maximize solar gain in the winter and minimize solar gain in the summer.

We can describe the sun’s position in two ways: the altitude above the horizontal and the direction, measured in compass bearings or azimuths. As shown in the diagrams below, the sun in the northern hemisphere moves from sunrise to sunset through the south rather than directly overhead. In the winter, the sun’s altitude remains low and the sun comes up in the southeast and sets in the southwest. In the summer the sun reaches a higher altitude, and it rises and sets much further north of east and west than in winter. (Click to enlarge.)

Usefully, this means that in the summer, the roof and the east and west walls receive more solar gain than the south wall. Minimizing glazing on the roof, and the east and west walls, therefore, will reduce overheating. A generous roof overhang on the south can further reduce direct sun on the south windows.

In the winter, the roof, east and west walls receive very little direct sun, and the south wall receives the highest level of direct sun. High amounts of glazing on the south wall–with minimal glazing on the north, east and west walls—makes good sense.   

Our landscaping will also make a difference over time. Shade from trees on the east and west walls will block excess solar radiation in the early morning and late afternoon without blocking winter gain. What about all those diagrams from 1970s passive solar books showing deciduous trees shading the south sides of homes in summer, dropping their leaves in the winter? According to the Chicago Urban Climate Study even the branches of deciduous trees cast so much shade in the winter that deciduous trees on the south sides of homes raise heating costs more than they lower summer cooling costs.  

All pretty simple, eh? Well, not quite. Unfortunately, our solstices—when we get our highest and lowest sun altitudes, June 21 and December 21–do not correspond to our highest heating and cooling needs. In practice in Maine, this means that it is easy to get too much sun in October and less than we’d like in March. We may find ourselves tilting the windows open and pulling the shades in October.

 Thermal Mass

Thermal mass is an important component of the solar heating plan for TerraHaus. Thermal mass has been compared to a thermal flywheel, evening out the air temperature of a building.

 The concrete slab and kitchen island of TerraHaus absorb heat during the day, keeping the house from overheating. (Thermal mass releases heat during the day too, but on a net basis it absorbs more than it releases.) At night, this heat is released at a rate related to the drop in temperature, counteracting the loss of solar gain. Passive solar designers use various formulas to optimize the ratio of glazing to thermal mass.

While it was not available for purchase in the small amount we could have used for TerraHaus, we are interested in trying a new product when it becomes more available: phase change drywall. One current version incorporates cells of paraffin which absorbs high amounts of heat as it melts and releases this heat as it cools at night. Passive solar homes can use this on wall surfaces that receive direct sunlight.

Doug Fox, Director, Center for Sustainability and Global Change

The Heat Contribution of our Passive Solar Windows

8.5.2011

In my last post I wrote about some of the characteristics necessary to achieve net gains in solar energy in cold climates. Since then I have corresponded with Alan Gibson, Principal, at GO Logic. He updated me on some of the final specs on the windows, which I’ll include here and correct in the previous post.

What is the contribution of passive solar to the success of TerraHaus? Using the specifications below, 70 degrees F as our temperature goal, and climate data from Bangor, Maine, the Passive House software model (PHPP) predicts that the south façade will contribute a net 7.8 million Btus annually to heating when all gains from sunlight and losses from conduction and radiation are included.

How Important is a 7.8 million Btu Capture?

 It is this 7.8 million Btu net gain from solar energy captured through the windows that I want to put into perspective. How much energy is this, and how much of the heat load of TerraHaus does it cover?

First, this solar gain contributes one half of the energy needed to keep TerraHaus at 70 degrees F.

But what would this mean for those of us who burn oil or wood? A gallon of #2 fuel oil contains 138,500 Btus, but furnace inefficiency means that we probably capture only 115,000 Btus. Therefore, the 7.8 million Btu solar capture is the equivalent of about 68 gallons of fuel oil or about ½ cord of wood.

While this is ½ the space heating need for TerraHaus, it would not be as significant in most of our homes due to their higher heat requirement. Efficiency Maine reports that a typical weatherized home of 2000 SF in Maine will use 870 gallons of oil (about 6.5 cords of wood) to heat the home to 70 degrees for one heating season. (Many of us burn less, but our homes don’t average 70 degrees in the winter!) Compared to the heat load required for our homes, the amount of solar gain is not strikingly high.

Second, when thinking about windows note that most of the windows in our houses lose much more heat than they gain. For most houses, the function of windows is to provide daylighting and views to the outdoors rather than solar gain. These functions cost us in fuel use. Whether a building is passive solar or not, minimizing losses through high R-value, low infiltration, and minimal north-facing glazing  is key regardless of any net gain. I’ll play with the PHPP model some more then publish some numbers on the insulation value of the TerraHaus windows over some standard options.

In Conclusion

TerraHaus’ remarkably low energy requirement is achieved through multiple factors, each contributing a modest energy savings. How do we achieve a 90% reduction in energy use as compared to typical code-compliant homes in our area? GO Logic’s Alan Gibson estimated the following from their GO Home Prototype in Belfast, Maine.

• 29% from improved insulation (R 50 walls, for example)
• 27% from improved air sealing
• 24% through carefully specified windows (reduced losses as compared to standard windows, standard glazing size and position, plus gains from solar) 
• 10% from heat recovery ventilation

The Final Specifications (for my technical readers):    

 Large, South-facing Passive Solar Windows:

     All Panes: U-value= 0.106, therefore, an R-value of 9.46

    Tempered glass Panes (lower windows): SHGC = 0.5

     Non-tempered glass (upper windows): SHGC = 0.61

     All Frames: R = 5

     R value of full window with all thermal bridging accounted for = 6.6

Other Windows:

     R 11 glass and SHGC = 0.5

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

Passive Solar and “Tilt and Turn” German Windows

7.27.2011

7.27.2011

Why did we haul windows all the way from Germany for TerraHaus? First, achieving the Passive House standard of 90% less energy use than a code-compliant home of the same size necessitates the use of the best technology available. Second, we hope to promote some of the features of these windows to influence North American window manufacture and even energy policy. GO Logic’s Alan Gibson and Matt O’Malia came across these windows, made by Kneer Sudfenster, at a green building trade show in Germany, home of the Passivhaus standard.

Windows can capture solar energy to warm a home, but they also lose energy, partly through radiant losses (reduced through the use of low-E glass), partly through conduction due to the low R-value of glass and many types of window frames, and partly through convective losses due to air leakage.  The goal in a passive solar home, therefore, is to gain more heat energy in the form of solar radiation than is lost through the same window.

Many Americans have some understanding of R-value, the insulation value of building materials, and those with leaky windows are sensitive to how they let in the cold, but few probably understand a relatively new measurement of window performance known as the Solar Heat Gain Coefficient (SHGC). The SHGC is a measure of how much of the sunlight hitting a window passes through the glass. While it may seem like all the incident light should make it through to the house interior, for most glazing only 30-40% of the radiation may be transmitted. Triple pane, low E windows, used for their insulative quality, generally block high amounts of incident solar radiation.  If we want to use solar energy to heat our homes we need south-facing windows that transmit 60% or more of the radiation hitting them to make up for the heat loss from the same windows for a net energy gain.

South Facing Window

With a SHGC of 0.5 to 0.6 (meaning 50%-60% of incident solar radiation penetrates the glass) our south-facing TerraHaus windows can pick up a lot of solar radiation. Remarkably, these windows also have a very high R value (9.46 for the panes) and low level of radiant heat loss. Somehow those clever Germans have managed to embed low-E material into their glass in a way that doesn’t block the short wave radiation from the sun. And those of you fortunate to take a tour of the TerraHaus this fall will want to experience how positively tight they close, latching in multiple places, keeping warm air in and cold air out. Finally, these windows achieve their very high R value due to their thick wooden frames equipped with sealed air channels for extra insulation and aluminum cladding on the outside that is attached in a way that does not form a thermal bridge to the inside.

In addition, TerraHaus will demonstrate increasingly popular European-style “tilt and turn” windows on the north, east and west. These windows close much tighter than double hung windows, and they are much easier to clean than standard casements. In their tilt position, they offer summer ventilation while blocking drafts and rain. These windows also offer extra security and safety: In the turn position they offer easy egress, and in the tilt position it would be virtually impossible for an intruder to enter or a young child (or college student?) to fall out.

Tilt Position

"Turn" Position

How do we hope to influence energy policy through use of these windows? While you will frequently see Unity College promoting Energy Star products, the Energy Star program and the other programs that rely on Energy Star ratings have a flaw when it comes to windows. The very windows we most need in the Northeast—high SHGC windows for southern exposures—are not Energy Star rated and do not qualify for federal and state incentives! The Energy Star program promotes low SHGC windows, which makes sense for homeowners in the South where these windows help keep homes cooler.

I apologize for the poor photos on this post. I’ll replace them soon with better photos, including a view of a cut-away demonstration model that shows the features of the framing very well.

Next: Solar Orientation, Use of Thermal Mass (including phase-change drywall)

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

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

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.

TerraHaus

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

 

Air Sealing TerraHaus

June 24, 2011

6/30/2011

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!

 

Footnote:

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

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