What is truly important when designing and building a green home? Some of the many existing programs don’t go far enough, some are accused of going too far, and some just miss the mark. What should be included in a Pretty Good House?
We had a pretty good turnout, 50 people or so, at the most recent building science discussion group, held each month at Maine Green Building Supply in Portland. Plugs were made for the upcoming Maine Indoor Air Quality conference and the NESEA conference in Boston. We did a quick round of self-introductions, and then we got down to business — Part 2 of the Pretty Good House discussion.
More suggestions for a PGH
Dan Kolbert, builder and moderator, kicked it off by briefly recapping the results of the first discussion and the comments to Part 1 of this blog series. He clarified that, in his mind at least, the PGH is not meant to be a prescriptive measure for all climate zones (and that in fact he’s not really sure what it’s supposed to be at all), but that we should try to focus our discussion on our zone (Climate Zone 6) and to consider how other climates might relate.
With many sharp minds in the room and no strong agenda, the discussion wandered around, and as usual there was no consensus on what the PGH metrics should be or even what the PGH concept really means. Good things came out of the discussion though, including the following ideas of what should be included in a Pretty Good House:
Near 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.. Jason Peacock suggested this, and he practices what he preaches. He propsed that a PGH should have utility bills of no more than $500 to $700 a year, and that no house should be built without renewable energy systems as part of the mechanical mix.
Zoned heating system to reduce the load. Jim Godbout, one of Maine’s premier plumbing and heating experts, says that one relatively inexpensive way to reduce demand on the heating system is to provide separate zones for different parts of the house. He said that if you are using a boiler, a popular choice in Maine, you can also use the boiler to heat domestic hot water — an approach that usually requires a boiler rated at 80,000 BtuBritish thermal unit, the amount of heat required to raise one pound of water (about a pint) one degree Fahrenheit in temperature—about the heat content of one wooden kitchen match. One Btu is equivalent to 0.293 watt-hours or 1,055 joules.
/h or more. He says that in a tight, well-insulated house, the heat load could be reduced to 20,000 Btu per square foot per year or less, at which point electrically supplied heat can make sense — but you will need another heat source to supply domestic hot water.
Mechanical ventilation should be a given. An HRV or ERV, or possibly an exhaust-only ventilationMechanical ventilation system in which one or more fans are used to exhaust air from a house and make-up air is supplied passively. Exhaust-only ventilation creates slight depressurization of the home; its impact on vented gas appliances should be considered. system, is required with the airtightness level expected of a PGH.
At the previous discussion, Mike Pindell of IS Insulation had suggested 2 ach50 as a reasonably easy target to hit; informal feedback seems to indicate that tighter levels may be preferred. Mike says, “We’re standing here in rarified air,” arguing about the difference between 1 and 2 ach50, when the vast majority of people out there are nowhere near these numbers. Is this a case of building nerds being nerdy, or are supertight blower door numbers really necessary?
No fossil fuels. Phil Kaplan offers this concept, and has achieved it in his firm’s Bright Built Barn, which produces more power than it uses. Using no fossil fuels at all may be more than Pretty Good, so the suggestion was amended to “no fossil fuels burned on site.” Affordably sized grid-tied renewable energy system, here we come.
Renovating vs. new construction. Dan states that until we get our heads around the concept of the PHG, let’s stick to new construction. Architect Liz Newman argues that in 50 years, 90% of the housing stock will be stock that exists now, so thinking about retrofits is vitally important. Margo Billings of Horizon Energy Services asks whether retrofits should meet the same standards as PGH specifications for new homes. Clearly this group is focused on retrofits, and further discussion about retrofits, when we get to it, will be interesting.
Client concerns. Sam Zuckerman of Solaris says that we should bring the discussion back to the customers. Their concerns about achieving a reasonable return on their investment are something he hears about all the time when talking to people about energy upgrades and installations. Should the PGH specifications be a list of “Do this, don’t do that,” or should it be about giving the client the best house you can for the money they are willing to spend?
On one hand, Dan relates a story about a leading green builder in New England, who tells customers: If you already have 500 square feet per person, I will not build you an addition. On the other hand, Bob Earnest of Spring Island Builders says, “If you can help people build a greener house than they would have otherwise, who cares how big it is?” A house that uses little fuel will cost less to operate, allowing for a bigger mortgage, and energy incentives also make a difference in what homeowners can afford, so the cost vs. square foot debate is a moving target.
Client education is a big part of this too; Dan suggests a Pretty Good House coloring book to help all parties visualize what is important.
Energy-efficient assemblies. Wes Riley, an energy rater and consultant, suggests we follow the latest energy code requirements — specifically the 2012 IECC International Energy Conservation Code., which has some interesting changes over past iterations. As most of us are not yet aware of the upcoming changes, he says that there is going to be a move from insulation R-valueMeasure of resistance to heat flow; the higher the R-value, the lower the heat loss. The inverse of U-factor.
to overall wall assembly U-factorMeasure of the heat conducted through a given product or material—the number of British thermal units (Btus) of heat that move through a square foot of the material in one hour for every 1 degree Fahrenheit difference in temperature across the material (Btu/ft2°F hr). U-factor is the inverse of R-value.
, taking into account thermal bridgingHeat flow that occurs across more conductive components in an otherwise well-insulated material, resulting in disproportionately significant heat loss. For example, steel studs in an insulated wall dramatically reduce the overall energy performance of the wall, because of thermal bridging through the steel.
and window and door performance. Wes suggests that any house with a HERSIndex or scoring system for energy efficiency established by the Residential Energy Services Network (RESNET) that compares a given home to a Home Energy Rating System (HERS) Reference Home based on the 2006 International Energy Conservation Code. A home matching the reference home has a HERS Index of 100. The lower a home’s HERS Index, the more energy efficient it is. A typical existing home has a HERS Index of 130; a net zero energy home has a HERS Index of 0.
The older versions of the HERS index were based on a scale that was largely just the opposite in structure–a HERS rating of 100 represented a net zero energy home, while the reference home had a score of 80. There are issues that complicate converting old to new or new to old scores, but the basic formula is: New HERS index = (100 – Old HERS score) * 5. performance rating of 40 or less is Pretty Good.
Margo agrees that the tipping point is right around HERS 40; that’s where you start to see serious reductions in energy use. Wes says that the easiest thing you can do is to minimize thermal bridging; Sam agrees that it doesn’t cost a lot and makes a big difference in wall U-factors. Wes says that in the 2012 IECC, air leakage is also going to be a major factor.
Tom Fullam points out that in any wall assembly you need to be aware of moisture management issues — there’s no sense in building a superinsulated house if it’s not going to be durable.
Prescriptive vs. performance. Jim Godbout says that he recently went to a meeting of ten reputable builders — these are guys who are proud of the way they are building — and all ten were insulating in different ways: proof that prescriptive paths don’t work.
Mike Pindell and Chris Corson agree that if the PGH is going to require high performance levels, the standard has to be performance-based. Chris, who recently built a house that “killed the Passivhaus standard,” says that PGH could be the beginning of a potential paradigm shift. Sam agrees, and says that there doesn’t need to be one solution with a ribbon around it; simply planting five ideas in someone’s head will make a big difference.
Steve Konstantino, owner of Maine Green Building Supply (our gracious host and provider of delicious sausages and other snacks), thinks that including an energy model up front is very important, in addition to testing performance at the end of the project.
What’s the point?
At the end of the night, there seemed to be a few recurring questions:
- Should you quantify the PGH, and if so, how?
- And what is the purpose of the PGH?
The best response so far to the second question came from Shepard Bosworth, a builder: you get a Pretty Good Plaque. But I think there may be other good answers to both questions. Let’s hear them.