You may already know about the Pretty Good House concept, the result of a question that moderator Dan Kolbert asked back in 2011, partly as a joke, at the long-running building science discussion group at Performance Building Supply in Portland, Maine. Fed up with other building standards, from the wimpy and under-enforced building code to the nit-picky Passivhaus, Dan asked, essentially, what you should include in a house that does right for its inhabitants and the planet, but that does not go beyond reasonable environmental or financial payback.
We developed a list, which I shared in a blog post on Green Building Advisor, and since then the idea has taken on a life of its own. (A list of GBA articles discussing the “pretty good house” concept can be found on the “Pretty Good House” category page.) A lot has changed since 2011, and — unfortunately, perhaps — the time has come to revisit the Pretty Good House, also known as PGH.
Reducing embodied carbon in buildings — especially in Pretty Good Houses — was the topic at the last two building science discussion groups in Portland, then at one of the BS + Beer events I moderate in Liberty, Maine. (“BS” for Building Science, of course.) Right now is the worst time in the history of our species to dump a lot of carbon into the atmosphere, but that’s exactly the result of many construction practices. Even builders concerned with energy efficiency often front-load enormous amounts of carbon-intensive materials with the expectation of saving over the life of the building. But if we only have one or two decades to mitigate the worst impacts of climate change, what should we do instead? The following ideas are a summary of our discussions.
Like everything PGH, the 2.0/Low Carbon Edition is meant for thought, discussion, and action, not for check boxes, awards, or membership dues. As always, however, if you feel that you deserve a plaque, feel free to buy yourself one.
Include these features
In no particular order, a PGH 2.0/Low Carbon Home should:
• Be as small as possible. Ideally with multi-family or multi-generational occupants.
• Be PV-ready or include photovoltaic panels. PV-ready means designed, built, and sited in such a way that a reasonably sized photovoltaic array can handle all of the home’s energy needs on an annual basis. (PV panels pay their carbon debt in 2 to 4 years.)
• Be simple and durable. Simple shapes are easier to air seal and insulate, perform better in harsh weather, and require fewer materials and less maintenance than more complicated buildings. If you need to bring in a structural engineer, your design might be too complicated. Invest in the parts that are hard to change later.
• Use wood and wood-derived products as construction materials. Just make sure that the wood is sustainably harvested, locally if possible. Otherwise the trees are better left to remove CO2 through photosynthesis. The more materials are processed, in general, the higher their carbon footprint.
• Use air-source heat pumps. Minisplits can be efficient to -15°F or below, affordable (especially for the sizes needed in a PGH), and relatively simple to install. For those who can’t stand the look of an appliance on the wall, there are slim-duct, ceiling cassette, and floor-mounted versions. But the wall-mounted units are the most efficient, so learn to love them. Heat-pump water heaters are a no-brainer for most homes.
• Invest in the envelope. Insulation and air-sealing should be good enough that heating and cooling systems can be minimal, with indoor air quality and comfort levels that are very high.
• Be affordable, healthy, responsible, and resilient.
• KISS: Keep It Simple + Safe — easy to operate and understand. Use owner-proof systems to get around operator influence.
• Consider traditional, non-flashy approaches: deciduous trees shading south and west walls; cooling via fans and natural convection instead of air conditioners; use biomass secondary water heating (i.e., let your wood stove heat your water); air-dry your clothes.
• Be part of a sustainable community: have access to community solar, jobs, and services nearby that minimize driving and provide shared infrastructure costs, to name a few advantages. A one-hit wonder in the middle of the woods often comes with a bigger carbon footprint than a community-based home.
Minimize or avoid these features
A PGH 2.0/Low Carbon Home should minimize or avoid:
• Concrete, which contributes 10% of man-made global warming emissions, partly through fuel to heat and move minerals, but 60% from release of carbon dioxide (CO2) from limestone (CaCO3) to get calcium oxide (CaO) for Portland cement. One concrete-reducing technology that is gaining ground is the use of helical metal piers, which are screwed into the soil to support decks, houses, and more. Some engineers and builders have doubts, but with many thousands of installations, they have a proven track record.
• Foam, especially HFC (hydrofluorocarbon)-blown closed-cell spray foam and XPS (extruded polystyrene) rigid insulation. When building a new house, there should be no need to use foam above grade. (For more information on this topic, see “Building a Foam-Free House.”)
• Combustion appliances, especially those that burn fossil fuels. You can have a wood stove in a PGH, but make sure it’s EPA-certified and includes dedicated makeup air.
• Unhealthy materials.
Some bigger ideas to consider:
- Straw bale construction. Although straw bale construction is dismissed by many as low-tech, low-R, and prone to moisture damage, experienced straw bale builders have developed effective ways to use this carbon-sequestering approach to building.
- Phenolic rigid foam. Zero greenhouse-gas emissions, extremely high R-value — what’s not to love? The fact that it’s impossible to get.
- Mycelium insulation. (Cue “fungus among us” jokes.) But seriously — it sequesters carbon and traps air, so why not use it to insulate homes?
- “Smart” materials. Variable-permeance membranes have made many of us more confident about simple but theoretically risky assemblies like double stud walls and building without foam. Glazing has come a long way in the last 10-20 years, but we could benefit from more glazing and other materials that respond passively to changes in conditions.
- Offsite fabrication. There is a lot of carbon burned getting workers and materials to job sites, and a lot of efficiency and quality control possible in a factory setting.
The original PGH had simple rules for insulation and airtightness, borrowed from Dr. Joe Lstiburek of Building Science Corporation as his recommendations for efficient homes in a cold climate. For PGH 2.0, we need to update the rules a bit to account for easy access to better windows and a better understanding of embodied carbon.
In a cold climate, DOE climate zone 5 or 6, use:
- R-5 to R-8 windows (U-0.20 to U-0.13); the higher the glazing-to-wall-area ratio, the more important it is for the windows to have a low U-factor. Even the best windows make lousy walls, so don’t over-glaze.
- R-10 sub-slab insulation (either EPS — that is, expanded polystyrene — or mineral wool or recycled XPS).
- R-20 foundation wall, frost wall or slab perimeter insulation (or build on piers).
- R-40 above-grade walls.
- R-60 roof.
- The wall and roof values should be lower if you’re using foam, due to its long carbon payback, but in a PGH 2.0 there is no reason to use foam above grade.
- Airtightness: 1.0 ACH50 (air changes per hour at ±50 Pascals pressure) is the maximum air leakage target many of us are using, but others say 1.5 or 2.0 ACH50 is tight enough. Definitely stay well below code-minimum requirement of 3.0 ACH50. Going tighter than 1.0 ACH50 gets you cool-kid points but may not add significantly to your home’s performance.
Performance-based approach (optional)
Use energy modeling to optimize designs, especially for fine-tuning window performance values. (BeOpt is a good, simple, free program for this.)
Should there be a standard PGH energy-use target, such as xx% better than code-minimum, xx% of Passive House levels, xx Btu/ft² or xx BTU/occupant?
Our generation is the only one who can fix the climate change problem. We can’t opt out; this is our only chance.
Aim for the biggest targets; don’t get lost in the weeds.
If you’re a designer or builder, sell the comfort aspect of a PGH; many clients do not understand or want to hear about technical details or climate change.
We need a carrot-and-stick approach: improve building codes and enforce them.
We need affordable and effective PGH 2.0 retrofits.
Read the Project Drawdown website or book for 100 more ways to reach carbon-neutral emissions.
Read Bruce King’s The New Carbon Architecture to learn more about reducing embodied carbon in buildings.
What would you include (or avoid) in a Pretty Good House 2.0/Low Carbon Home?
Michael Maines designs Pretty Good (and better) homes and renovations throughout New England, and also builds them in central- and midcoast-Maine. He writes the “Building Matters” column for Fine Homebuilding magazine, and the “Building Science 101” column for Green and Healthy Maine Homes. Email him through michaelmaines.com to join the BS + Beer discussion group mailing list, or join facebook.com/groups/BSandBeer. To join the mailing list for the Portland discussion group, send an email to [email protected]