A highly-efficient house– the Solar House at Florida International University.
What makes a perfect house, in terms of energy efficiency, environmental concerns, and design and construction techniques? In the last month I’ve attended two different panel discussions on the topic, and have come away with some interesting insights. It turns out that this is an enormously complex and changing topic (like everything else!), and it also turns out that my own house, which I thought was fairly sound in these areas, might have a number of problems that I didn’t realize. It’s a big subject, but let me attempt to distill some of my realizations here:
— Air-tightness is really, really important in order to have an efficient building. It turns out that the easiest way for energy to escape from a building is if it leaks air, so highly-efficient buildings are built with a continuous “air-barrier” that is integral to the building envelope. Now, what I didn’t realize is that this barrier isn’t always a discreet item, like a layer of impermeable plastic, but is sometimes just a boundary that is sealed—say, the exterior sheathing, and then across the ceilings with gypsum board. Adhesive tapes and caulks can be used to seal seams and gaps, plastic sheeting is used in basements and under slabs, and sometimes large adhesive sheets are used that adhere to the exterior of sheathing materials. Regardless of the approach, the barrier needs to be continuous, including under the floors, because even small leaks can have large overall effects on building performance. Ideally, all mechanical air-handling systems are within this envelope, to reduce the number of potential air leaks, and to lessen the chance that negative pressure in return ducts will suck in contaminated or unconditioned air. Then, electrical outlets, fixtures, plumbing—anything at all the goes through the air barrier—must be sealed. In addition to efficiency, though, really good air barriers make houses more comfortable, AND make them more durable, because air carries moisture, and if that air goes across a temperature differential then condensation can occur within a building’s walls.
To test how well a building is sealed, a blower-door test is used, where a fan puts negative pressure on the building, and leakage is measured in the number of air-changes per hour. Highly-efficient buildings will typically score between 1 and 3 air changes per hour (at a reference pressure of 50 Pascals), as compared to ranges of 7 and higher for conventional construction in decades past. (A good article on blower-door testing here.)
Plastic sheeting being used as an air barrier in wood-frame construction. In cold climates, air barriers are often used on the inside of buildings to prevent warm inside air from exfiltrating to the exterior, where it might cause condensation problems. Note how seams are taped, so a continuous barrier is formed.
— At any reasonable air-tightness level, mechanical ventilation is required to maintain indoor air quality, because the downside to really sealing up a house is that the indoor air quality can plummet. To compensate for this, mechanical air exchange systems are used, to vent stale indoor air outside, and draw fresh air in, and they incorporate heat exchangers to keep the incoming air as close as possible to the temperature of the outgoing air. The best of these systems measure and control for a wide variety of air-quality indicators, and will circulate air within the building, control for humidity, and/or use heat pumps to boost the temperature of the incoming air. Most systems operate automatically with efficient variable-speed fan motors, but are also activated manually at times, as when a bathroom exhaust switch is turned on. Though air exchange systems use electricity to run, the savings from reduced heating and cooling loads in a tight house generally outweigh those costs. (1 May 2015 Note: the most common names for this piece of equipment seems to be “ERV”, or Energy-Recovery Ventilator”, or “HRV”, for Heat-Recovery Ventilator. The two are slightly different. There is a good overview of these here.)
A mechanical ventilation system with cross-flow heat exchanger.
— There’s a “sweet spot” with regard to cost-effectiveness when it comes to designing and building highly-efficient houses. Efficiency improvements generally pay off economically, but at some point there are diminishing returns. As an example, several of the panelists discussed how going all the way to Passive House standards (a very strict standard popular in Europe) won’t always result in a monetary payback here in Vermont with its relatively severe winters, because the levels of insulation required to meet that standard here can be extreme. Economic returns can sometimes be maximized by stopping short of the highest levels of insulation and air-tightness, and spending money instead on photovoltaics and cold-climate heat pumps to make up the difference.
— It’s easier to achieve high performance in new construction. On one hand, few improvements pay dividends faster than basic air-sealing in existing homes, as returns on investment can be extremely short. On the other hand, it is difficult to continue to completely “fix” an older home in ways that remain cost-effective. Additional efficiency, if added to a new home during the construction phase, can pay off financially, but this is not always the case with older construction. One of the panelists used an example of a “70’s ranch-house with 2×4 stud walls and R-19 fiberglass batts” as a house that could not be cost-effectively fully upgraded. Note, however, that it is possible to bring such houses up to the highest standards if financial payback isn’t a driver, and that basic weatherization or efficiency improvements could still pay off, and any resulting deficits could be closed with added PV generation and/or heat pumps.
A rough sketch of double-walled construction. This technique is often used in deep-energy retrofits, where an additional stud wall is added to the interior of older construction, and the resulting wider wall space filled with insulation. Note the continuous air barrier that wraps up just behind the inner stud wall, and down on top of the sill plate and into the basement. Double-wall construction also helps avoid thermal bridging, which results in higher actual R-values across entire wall assemblies.
— Energy consumption of a building needs to be measured separately from its energy production in order to get a full picture. Related to this, “net-zero” is a tricky term. Here’s why—if a home has its energy needs met with PV panels on the roof, so that no additional purchased energy is required, then it is usually considered “net zero” (and has a lower HERS score). But what if those panels were in the yard, and tied to the house with net-metering? Most people would still consider this a net-zero property. But after this is gets fuzzier—what if the panels were a block away? What if they were part of a community solar array? It becomes a slippery slope—few would call a leaky old farmhouse “net-zero”, just because its electricity was produced from a community solar array three miles over (though some would indeed feel this term was valid in this case).
In a way, these arguments miss the point, or at best conflate two things that might be better viewed separately. Thing one—how much energy does a particular house use? And, thing two—how much of its energy needs are met with renewable power? And as for how much energy a building uses, one of the panelists held the position, and I would tend to agree, that it is best to use actual amounts of energy, measured in millions of Btu’s per year (MMBtus/yr), as this might be more meaningful than, say, a HERS score, which normalizes energy use depending on the size of the building. A tiny little house and a McMansion might have the same HERS scores, but even if they were good scores, the mansion would use far more energy. So, perhaps the sustainable goal for all of us would be to have efficient buildings that use as little energy as possible, and then to provide 100% of that energy from renewable sources.
Infrared image of a skylight during a blower-door test, showing clear air leakage.
The same skylight without the infrared camera.
— And finally, insulation choices matter because of their highly varying environmental impacts. This one was news to me—some insulation types have horrible environmental impacts on the production side, to the point that from an environmental perspective they can virtually never be “paid back”. In fact, two types of insulation were far, far worse than all the others. Unfortunately, they are both quite common. One is XPS, or extruded polystyrene, more commonly known as “blue board” or “pink board”. The other is ccSPF, the most common type of spray foam. Unfortunately, both of these types are common because they are extremely useful– XPS doesn’t absorb water, and can but used underground or underneath poured concrete slabs, and spray foam provides high R-values per inch, doesn’t absorb water, molds and sticks to virtually any surface, expands to fill cracks and gaps, and can actually provide structural support in some cases (good article at BuildingGreen.com on this subject, “Avoiding the Global Warming Impact of Insulation“). This isn’t my field, so I’m not completely sure what the work-arounds are here, but two insulation types that came up repeatedly at the panel discussions were mineral wool and dense-pack cellulose. The cellulose in particular is probably the best all-around insulation in terms of the environment, it has low embodied energy, a low greenhouse gas footprint, and is virtually chemical-free. It must be kept dry, though, so even with air and vapor barriers it isn’t an all-around substitute. But in the words of one of the panelists, “Cellulose is a great product”.
So, all this information presents some challenges. For my own house, two big problems are apparent. First, while our house is quite tight, it does have some leaks, but I’ve always felt that it didn’t make sense to tighten it up further, only to find myself needing a mechanical ventilation system. It appears I was wrong. The leaks in some places are causing condensation problems behind some of the fascia boards, which is causing them to rot, and the overall humidity in the house is too high virtually year-round. This problem is exacerbated by our use of a propane stove and oven, which were necessary choices from our off-grid days. The combustion of propane produces water vapor, in addition to other pollutants that stay in the air. To worsen this even a bit more, “leaky” houses don’t reliably change indoor air; they only “self-ventilate” when pressure differentials are high, in the cold winter days and the hottest summer days (and in the case of the latter, when the windows tend to be open anyway). All those other days “in the middle” are probably days without enough ventilation. So, as part of my net-zero project, I think I need to work on fully tightening up the house and finish all the insulation work in the basement, and then to add a mechanical air exchanger.
The second problem here can’t really be fixed—we used a lot of pink board when we built the house; the entire basement slab sits on several inches of it, and the basement walls are insulated with it behind the stud walls. It will eventually save enough energy to pay back its environmental footprint, but I might not live to see it. So, live and learn; we all need to use what we know to do better next time.
With regard to society as a whole, it doesn’t bode well that older homes can’t be cost-effectively brought up to the highest efficiency standards, because buildings account for nearly half of the energy use in the US, and we have a heck of a lot of older buildings. As we get closer to an economy that is powered with renewable energy, we might need to decide as a society that all these homes need fixed, and support government programs that help offset the costs. There are a lot of homes out there, and it won’t be cheap, and we would need some real leadership from our politicians. Of course, this is assuming that energy prices won’t increase much. If they do, then deep-energy retrofits might indeed pay off, for virtually all buildings.
Then, too, there is that other option—a carbon tax where the proceeds are used to help increase efficiency. Interestingly, Vermont is currently considering just such a tax.
In the end, there are paths forward, but none of them are completely clear.
(See Part 2 of this post here)
Top image credit: Junior Henry, “Energy Blues”, Flickr Creative Commons.
Air barrier: jp1958, “Wood Frame Construction”, Flickr Creative Commons.
Air exchanger: David Dodge, Green Energy Futures, Flickr Creative Commons.
Stud wall: Jenny Cestnik, “Sketch 054”, Flickr Creative Commons.
Blower door tests: Sonke Krull, Wikimedia Commons.