In Europe, builders interested in energy efficiency are gravitating to the Passivhaus standard. Meanwhile, American researchers — and a few American builders — have developed a fascination with the idea of the net-zero-energy house. The U.S. Department of Energy has established as a goal that new buildings in the U.S. will be built to a net-zero-energy standard by 2030.
Passivhaus buildings and net-zero-energy buildings have a lot in common. Both types of buildings aim to reduce the amount of energy used for space heating or cooling by designing envelopes with a low rate of air leakage, thick insulation, and high-performance windows.
While Passivhaus designers are content with achieving a very low energy budget, net-zero-energy home designers add frosting on the cake by including a rooftop photovoltaic (PV) array sized to produce enough site-generated electricity to balance the home’s annual energy use.
While both approaches have merit, both approaches are also open to criticism.
Advantages of the net-zero-energy approach
The best aspect of the net-zero-energy approach is the fact that, in order to balance energy loads with energy production, designers are forced to evaluate the cost-effectiveness of each energy-efficiency measure and compare it to the cost of a PV array. The aim is to find the least-cost path to building optimization.
Here’s the way such analysis works. Say you are building in Syracuse, N.Y. Designers know that a 1-kW PV array — that is, an array that now costs about $7,000 to install — will generate 1,123 kWh per year in Syracuse. In other words, each $1,000 you invest in PV will reduce your energy expenditures by 160 kWh per year. [Update: in September 2012, the cost of a 1-kW PV system has dropped to $3,500. That means that a $1,000 investment in PV will generate twice as many kWh per year — about 320 kWh — as the calculations shown in this article.]
Using that investment in PV as a benchmark, it’s possible to evaluate other $1,000 investments. For example, what will be the effect of adding $1,000 worth of extra cellulose insulation to your attic floor? With a good energy modeling program, it’s easy to do the math; if the cellulose saves more than 160 kWh per year, it’s a good investment compared to PV.
Once you’ve designed a good shell, each incremental improvement adds to the cost of construction, but saves less and less energy. As thicker insulation or additional layers of glazing begin to cost more than PV, it’s time to question the logic of the investment. If you’re building a zero-energy house — that is, a home with a PV array on the roof — it doesn’t make much sense to invest in insulation upgrades unless the investment yields more kWh savings than a PV array.
In 2004, engineers at the National Renewable Energy Laboratory in Golden, Colorado developed a software program, Building Energy Optimization (BEopt), that performs the calculations necessary to determine the least-cost path to building a zero-energy home. For more information on BEopt, see BEopt Software Has Been Released to the Public.
Apples to oranges
Most Passivhaus builders defend investments in envelope improvements that cost considerably more than PV. Their main argument: comparing insulation to PV modules is an apples-to-oranges comparison. While PV modules may wear out in 30 or 40 years — and may require maintenance or repairs along the way — insulation is likely to last far longer and is virtually maintenance-free.
The argument has merit. Nevertheless, when large amounts of insulation are used to save only a handful of kWh per year — the classic example being a very deep layer of rigid foam insulation, in some cases up to 14 inches deep, under a slab foundation — it’s worth stepping back and considering the situation from a neighborhood perspective.
Far more energy will be saved when two houses are each equipped with 7 inches of sub-slab foam than when one house has 14 inches of foam and the other has none. This example raises the question of whether installing very thick layers of insulation in a handful of houses is a good use of the world’s limited resources.
Advantages of the Passivhaus approach
The best aspect of the Passivhaus approach is that it doesn’t fall into the trap of assuming that electricity production is best performed on a residential roof.
It’s hard to understand why so many researchers in the U.S. have concluded that homeowners need rooftop PV. In fact, generating electricity on residential roofs rarely makes sense, for the following reasons:
- Residential roofs are often shaded by trees or neighboring buildings.
- Many residential roofs don’t have the optimal slope or orientation for a PV array.
- A rooftop PV array greatly complicates re-roofing.
- Most homeowners don’t want to be responsible for maintaining and repairing energy-generation equipment.
- It’s far cheaper to generate electricity from utility-scale wind turbines — or even from a solar thermal plant in the desert — than from small PV arrays.
- The cost of tax credits and subsidies doled out to homeowners who install PV arrays increases the tax burden and the cost of electricity for the general population.
- Programs that encourage the installation of residential PV arrays draw investment dollars away from more logical investments (like improved air-sealing measures) which yield more energy savings per dollar invested.
- There are far cheaper ways to reduce carbon emissions — for example, upgrading old coal-fired power plants with cleaner technology — than the installation of PV arrays.
Combining the best of both approaches
Although I’m well aware that the expected service life of insulation is longer than that of a PV array, I think that it’s sensible to use the cost of PV as an upper limit or “reality check” when considering the cost of any envelope improvement. It’s a useful way of reining in an out-of-control designer who’s about to go over the cliff.
That doesn’t mean, however, that a superinsulated house will necessarily benefit from a rooftop PV array. If you do the math, you’ll discover that homeowners who invest in PV pay more for their electricity than homeowners who buy their power from the grid. If we go back to the example of the house in Syracuse, NY, we discover that a $1,000 PV array saves only $19 per year, assuming that grid electricity costs 12¢ per kWh.
In other words, these homeowners are deliberately choosing an expensive source of electricity. (There is an exception to this rule: in areas of the country with generous PV subsidies, tax credits, or feed-in tariffs, the installation of a PV array can sometimes save a homeowner money. That’s only possible, however, when the homeowners pass along some of the cost of their PV array to utility ratepayers or taxpayers — in other words, their neighbors.)
The fact that PV-generated electricity is very expensive is a further reason to be wary of any investment in insulation or windows that yields fewer annual kWh savings per dollar invested than PV.
A designer who advocates installing insulation that costs more than PV is anticipating a future with fuel costs that exceed the current cost of PV-generated electricity. That’s an unlikely scenario, considering the fact that the cost of electricity generated by utility-scale wind turbines is now much lower than PV-generated power, and considering the fact that PV prices are still dropping.
If a homeowner borrows money to pay for insulation that costs more than PV, the mortgage amounts to an investment that only pays off if future fuel prices exceed the price of today’s PV-generated power. To me, that’s a risky stock to invest in.
So here’s my recommendation: design your house using the net-zero-energy approach to cost optimization — but don’t buy or install the PV array.
Are some elements of the Passivhaus standard arbitrary?
Passivhaus proponents have been known to bristle when energy experts suggest that a few elements of the Passivhaus standard — for example, the airtightness limit of 0.6 ach50 or the annual space heat limit of 15 kWh/m²/year — are arbitrary.
Passivhaus proponents have proposed the following explanations for the 0.6 ach50 limit and the 15 kWh/m²/year limit:
- The purpose of the 0.6 ach50 limit is to avoid structural damage to the building.
- The purpose of the 15 kWh/m²/year limit is to make it possible to deliver space heat through a home’s ventilation ductwork.
For example, on the first point, here is what Dr. Wolfgang Feist had to say: “The airtightness is one of the things that we really have to stick on in almost all climates. There are only a few climates where this might not be a [requirement], but very few — like in San Francisco. In San Francisco you might not need to have it airtight, but in almost all other climates you need that. A major part of the airtightness requirement is to avoid structural damage. You have bad indoor air with humidity, and if there is an exfiltration through the construction you get really big problems of condensation in the structure. This is the major reason to make it completely airtight, and even in subtropical climates and of course in tropical climates, it has to be airtight because you get structural damage without airtightness.”
Feist’s statement doesn’t bear up well to close scrutiny, however. Plenty of wall systems are robust enough to be fairly immune to structural damage related to air leakage — for example, ICF walls. Moreover, there have been examples of extreme structural damage without air leakage — the most famous being SIP homes in Juneau, Alaska, that suffered catastrophic roof damage due to convective loops through the SIP seams. These convective loops led to condensation and rot, even when no exfiltration occurred.
Finally, almost all wood-framed homes with air leakage rates of 2 or 3 ach50 are doing fine, with no evidence whatsoever of structural damage.
Why does heating energy use need to be so low?
When it comes to the 15 kWh/m²/year limit, there is a good deal of evidence that the limit was chosen to allow space heat to be delivered through ventilation ductwork — at least in central Europe.
According to the Passipedia Web page, “A Passive House is a building for which thermal comfort (ISO 7730) can be achieved solely by post-heating or post-cooling of the fresh air mass [ventilation air] which is required to achieve sufficient indoor air quality conditions – without the need for additional recirculation of air. … All airtight buildings (any low-energy building needs to be airtight) require the use of an efficient ventilation system. In Passive Houses this system is also used for heating purposes, without the need for additional ducts, major technical interfaces, auxiliary fans etc. … The way to go therefore involves cutting back on one of the two systems: either on the ventilation system, e.g. by installing an exhaust system only; in this case the building will become a low-energy house with conventional heating; or on the heating system by using the ventilation system for heating as well – in this case the building will become a Passive House. This heating concept automatically implies extremely low energy consumption. After all, using the fresh ventilation air for heating without an additional heating system can only work in buildings with minimal net losses.”
In the colder regions of Scandanavia and North America, it’s extremely difficult to deliver enough space heat through ventilation ducts to keep a Passivhaus building warm in cold weather. This needn’t be a problem, however, since Dr. Feist says that it’s perfectly acceptable for a Passivhaus building to deliver space heat through other mechanisms. Feists’s concession is certainly useful. However, once it becomes permissible to use any type of heat delivery system, the justification for the 15 kWh/m²/year limit loses its original importance.
Thousands of dollars of insulation to save just a few BTUs
Ultimately, it really doesn’t matter whether Passivhaus limits are arbitrary or firmly based in a consistent philosophy of conservation. What does matter is whether the extremely thick (and expensive) layers of insulation needed to meet the standard in a cold climate can be justified by anticipated energy savings.
In northern areas, meeting the 15 kWh/m²/year limit requires insulation levels that are hard to justify. For example, Phil Kaplan, an architect in Maine (and one of the Podcasters in GBA’s “Green Architects Lounge” series), is designing a superinsulation retrofit project for Claudia King, a homeowner in Falmouth. Working with energy consultant Marc Rosenbaum, Kaplan proposed a series of retrofit insulation measures (including 4 inches of polyisocyanurate foam on the exterior of the walls) to lower the home’s energy use. After running the numbers, the team found that they were still short of the Passivhaus goal.
By adding more insulation — including 2 additional inches of polyiso on the walls, for a total of 6 inches — the house could meet the Passivhaus standard. But these heroic measures would add at least $2,880 to the cost of construction, while only saving 950 BTU/sf/year. Assuming that heat is supplied by a ductless minisplit heater with a COP of 2.0, the extra insulation would have a simple payback period of about 58 years.
“We wanted to know if we could meet the Passivhaus standard,” Claudia King told me. “We thought it would be really cool if we could get certified. We did all the figuring, and we found that we needed to do a little more upgrading — to add more insulation in the cellar and more foam to the outside walls. We calculated that the cost of doing wasn’t worth it for the gain we would get — the savings were not worth the extra investment. We could get the same gain for less investment by putting up some more PV panels, so the expense could not be justified.”
For more on this topic, see “Are Passivhaus Requirements Logical or Arbitrary?”
This article has been translated into Serbian: Dva Pristupa Energetski Efikasnim Zgradama