The Passivhaus (or nearly zero energy building) vs. net zero debate has become an ongoing discussion that rears its nerdy head every few months. It really first took grasp shortly before Martin Holladay published his Net Zero versus Passivhaus blog. Recently, the topic has made its way into several conversations – and my arguments for nearly-zero-energy buildings (NZEBs) or NZEBs + renewables always spark a lively conversation. I was asked to put together a post collating my thoughts on why achieving Passivhaus should take priority over zero-energy buildings, or at least before adding renewables.

It may be worth mentioning that I view this as an apples v. brownies issue (I’ll leave you, dear reader, to decide which is which). On one hand, extreme comfort and consumption reduction (Passivhaus); on the other, energy production (which may or may not entail significant energy conservation measures). Passivhaus + renewables was an active (and lively) topic at the May 2012 Passivhaus conference in Hannover, Germany, and one of the more interesting aspects for me is where people fall on the spectrum –

As stated numerous times, significant energy reductions should be a priority (hence, Passivhaus). From an economic standpoint, I understand how a photovoltaic (PV) array makes sense for a home or business (albeit, a heavily subsidized one). However, a handful of zero-energy buildings in a sea of inefficiency does little in terms of actual CO2 reduction.

Instead, let’s be smart and have the foresight to retrofit (enerPHit!) and build an armada of efficiency (Passivhaus buildings, Minergie buildings, Living Building Challenge buildings – when Passivhaus!) and then, then! – once that’s been attained – let the energy companies do their thing (which they already do better than anyone else, anyway) to develop larger scale renewable projects, to decommission coal plants, and to green the damned grid.

This would have a far greater impact at lesser (collective) cost. It’s also on the radar for much of the European Union – where in the UK, addressing demand could cut up to 22 power stations, and where the Swiss are aggressively planning to reduce consumption by half (!) over the next 30 years.

Those aims, not coincidentally, parallel the goals of the incredibly ambitious Passive House Regions with Renewable Energies (PassREg) program. For those that don’t know, PassREg is yet another program where the leadership of the Passivhaus Institut has been superb. The program will leave the U.S. even further behind Europe. It’s literally the coolest thing since sliced bread, and makes me want to move back to Europe just so I can work on these, too.

The Passivhaus Institut states that PassREg will “… take a critical look the successful models employed by front runner regions, identifying the stakeholders involved, evaluating the driving factors and collecting appropriate solutions that might be applicable in other urban and economic contexts. In turn, the opportunities and existing barriers to the introduction and implementation of PassREg concepts in aspiring regions will be examined. Through PassREg, the wealth of knowledge that arises from this examination of the regions’ models as well as through the case studies of buildings in each region, known as beacon projects, will help aspiring regions to shape success models of their own and front runners to optimise what they already have…”

That’s not to say I abhor zero-energy buildings. We’d love the opportunity to crack open a few more Passivhaus buildings or zero-energy buildings here at Brute Force Collaborative. But if we’re going to shoot for zero-energy buildings, it really makes the most sense in terms of comfort and economics if paired with Passivhaus first.

Thus, I vote Passivhaus first…

It can be difficult for urban buildings and multifamily housing to reach zero energy

Even if achieving Passivhaus levels of efficiency, urban buildings and multifamily projects have proportionally smaller roof areas than detached housing, making it pretty difficult to achieve a zero-energy goal.

And that’s before Donald Trump blocks your solar access with a brass-bedazzled tower. Additionally, urban rooftops are typically utilized as a deck or HVAC parking spot, which can interfere with incorporation of a rooftop PV array. +1 Passivhaus

Not all houses are “solar-ready”

Not all buildings are “solar-ready,” and enforcement/encouragement to make houses solar-ready can have some aesthetically dis-pleasuring consequences.

Do we really want to all have south-facing shed roofs? Who wants to live in that world? Not this guy! What if my gabled facades want to face east-west?

Much of the existing housing stock doesn’t have the roof area, orientation, or solar access to make a zero-energy building feasible, whereas updating to enerPHit or near-enerPHit levels may not be entirely difficult. Pushing the existing housing stock to the level of zero energy with rooftop PV is so difficult, it’s only been attempted by a few projects.

Furthermore, obstacles like trees (and power poles, overhead wiring) can seriously affect the output of a PV array (even when bare in winter, as Marc Rosenbaum recently attested). Whereas, trees are a pleasant thing to look at in a warm, comfortable Passivhaus, and can provide shade from overheating in summer. +1 Passivhaus

Passivhaus buildings need a smaller PV array

Achieving Passivhaus levels of efficiency is one of the surest ways to ensure your PV array doesn’t have to spill over the boundaries of the building.

This is true for homes and commercial buildings. As we noted a few years back, the Bullitt Center’s PV array could have avoided intruding into the public domain had the building met Passivhaus. The amount of PV needed for North American Passivhaus homes to become zero-energy buildings is pretty small, especially compared to what’s necessary in solar-deficient Central Europe.

In looking at some of the Building America projects incorporating PV, in nearly every case, a Passivhaus would have resulted in significantly less PV than what was utilized (!). The PV arrays for many net-zero projects are much larger than would be needed for a worst-case Passivhaus, and significantly larger than an aggressively efficient Passivhaus.

Instead of having PV bleeding out over the entire roof and adjacent garage, you could conceivably get by with just adding PV on your garage. +1 Passivhaus

Retrofit projects could be cheaper if they aimed for PH + PV

When you have an existing project that could be considered solar-ready – e.g., one with a generous south-facing roof or a large enough flat roof – it may actually be less expensive to achieve Passivhaus/enerPHit and add renewables than to just add lots of PV.

Those existing projects in the U.S. that have been able to achieve zero-energy status seemingly do it on the backs of large tax credits and incentives (as was the case for the Grocoff Net Zero project: $49,000 out of pocket, $43,000 incentives/credits!). In a brief breakdown of their numbers, an EnerPHit-type reduction actually would have resulted in avoiding the ground-source heat pump ($21,000), and a “Passivhaus + PV” system would have been anywhere from 33% to 59% smaller. Even tripling the insulation/airsealing cost, adding minisplits and $100/sq. ft. Passivhaus-certified wood windows, the Grocoffs would have spent much less money to achieve ZEB – which by the way could have allowed those generous tax credits and incentives to be spread around to more projects and have even greater economic and environmental impacts. +1 Passivhaus

New buildings could be cheaper with the PH + PV approach

We’ve looked at this on a couple of buildings here in the Northwest that aimed or are aiming to be zero-energy buildings. Two of the more high-profile ones (Bullitt, zHomes) could have actually saved quite a bit of money (through PV/mechanical costs) by achieving Passivhaus first.

We’ve been stating this for years, as have others in the Passivhaus community – but I think we’re finally at the point where the cost to achieve Passivhaus is exceeded by the reduction in PV costs through meeting Passivhaus. Onion Flats Rowhouses in Philladelphia ($129/sf!), which took the PH + PV approach, are a terrific example of this (see Image #1, above).

For the Salem Passivhaus, the size of the needed PV array would only be 2.8 kWp (PHPP) – 3.8 kWp (actual) – to offset the energy consumed. In Germany, where installed PV prices are even lower than the U.S., the emphasis is still on “Passivhaus first, then renewables.” They’ve been doing this in solar-deficient Austria, Switzerland, and Germany for a decade now.

They’ve even surpassed net-zero through the incorporation of Passivhaus: Passivhaus + renewables = Plusenergiehaus (a.k.a. producing more than consuming) – and this was also a large focus at the Hannover conference.

As the cost to achieve Passivhaus in the States drops due to familiarity, smarter designs, smarter Passivhaus integration, and the use of affordable and locally manufactured products (pretty please!) – this will be even more true. +1 Passivhaus

PV ain’t pretty

Yup, like semantics, aesthetics count. The nasty array on the banal house below isn’t even an extreme example.

Sure, there are a few projects with building-integrated PV, as well as a few zero-energy projects, that incorporate attractive PV installs (next post!) – half of them are Passivhäuser. But let’s be honest: most PV installations are an aesthetic abomination.

PV arrays tend to be rather crude, don’t work well with certain roof types (mansard! hip!) – and if the roof angle ain’t where it should be for optimal performance, racking the panels up or down on a tilted array just makes the installation look worse.

This goes doubly for solar-hot-water installations, which except in certain applications don’t really make economic sense anymore, anyway (as Martin Holladay claims, solar thermal is dead!).

I just don’t get the appeal of looking at roof-mounted PV arrays, when they’re usually so poorly installed that even the aesthetically ridiculous spoiler on a VW Beetle looks light years better. +52 Passivhaus

Passivhaus wins on the embodied energy argument

As we’ve blogged (and the Passivhaus Institut and others have shown), the additional embodied energy required to hit Passivhaus is nominal, and is recouped in a few years at most (or less – if using natural materials).

Whereas, studies on the embodied energy payback of PV systems are on the order of nearly a decade (or more) once accounting for the structure, inverter, and location/orientation/system efficiency. +1 Passivhaus

Passivhaus wins on the quantity of rare earth minerals required

You can build a Passivhaus almost entirely without rare earth minerals, but a photovoltaic panel?

Not so much. If mountaintop removal is environmentally devastating, if the tar sands are an environmental desecration – should we really give rare earth minerals a pass because iPhones and androids are überkůl? +1 Passivhaus

A Passivhaus is a resilient house

Resilient buildings have been getting a lot of (deserved) attention lately, thanks to Alex Wilson’s latest venture. In a lot of respects, this is. A certified Passivhaus should have few issues with durability (part of the reason for the extreme airtightness).

A grid-tied PV won’t keep you warm at night during a blackout. Roger Lin’s Arlington Passivhaus lost power for nearly two insanely hot and muggy days last summer. How’d the house perform? Extremely well:   “While the outdoor temperature was 92 degrees, the basement was a comfortable 73 degrees.  First floor was a warm but not unpleasant 81 degrees.  Second floor was 79 degrees… Insulation really works. First, all the insulation in the walls and roof is effectively isolating the indoor environment from the outdoor elements, slowing down the effects of extreme outdoor temperature changes, i.e. after 40 hours of power loss, the first floor only warmed up by 6 degrees (75°F -81°F) and second floor by 2 degrees (77°F-79°F).”

And, as Greg Duncan recently tweeted, the “Winter of ‘extreme storms’ is new norm. Passivhaus ensures comfort from drafts, cold; resilience against outages and respite from loud winds.”

Whether it’s holding in heat, keeping the heat at bay, or reducing external noise, Passivhaus bests a zero-energy building in nearly every case. Another way to think about it: Don’t like living next to that loud street or in that airport approach? Maybe you could use your solar panel as a sound barrier!  +1 Passivhaus

Zero-energy buildings are not bound by Quality Control

Finally, and I think this is one of the most important reasons, Passivhaus (at least in Europe) is much revered for being the “quality control” standard (despite PHIUS’s ridiculous proclamation to the opposite).

Recently, a costly zero-energy building wasn’t built well, and during the monitoring process, it became clear there were performance issues. This Living Building, the Tyson Living Learning Center, required an envelope audit – less than six months post-construction!

Shooting for Passivhaus would not only have saved them the headache of the audit, it would also have saved some serious coin. The Center as a “worst-case Passivhaus” would have reduced the amount of PV required by two thirds (!) compared to what eventually was installed/needed to meet LBC.

This debacle led to our critique of the Living Building Challenge program, which we actually support and would love to see heavily adopted – just in conjunction with Passivhaus. +1 Passivhaus

So fix yer damn U-values!

Focus on your air sealing. And build a stunning Passivhaus.  Because then – without any  horrendous, oversized, budget-busting photovoltaics slapped on top – your house can truly be a very, very, very fine house!

UPDATED 12/7/2012

Most green builders include some type of mechanical ventilation system in every home they build. That’s good. Since green buildings usually have very low levels of air leakage, mechanical ventilation is usually essential.

Unfortunately, several research studies have shown that a high number of mechanical ventilation systems are poorly designed or installed.  Among the common problems:

• Ventilation fans with low airflow because of ducts that are undersized, crimped, convoluted, or excessively long.
• Ventilation systems that ventilate at too high a rate, or for too many hours per day, resulting in a severe energy penalty.
• Ventilation systems that waste energy because they depend on inappropriate fans (for example, 800-watt furnace blowers).

It’s disheartening to learn that many green homes waste energy because of poorly designed ventilation systems that were improperly commissioned.

If you’re unfamiliar with residential ventilation systems, it’s a good idea to review the ventilation information in the GreenBuildingAdvisor encyclopedia.

The ASHRAE standard

ASHRAE’s residential ventilation standard (Standard 62.2) sets the minimum ventilation rate at 7.5 cfm per occupant plus 1 cfm for every 100 square feet of occupiable floor area.

Systems complying with ASHRAE 62.2 have ventilation rates that are relatively low; for example, a 2,000-square-foot house with three occupants requires 43 cfm of mechanical ventilation. That’s less airflow than is provided by a typical bath exhaust fan.

Since ventilation airflows are typically quite low, ventilation ductwork needs to be impeccably sealed. If ventilation ductwork is leaky, fresh air won’t reach its intended destination.

Prominent building scientists are now debating the merits of the ASHRAE 62.2 ventilation rate. Max Sherman, former chairman of the ASHRAE 62.2 committee, defends the existing ASHRAE formula. On the other hand, Joseph Lstiburek, the well-known building scientist and gadfly, argues that the existing ASHRAE ventilation rate is too high, resulting in unnecessarily high energy costs — especially in hot humid climates, where the introduction of high volumes of outdoor air increases the need for cooling and dehumidification.

Lstiburek and Armin Rudd, a fellow engineer at the Building Science Corporation, advise designers of Building America houses to ventilate at a lower rate. “These [Building America] homes have roughly 50 to 60 percent of the ventilation rate required by ASHRAE standard 62.2,” Rudd has written. “The lack of complaints by occupants indicates that the systems are working to provide indoor air quality acceptable to the occupants.”

The “great rate debate” is far from settled; stay tuned.

Do we really need mechanical ventilation?

As more and more local building codes include ventilation requirements, fewer builders are able to get away with building new homes without mechanical ventilation. However, a few die-hard holdouts defend homes without mechanical ventilation.

One reason why homes without mechanical ventilation systems work better than expected is that many common household appliances act just like exhaust-only ventilation systems. Such appliances include:

• Power-vented water heaters (50 cfm),
• Clothes dryers (100 to 225 cfm),
• Central vacuum cleaners (100 to 200 cfm), and
• Wood stoves (30 to 50 cfm).

When these appliances are operating, fresh outdoor air enters a house through random cracks to replace the air that is exhausted.

However, homes without ventilation systems are homes of the past. The building science community has reached a consensus: build tight and ventilate right.

What are my choices?

After two decades of experimentation, builders have narrowed ventilation options down to three main options:

• The simplest system is an exhaust-only ventilation system based on one or more bath exhaust fans.
• For better fresh air distribution, choose a central-fan-integrated supply ventilation system.
• For the lowest operating cost, choose a heat-recovery ventilator (HRV) or an energy-recovery ventilator (ERV) connected to a dedicated duct system.

Can I install a supply-only ventilation system in a cold climate?

Some builders worry that a supply-only ventilation system (for example, central-fan-integrated supply ventilation) won’t work in a cold climate, because the ventilation fan will drive interior air into building cavities where moisture can condense.

This worry is needless. As energy expert Bruce Harley explains, “The upper portions (walls and ceilings) of every home — typically most of the second floor in two-story homes — already operate under positive air pressure in cold weather, due to the stack effect. The relatively small airflow of most supply-only ventilation systems (75 cfm to 150 cfm) will have little effect on this situation other than to shift the neutral pressure plane down slightly, in all but the very tightest of homes. … In cold climates, I believe that distributed, supply-only ventilation such as that supplied by a ducted distribution system controlled by an AirCycler, or other ducted low-flow supply ventilation, is vastly preferable to single or multi-port exhaust-only systems, except in extremely tight homes (in which case balanced supply and exhaust ventilation is the best choice).”

What’s wrong with exhaust-only systems?

As Harley’s comments make clear, many energy experts (including Lstiburek) disparage exhaust-only ventilation systems. The main argument against exhaust-only ventilation systems — for example, a Panasonic bath exhaust fan controlled by a timer — is that they don’t provide adequate distribution of fresh air. As a result, some rooms have plenty of fresh air while other rooms remain stuffy.

According to some ventilation experts, ASHRAE 62.2 — which currently lacks any provision requiring fresh-air distribution — should be revised to include a distribution requirement. Armin Rudd has written, “I think distribution of ventilation air is an important issue. Bringing in ventilation air and hoping that it will provide adequate indoor air quality throughout the whole house is just a hope and a prayer.”

Research shows, however, that in some homes — especially small homes with an open floor plan — exhaust-only ventilation systems work well. If the exhaust fan is well chosen — my own favorite is the Panasonic Whisper Green fan, which uses only 11.3 watts to move 80 cfm — exhaust-only ventilation systems have very low installation and operating costs.

If you choose this type of ventilation system, it’s important to remember to undercut the bathroom door.

Do I need passive air inlets?

If you do install an exhaust-only ventilation system, don’t bother installing passive fresh air inlets in the walls. Fresh air will find its way into the home through random cracks.

A 2000 Vermont study (“A Field Study of Exhaust-Only Ventilation System Performance in Residential New Construction In Vermont”) by Andy Shapiro, David Cawley, and Jeremy King, investigated whether passive fresh air inlets make any sense. The researchers studied 43 new homes (22 of which had passive fresh air vents) with exhaust-only ventilation systems. They wrote, “When the EOV [exhaust-only ventilation] fan was operating, 35% of the vents were exhausting inside air, 48% were supplying outside air, and 17% of the vents were not moving air.” The explanation? “The pressures induced by fans in these [studied homes] … were low relative to pressures induced on a house by natural forces, including wind and temperature-driven stack effect.”

Central-fan-integrated supply ventilation

For years, the engineers at the Building Science Corporation have been singing the praises of central-fan-integrated supply ventilation systems. These systems can only be used in homes with forced-air heating or cooling systems. The systems include three important components:

• A duct that introduces outdoor air to the furnace’s return-air plenum;
• A motorized damper in the fresh air duct;
• An AirCycler control to monitor the run-time of the furnace blower and to control the motorized damper.

The AirCycler control (also known as a FanCycler) prevents both underventilation and overventilation. When the AirCycler notices that the furnace fan hasn’t operated for a long time, the control turns on the fan to prevent underventilation. When the control notices that the fan has been operating continuously for a long time, the control closes the motorized damper to prevent overventilation.

During the swing seasons — spring and fall — the furnace blower will need to operate for ventilation purposes. In most climates, about 15% of the annual blower run time for such systems will be devoted to ventilation only. If the system is properly commissioned, the furnace will supply a 7% outside air fraction during ventilation mode.

The big downside to central-fan-integrated supply ventilation is that the installer needs to understand how to design and commission the system. HVAC contractors capable of this task are rare. Unless the designer of a central-fan-integrated ventilation system takes great care when specifying the furnace and programming blower operation, such a system can have unreasonably high operating costs.

A well-designed central-fan-integrated supply ventilation system needs a furnace with an energy-efficient ECM blower. Such furnaces cost between $1,000 and $1,500 more than conventional furnaces. If you end up using a furnace with a conventional blower motor — that is, one that draws 700 to 800 watts — the ventilation system will incur a big energy penalty. (For purposes of comparison, a Panasonic exhaust fan draws 11.3 watts, and most HRVs draw 100 watts or less).

Duct systems and fans designed for heating and cooling are not optimized for ventilation. While ventilation airflow is typically in the range of 50 to 100 cfm, furnace fans move as much as 1,200 to 1,400 cfm. One study (Robb Aldrich, Chicago, 2005) found that a poorly designed central-fan-integrated supply ventilation system in a house with an 800-watt furnace fan used 347 kWh of electricity for ventilation during a swing-season month. During the same month, an identical home with an exhaust-only ventilation system used only 6% as much electricity for ventilation. Although the researchers were somewhat worried that the exhaust-only ventilation system might be ineffective, the data were reassuring: all of the rooms had very acceptable CO2 readings.

Will cold outdoor air damage my furnace?

Some builders worry that central-fan-integrated supply ventilation systems won’t work in a cold climate, where cold outdoor air might damage the furnace. According to Armin Rudd, such concerns are baseless — as long as the ventilation system is well designed.

Assuming a high outdoor air fraction (15%) and a low outdoor temperature (-30°F), the furnace will experience mixed return-air temperatures no colder than 55°F, as long as the thermostat is set to 70°F. Even in Chicago, such systems work well.

Do I really need the AirCycler and motorized damper?

To reduce costs, some builders install the lazy man’s version of a central-fan-integrated supply ventilation system — one that includes a passive fresh air duct to the return-air plenum, but without a motorized damper or AirCycler control.

What’s wrong with this approach?

• During the swing seasons, when the furnace fan isn’t operating, the house won’t get enough fresh outdoor air, and homeowners may complain of stuffiness.
• During the rest of the year, when the furnace fan is operating regularly, the house will be overventilated, resulting an a severe energy penalty. During the winter, all that unnecessary cold air will need to be heated; during the summer, all that unnecessary hot air will need to be cooled and dehumidified.

An HRV with dedicated ventilation ductwork

The best ventilation performance and lowest operating cost comes from an HRV or ERV with dedicated ventilation ductwork. Such a “gold standard” system should be designed to pull stale air from bathrooms and laundry rooms, while introducing fresh air to the living room and bedrooms.

Although HRVs and ERVs save energy compared to exhaust-only or supply-only ventilation systems, they are expensive to install. The high cost of these systems raises questions about their cost-effectiveness, especially in mild climates. To learn more about this issue, see Are HRVs Cost-Effective?

For ventilation purposes, either an HRV or an ERV can work well in any climate. The presumed advantage of ERVs over HRVs in hot, humid climates is not based on research or field data. As Max Sherman has written, “Almost all hot, humid climates have hours when it is dryer outside than inside, and then ERVs actually make the [indoor] moisture problem worse. The net effect this that ERVs are about a wash [compared to HRVs] for humidity control in those climates.” (For more information on this topic, see “HRV or ERV?”)

To commission a ventilation system, you need to measure airflow

Anyone who commissions a ventilation system needs to learn how to measure airflow. Manufacturers offer an array of accurate (and expensive) instruments to measure airflow. However, builders who need to troubleshoot problems may be interested in several low-cost methods of measuring airflow:

• The August 2002 issue of Energy Design Update describes how to build a homemade flow hood using a cardboard box and a $90 digital anemometer.
• Two Lawrence Berkeley National Laboratory engineers, Iain Walker and Craig Wray, have written a paper describing a method of measuring airflow with a “calibrated” laundry basket and a manometer.
• Terry Brennan promotes a method of measuring bath exhaust fan airflow with a cardboard box and a credit card.
• The easiest way to measure airflow at a supply register is the garbage bag technique developed by Don Fugler of the Canada Mortgage and Housing Corporation.