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To PHI or to PHIUS

The term “passive house”, once obscure jargon used solely by engineers, is now being heard more and more among construction professionals. The concept is gaining ground, here in Quebec as in the rest of the world. After all, who wouldn’t want a more efficient house — one that would radically reduce one’s energy bills? Building to passive house standards is, without a doubt, a serious technical challenge. To further complicate matters is the fact there is not one but two certification bodies in North America, PHI (Passive House Institute) and PHIUS (Passive House Institute US).

Back in 1990, the father of the Passive House Institute (PHI), Dr. Wolfgang Feist, inspired by the Saskatchewan Conservation House, built the first “passive” house in Darmstadt, Germany using the latest building science to dramatically increase the building’s energy efficiency. PHI then put together a certification system based on those very high performance standards — guidelines to help others achieve the same results. Ten years later, at Dr. Feist’s urging, Katrin Klingenberg, a German architect, introduced these concepts to the United States. From the start it was clear that some of the PHI methodology would need adapting to carve out a place for itself in the North American market. Just the simple process of converting all the measurements from metric to imperial in PHI’s Passive House Planning Package (PHPP) software boosted its popularity. Klingenberg and other members of PHIUS then launched a collaboration with energy sector leaders to adapt the PHI performance standards to North American climate realities (we’ll get back to that) to try to make the standard accessible to all. Across the pond, PHI was not at all keen to see these collaborations, even less to see their standards converted or deviated from and they officially cut the cord in 2011. Debate among ecological builders has been raging ever since about which certification constitutes the way to build a passive house.

Common ground

The Passive House concept is based on three ideas that both certifications share. First off, a fabric-first approach of reducing heat loss through the building envelope achieved through thermal bridge-free construction, super insulation and air tight construction. Heat loss through the envelope not only hampers a building’s energy efficiency but can also bring about damage to its components (via humidity and mould). The second principle is to optimize and balance energy gains by, for example, strategically placing the windows. The goal is to optimize solar heat at key moments of the day while avoiding the building becoming a furnace once the setting sun reaches deep into the house (balance). Choosing the right windows is key. Heat gain mustn’t be offset by losses caused by badly insulated glass or air leakage in the frame. The third theme is efficient systems. After all, the best way of cutting back on energy needs is installing mechanicals (air exchanger, appliances, heating, water heater and lighting) that are incredibly efficient to begin with.

These are the building blocks of both PHI and PHIUS. The primary goal of both bodies is the same — to design buildings that use next to no energy. The point where they diverge is in how to evaluate the performance standards.

Performance standards

The PHI standards are clear, precise and set in stone. One size fits all. We’ll get into the nitty-gritty below but to summarize :

Airtightness : ≤.60ACH50 (Air changes per hour at 50 Pascals)

Heating/Cooling : Annual consumption ≤ 15 kWh per square metre of living space (Treated Floor Area) in heating and cooling OR a peak demand of ≤10 W per square metre of living space (TFA)

Primary Energy: Annual consumption in Primary Energy ≤ 120 kWh per square metre of living space (TFA)

What does all that mean? The airtightness of a building is calculated by measuring leakage — the volume of air (in cubic feet per minute, CFM) entering a building pressurized at 50 Pascals and dividing that measurement by the net air volume of the house. Simple enough. But does it make sense? After all, air leaks happen through a building’s shell (the surface) and two buildings of identical volumes may have entirely different surfaces. One wouldn’t estimate how many gallons to buy to repaint a room by calculating the room’s volume. And like in painting, when it comes to air leakage, it’s all about the square feet of the surface. Growing a geometric shape affects surface and volume differently. Volume, with the added dimension of depth, grows exponentially as the building grows, so dividing the air leakage by the volume as opposed to the surface of the building makes it far easier for bigger buildings to achieve the standard. The passive house movement is, in essence, an ecological one so if buildings are not going to be measured by the same yardstick, should it not favour those of more modest proportions?

PHIUS+ uses a different standard, moving from ≤.60ACH50 to 0.05 CFM/square foot of surface area. It starts with the same metric — the total of air leakage (measured in CFM) but divides it by the total surface area of the building.

As for heating and cooling load standards, PHIUS again takes exception to PHI’s approach. The number of kilowatt hours permitted annually as well as the number of watts in peak load were established for German climate realities (the birthplace of PHI). The standards are well-suited to central Europe’s modest temperature variance and humidity levels but don’t work in other climates. PHIUS’s engineers —struggling to establish the program in North America — quickly realized that expecting a building in the Yukon to have the same annual heating load as a building in Hawaii just wasn’t realistic. That is why, in collaboration with the Building Science Corporation and the US Department of Energy, PHIUS established new “climate-specific” targets that are tailored to their locale. A fairly exhaustive map of North American locations and their targets is housed on the PHIUS website but here’s a little snapshot:

A passive house owner in Juneau gets to use more energy heating their home than a homeowner in El Paso while the Alaskan has a much smaller allowance in the cooling department than their counterpart in the south.

Were one to give a pair of sandals and a pair of winter boots to an Alaskan and a Hawaiian, would it be reasonable to expect them to use each pair for six months of the year? If an equivalent amount of energy and resources went into creating the pair of boots and the pair of sandals, why not allow the Yukon resident to wear his boots eleven months of the year and his sandals for one while allowing the Hawaiian to do the opposite? In the end, is it more important for the planet that they use the same amount of energy or that they use their allotted energy in an identical fashion?

As for Source Energy, the important thing to understand is that we’re talking about energy at its source. The bulk of energy gets lost in transport — up to two thirds of it here in North America. For every three kilowatts that leaves the power plant only one makes it to the house. PHIUS calculates that ratio, what they call the Source Energy Factor, at 3.16. PHI, again based on EU data, sets that ratio at 2.6 worldwide (pretending the North American grid is 20% cleaner than it actually is — a curious practice for a standard aimed at carbon reduction). In normal circumstances, this factor grows or shrinks depending on how dirty (coal, nuclear) or clean (hydroelectric, wind) the energy is and how vast the network is (e.g., a hydroelectric dam may be hundreds of kilometres away from the houses it supplies). Because this factor varies so much depending on one’s location and energy source, builders lobbied PHIUS to tailor the standard according to the energy source. PHIUS’s response was to change the whole equation. Rather than evaluate consumption on a per square metre basis they made it about how many people live in a house, deeming the reduction of CO2 emissions a responsibility to be shared by every person (no matter what their project or energy source). PHI’s standard of ≤ 120 kWh/metre²/year became PHIUS+’s ≤ 6200 kWh/person/year. The number of residents is counted as the number of bedrooms plus one. For non-residential buildings the ≤ 120 kWh/metre²/year metric remains in effect. This way of calculating gives “right-sized” projects a distinct advantage because it makes it very difficult for someone living alone in a mansion to hit the targets.

Over the next few years, PHIUS aims to gradually reduce the standard to ≤ 4200 kWh/person/year, lowering the threshold for a single resident to 8,400kWh and to 16,800kWh for a family of four.

The only renewable energy that PHI’s PHPP software allows for is solar-thermal energy (e.g., solar hot water). All other renewable energies are disqualified — something PHIUS changed by including on-site energy production (photovoltaics or wind turbines) used on site in their assessment.

PHI and PHIUS both believe that the Source Energy Limit should be the the most stringent but PHIUS’s is based on the number of residents rather than on square footage of living space. This doesn’t make the standard easier to hit. It actually makes it harder. The fewer the number of residents and the bigger the living space, the more challenging it is to achieve the standard.


Another problem PHIUS has with the PHI method is their certification of air exchangers (HRV and ERV). PHI set up its own proprietary method of evaluating air exchangers to help users make an enlightened choice among certified models. If, however, one wants to use a model that hasn’t been evaluated (aka not PH-certified) PHI uses the manufacturer’s specifications and cuts 12% from its efficiency. PHIUS’s Technical Committee frowns upon this practice, deeming it unfair discrimination against some very efficient machines simply because they aren’t PH-certified. North America has its own certification program for air exchangers (done by the Home Ventilation Institute) so PHIUS launched its own modeling protocol incorporating the best elements of PHI and HVI to make more available models eligible. The goal was not to dilute the standard but to give builders more paths to certification.

It is said that PHIUS is easier to achieve than PHI. This is a broad overgeneralization. Certain objectives may be easier. Others are harder. It all depends on scale and location. PHIUS is definitely more adaptable than PHI but does that make it better? Perhaps the real question ought to be why we aim for certification in the first place? Is it for the prestige and exclusivity of the Passive House label or are we genuinely hoping to reduce CO2 emissions and our overall ecological footprint? Wouldn’t it make more sense to encourage the most buildings to achieve a certain threshold of energy efficiency rather than creating a very narrow path for very few?

There’s nothing wrong with builders espousing one certification over another. PHI, PHIUS, LEED, NetZero, Novoclimat all have their raison d’être and were all created with the intention of easing the stress we’re putting on the planet by pushing the industry toward environmentally sound, super-efficient construction. If their objectives are indeed the same, why not redirect the energy spent debating the merits of one over the other into finding solutions together and making healthy, high performance buildings that are accessible to all.

Charlotte Paquin Béchard Rocket Construction

translated by Sarah Cobb


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