The Passive House building system from Germany is one of my favorite benchmarks. I have yet to renovate a Brooklyn brownstone by Passive House standards, although the Brooklyn Green Show House of ours comes very close.
We hope to do a Passive House brownstone renovation very soon though.
A recent article by John Straube of Building Science Corporation raised some interesting debate and a great response from Katrin Klingenberg of PHIUS. Both authors are very knowledgeable.
Here are Katrin’s remarks. It gives a good idea of Passive House. You will notice a lot of the system is focused on energy calculations, which can be very dry, but the end result is a very comfortable house.
She writes:
Here are some interesting points about Passive House building.
While we are glad that Building Science Corporation and the author of this
comparison John Straube, have taken the time and first steps to research the
Passive House Standard and its principles, we feel that the conclusions
reached and presented as facts are in need of correction.
Following John posting the article we have received quite many e-mails from
the PH community, many pointing out misinformation which might have led to
wrongful conclusions.
One consultant commented, based on his own comparison/calculations and
understanding of PH standard, that John’s assessment was off by about 50%.
Our own investigation of the comparison has found a similar discrepancy. At
this point we do feel that we need as the Passive House Institute US | PHIUS
to comment and clarify some of the misunderstandings and correct the
consequently incorrect values that are presented as facts in the comparison.
A comment upfront in regards to an e-mail received by us that understood
John’s posting as “Building Science versus Passive House.” Passive House is
firmly rooted in building science and physics principles. It does not
promote some “other” kind of physics or building science, it is building
science. The folks who know and design to PH standards today have understood
PH science and performance standards to be what is needed today to solve our
climate and energy crisis, nationwide as well as world wide. Its essence is
defining a quantifiable number and performance standard that matches our
current design constraints, challenges and crises, economically as well as
environmentally.
John Straube’s quote “There are, however, many recommendations in the PH
program that are not likely good decisions for cold climate North American
housing, and some are very impractical with little or no benefit to the
environment or the homeowner” is a surprising statement and worth evaluating
how he arrived at this judgment. He says “The unique focus of the PH
standard is an exceptional concern for heat loss by conduction and air
leakage through the building enclosure and a complete disregard for the
climate zone in its recommendations”.
PH focuses on creating a space which, by accurate design, approaches a
thermal equilibrium and assures occupant comfort. PH minimizes losses of the
thermal envelope to a point that internal heat gains (including potential
solar gains) can approach a thermal balance. By definition, this is very
climate specific, quite the opposite of what John concludes. I first pointed
that out to him when we were on a panel together discussing Passive House in
the US at the ’06 NESEA conference. The shell design is a direct response to
the climate that the Passive House is in. Passive House design has very deep
regard for climate zone, and to claim otherwise is baffling.
Where John is absolutely right is that the inspiration for the Passive House
Standard came from the US. William Shurcliff’s writings and publications are
spelling out most of all the details that are the relevant basis for PH
today.
John’s article goes back and forth between Passive House and Passivhaus. I
remember when the Passivhaus Institut first discussed with us what the
appropriate term for PH in the US would be. We argued for the English
because “passive” construction was a term that we understood to have a
history and a real resonance here, and many long-time high performance
builders are very familiar with it. We’ve always made an effort to
acknowledge and respect that history, and we had also hoped to avoid the
prejudiced positions that Straube and Holladay have taken.
But PH is also an energy standard. Its basics developed in the US during the
second half of the last century, but a huge leap evolved elsewhere. “Passive
House” describes a set of scientific principles that can be translated into
every language.
The UK on the other hand is going in a different direction. They have
resisted the recommendation from Dr. Feist of “Passive House” and insisted
on the German spelling largely, I believe, because they do not have the
history that “passive” has in the United States.
Requirements
John is correct in regards to defining the main certification criteria of PH
except for a few significant details:
Passive House criteria are fulfilled if either:
• a total heating & cooling demand of <15 kWh/m2/yr ( 4.75 kBtu/ft2/yr) each
is achieved
or
• the peak heating demand is under 10 W/m2 (3.2 Btu/ft2)
In addition, the building must meet:
• total primary (i.e., source) energy of <120 kWh/m2/yr ( 38 kBtu/ft2/yr)
• airtightness 0.6 ACH@50 Pa or less
Recommendations for building components U-values are climate specific. In
the case of windows, for central Europe it is the installed window U-value
of <0.85 W/m2K (0.15 Btu/ ft2/F, R-7.1). The chosen U-value is the result of
the comfort requirement inside of a Passive House. The comfort principle
specifies that the surface temperature difference between interior surfaces
and exterior surfaces should not be greater that 4 degrees Fahrenheit. That
temperature differential should be in the same range for every climate
(which I suppose could be one way in which we “disregard” the specific
climate zone!).
That comfort principle will determine the window quality based on the winter
design temperature of the specific climate. The U-value is chosen so that
the inside temperature of the window surface is within a certain temperature
zone on the coldest days, so that convection in front of it is eliminated
entirely and point heat sources under windows are no longer necessary.
Therefore: to maintain the interior comfort principle of warm surfaces we
have to choose U-values for all building components, not just the windows,
based on the specific climate.
To avoid creating a net energy loss through the introduction of mechanical
ventilation, highly energy efficient equipment is need. There are actually
two PH requirements to assure the energy efficiency of the mechanical
ventilation equipment. John is correct on the efficiency requirement of the
heat recovery of over 80% for Europe, but fails to mention that also a
highly efficient motor (ECM) is needed. The requirement here is 0.45 Wh/m³
(0.76 W/CFM). On the North-American market there is currently only one
ventilation system that meets both very stringent requirements. Here in the
US, because the efficiency of most equipment is very poor, we currently
still allow the old European threshold of <75% for the efficiency of the
heat recovery.
Systems not tested and verified by the PHI (all of the equipment here in the
States) are required to subtract 12% points from the highest Home
Ventilation Institute rating number, the apparent sensible effectiveness.
Testing to verify the accuracy of manufacturers’ efficiency claims has thus
far proven this subtraction valid.
To my knowledge there is currently no manufacturer in the States who uses a
counter-flow plate heat exchanger, which does provide recovery efficiencies
in the PH range (if you know of any please let us know). Counter-flow plate
heat exchanger cores are common for all PH rated ventilation systems in
Europe. They do not use dual core heat exchangers which, while better than
one core, do not result in the PH-required efficiencies. First improvements
have been made by some US manufacturers starting to introduce ECM motors.
Entering the counter-flow plate heat exchangers into the equation would be a
great next step.
John’s description of how the energy reference area in the Passive House
Planning Package is calculated is incorrect. This is a significant mistake,
as the exact calculation of the TFA (treated floor area - energy reference
area for a PH) will determine if the energy characteristic value calculated
per ft² is accurate. Wrongly calculated reference areas will lead to wrong
results.
Let’s correct the false assumption. It is not “the floor area is measured by
total conditioned area inside the cladding”. The TFA is defined as the
usable conditioned floor space in a dwelling. It is defined as the interior
floor area (not the footprint or exterior dimension), less interior walls
and columns over a certain size; and also less 40% for secondary spaces such
as storage, mechanical rooms and basements. The reasoning why basements are
counted at 60% is the realistic assessment that, even if a significant
portion is finished, there is usually a limitation to day lighting (a
basement is by definition partially underground) and there is a tendency to
use that space partially as storage. I believe, in many parts of the US
there are similar assessments of basement space taking it into account at a
reduced percentage into the finished floor area.
The result of these reductions is a much smaller energy reference area than
the one that is typically used in energy modeling software in North America,
which commonly uses the exterior dimensions of a building without any
reductions. The referenced TFA of a PH can be 30% or more different,
depending on the specific design. Therefore the energy characteristic values
of Passive Houses cannot be compared directly to US calculations as
attempted here. If the reference area of a typical modeling tool is used to
calculate Passive House values, then the energy characteristic values per
ft² will be significantly smaller than in the original PH calculation using
the TFA. This results in an even lower energy consumption value for Passive
Houses.
I consider John’s quote “Why, I can't understand; perhaps Germans don't
build basements you can live in like a modern basement in North America” as
polemic and really not helping a science based peer to peer discussion. Any
trip to Germany appears to me like a trip to the future these days in
regards to technology and efficiency. Maybe he has not been there recently.
Typical Passive House Approach
PH is aiming to create an energetic equilibrium through minimizing the heat
loss to a point where we take internal loads and solar gains into account
and a near balance point can be achieved. Contrary to some “conventional
wisdom”, that equilibrium does not happen at a prescriptive 20-40-60
everywhere. It’s that approach which disregards climate! In a PH, a designer
balances, analyzes and optimizes the effect of each component on the
performance as a whole, depending on climate.
This balancing act is done for the shell first, for the thermal envelope.
The first PH space conditioning criteria is our design constraint there.
Just as important and far from not being a concern to the PH approach is the
optimization that is done for the mechanical system: DHW, household
electricity, and auxiliary energy. Typically, the threshold of <120
kWh/m2/yr ( 38 kBtu/ft2/yr) is on the high end and a well optimized
mechanical system should allow a PH to come in generally around 80 kWh/m2/yr
( 25 kBtu/ft2/yr). (Again, all values based on the PH TFA, rather than
standard US modeling tool energy reference area - values would be further
significantly reduced if standard energy reference area was used).
In fact, the European PH community has identified the primary energy design
constraint of <120 kWh/m2/yr ( 38 kBtu/ft2/yr) to be too high to allow us as
a society to meet our CO2 emission reduction goals. It has been proposed to
significantly tighten these criteria over the next decade to well below 100
kWh/m2/yr, which many builders there are already doing.
In regards to the airtightness requirements: it certainly does help to
achieve the airtightness goal to choose a simpler shape, but in PH design
this is mainly done for energetic purposes, as the cube or even better the
geodesic dome has the best surface to volume ratio and is therefore the most
efficient shape to start with. However, requisite airtightness is regularly
achieved in far more elaborate building envelopes and is not a design
constraint from that perspective.
Solar heat gain recommendation: generally PH recommends for heating
dominated climates the choice of a conservative SHGC of around 0.5 for all
window orientations, but again, we’re a lot more flexible and responsive to
specific challenges than portrayed here. That recommendation varies also by
climate, as in cooling dominated climates a very low possible SHGC should be
used. SHGC can vary by orientation in mixed climates.
Heat recovery: as mentioned above, no dual core is used in PH equipment in
Europe. Earth tubes were used for the energetic gain, but more importantly
for the purpose of a defrost option. Earth tubes can contribute
significantly to pre-heating, pre-cooling and dehumidification. In the
recent years, earth tubes have been more and more replaced by closed ground
heat exchange systems that involve only the energy input of a very small
pump, eliminating concerns of condensation issues.
Heating of the ventilation air for space heating: One core principle of PH
design is to economize all components and their effects on each other,
energetically as well as economically. That means identifying components
that can be used for more than just one purpose. Minimizing the remaining
space conditioning to a point that it can be entirely provided through the
ventilation air without the need for additional point source heaters follows
that exact same principle. It is a preference of Passive House to be able to
use only fresh-air space conditioning and the small fan of the ventilator
for distribution. This distribution system used to be during the early years
of PH in Europe a requirement to meet the standard. Such a system can be
achieved, to economic advantage, in climates around 4000 HDD as is the case
in central Europe and in some climate zones of the US. In those cases, point
sources such as radiant heat and radiators are not necessary. Most PH’s have
a radiator in spaces where there might be the need for raising the
temperature quickly for a short period of time, as might be the case in a
bathroom.
John is correct that meeting the peak load requirement of 10 W/m2 (3.2
Btu/ft2) is next to impossible in cold and very cold climates in the US. In
those climate zones, in addition to the space conditioning that can be
transported through the ventilation air, small point source heaters become
necessary to meet the peaks.
Ventilation Requirements in a PH
The ventilation rate in a Passive House is not due to inexperience or based
on getting it right by accident [John Straube: “In Europe, higher
ventilation rates are often specified, likely because there is not a long
history of providing mechanical ventilation”] It is based on the hygienic or
Indoor Air Requirements, to assure exceptional indoor air quality. Dr. Feist
told me at one point that he paid close attention to compare the
requirements of PH to the requirements of ASHRAE and that measured PH indoor
air quality was equivalent to the IAQ of the highest ASHRAE rating.
The PH ventilation rate is determined by the expectation on IAQ in regards
to supply and exhaust requirements. On average that air flow rate comes out
to 80 CFM in a single family home. This does not reflect the fresh-air
requirement (PH and ASHRAE requirements for those are as far as I know
comparable) as John states, which in a single-family home is typically lower
and right around his proposed 50 CFM. The 80 CFM design air flow is due to
the second design constraint of a ventilation system, the exhaust air
requirement for kitchen and bathroom to safely exhaust pollutants and
moisture. Those requirements are also in line, as far as I am aware of, with
the ASHRAE requirements.
There is no compromise being made on the ventilation requirement in very
cold climates, and re-circulation defrost options in ventilation systems for
PH are uncommon.
Unlike John, in 7 years of working with the PHPP I have not seen a warning
pop up that warns of over-ventilation. The opposite is the case: a warning
pops up if the ventilation rate is not sufficient, as that affects
appropriate moisture removal as well as IAQ.
In regards to HRV/ERV performance the PHI has found deficiencies in the
standard industry testing in Europe as well as in the US. As I mentioned
earlier, they require the PHPP modeler to subtract 12% points off of the
standard rating if the unit has not been evaluated through approved testing
standards to account for those testing deficiencies. There are now
manufacturers on the European market who have achieved actual testing
results verified through the PHI of almost 90%!
One European HRV manufacturer tested by the PHI with an efficiency of 88%
has come onto the US market. The price for their unit is $1800. In my humble
opinion this is a very good price for what you are getting: superior and
healthy indoor air quality and comfort as well as an exceptional and
optimized energy performance overall.
HRV’s considered efficient by John are meeting Energy Star with a fan
efficiency of 1.5-2 W/cfm. This is twice the efficiency specified by PHI. If
this is the best we have in North America, we have long ways to go.
John’s assessment of “increasing ventilation rates to allow the use of
ventilation air as the only means of heating is at best highly restrictive
to a design and at worst simply impractical and antithetical to a low-energy
house” is inaccurate. In very cold climates, as we’ve already discussed,
peak heating loads cannot be met through fresh air heating alone. In fact, I
would move the cut-off to 5 degrees Fahrenheit instead of 0 as proposed by
John, and even that is a stretch. PH does not recommend increasing the
ventilation air to compensate and make it happen. The ventilation rate is
strictly determined by the IAQ requirements and it is wrong to say that the
PH approach advocates increasing the rate for the purpose of delivering
space conditioning in very cold climates.
Radiant heat, radiators, re-circulating air-heating are all fine solutions
to provide space conditioning in very cold climates in a Passive House as
long as the needed ventilation rate for hygienic and comfort purposes is
observed. Due to the tiny need for heating energy in a Passive House, the
temperature of radiant floors typically is set to a point where one can
hardly feel that the heat is on. Radiant floor in localized point source
applications is an excellent choice for a Passive House. My own house has a
radiant floor in the bathroom. Not sure where this characterization of
“dogmatic avoidance of using radiant floors” comes from, but it’s wrong.
Passive House addresses ducting in unconditioned spaces and strongly
recommends keeping all warm ducts in conditioned spaces. That said, one may
find it necessary to place warm ducts outside the thermal envelope, in
retrofit applications for example.
The typical BSC BA Home compared to Passive House
As outlined above, typically, when Passive House is compared to High
Performance homes in the US, significant differences in energy reference
areas are not taken into account. Therefore, if one attempts an
apples-to-apples comparison, one should be sure to understand the same base
assumptions.
Our Passive House, the Fairview House in Illinois based on PH specifications
(two story, slab on grade), calculates out to 4.46 kBtu/ft²/yr for heating
and a total primary energy of 34 kBtu/ft²/yr based on a mechanical system
using a solar thermal system and air-to-air heat pump/electrical base board
back-up calculated in PHPP.
If we change the assumptions to the 10-40-60 R values of a BSC BA home for
the climate in Illinois, use 3 ACH@50, same ventilation rate of 80 CFM to
maintain comparability at the same level of comfort and Indoor Air Quality,
installed U-value of 0.2 for windows for this particular climate, the energy
characteristic values are 14.2 kBtu/ft²/yr for heating and a total primary
energy of 45.3 kBtu/ft²/yr. This is three times the heating demand annually
and an increase of overall primary energy consumption of over 30%.
This is hardly comparable or even approaching Passive House. I’d be
interested to learn about those BSC BA homes that have been built in cold
climates that compare favorably to Passive Houses (which have been built in
10,000 HDD climates). I recently became aware of a Super-E home built in
Edmonton that comes very close.
If, as a next step, I insert a different climate data set we can determine
where in the United States a BSC BA home as specified above actually does
meet Passive House Standards. If the Illinois climate is substituted by
Raleigh, NC climate data, the BSC BA home envelope meets Passive House
criteria of 4.75 kBtu/ ft²/yr for heating, 2 kBtu ft²/yr for cooling, and a
total primary energy of 33.7 kBtu/ft²/yr.
I think I have stressed IAQ enough during this text and it has become
hopefully clear that IAQ is one of the initial design criteria for Passive
House. Good building science knowledge of the envelope construction and
moisture management is assumed, building for durability as well. One of the
reasons for the only qualitative requirement of Passive House, the level of
airtightness of buildings to as close to airtight as possible, has
durability implied in it as John correctly points out: “…damaging air
leakage condensation is likely controlled by the very low acceptable air
leakage.”
Energy consumption compared
A Passive House of 2000 ft² using a condensing gas boiler and a solar
thermal system, by my experience, would come in at around 80 kWh/m²/yr
primary energy consumption or better (only PHs using a lot of electricity to
heat with are right up against the upper limit of 120 kWh), and that is if
the PH TFA is used as energy reference area (typically used energy reference
area of exterior dimensions would further lower this primary energy
consumption value).
John’s comparison home at 160 kWh/ m²/yr uses at that point a minimum of
twice as much primary energy as a Passive House with the same mechanical
systems assumptions/primary energy conversion factors, depending on fuel
use.
John does allow that the overall consumption in a Passive House is typically
lower, but that this can be achieved in a North American home as well
“depending on occupants operating and maintaining the home in a low energy
manner.”
We have heard a lot recently on how important occupant behavior is. We heard
arguments that we have to “optimize occupant behavior”. One should be
careful with occupant behavior approaches to building performance
evaluation, because they can be readily gamed. ANY home can produce
terrifically low power bills by lowering comfort expectations.
Passive House Standard’s approach is different. Quality occupant comfort
levels are assured. Homes designed based on PH principles are designed such
that a building’s wasteful behavior is limited, by design, to a point where
it matters little how the occupants use their home. Some like it hot, some
like to save as much as they can. The critical challenge here is to come up
with a performance standard goal that on average, once the homes are
occupied, meets our societal and global requirements to reduce energy
consumption and green house gases to safe levels. We don’t want to have to
police people’s behavior either.
We need a design benchmark that allows people to live their lives as they
wish while providing the same level of comfort and meeting our environmental
and economical design constraints.
If now a Passive House owner installs a 2kW PV system or, better yet, buys a
share in a wind farm, we are very close to meeting or to exceeding our goal
of carbon neutrality in the near future. Passive House standard has been
recognized in 2007 by the International Energy Agency’s “Energy Efficiency
Policy Recommendations to the G8 2007 Summit in Heiligendamm,” recommending
conservation levels of the Passive House Standard or Zero-Energy buildings
in countries around the globe as energy code benchmarks. “Governments should
set objectives for PEH (Passive Energy Houses) and ZEB market share of all
new construction by 2020.”