Earth angel Passivhaus goes mainstream residential - AIRAH
←
→
Page content transcription
If your browser does not render page correctly, please read the page content below
FEBRUARY–MARCH 2021 · VOLUME 20.1
RRP $14.95
PRINT POST APPROVAL
NUMBER PP352532/00001
Earth angel
Passivhaus goes
mainstream residential.
F E AT U R E
How to push
the envelope
Over the past decades, the US has transformed its
approach to airtightness and ventilation. Building physics
luminary Max Sherman talks to Sean Maxwell, AM.AIRAH,
about the journey, and the lessons Australia could learn.
Max Sherman is an ASHRAE fellow and of space conditioning energy is due buildings if they aren’t designed properly.
distinguished retired senior scientist to air leakage in a traditionally leaky So yes, dedicated and properly designed
at the Lawrence Berkeley National home. Most ventilation in a leaky home ventilation is essential for healthy indoor
Laboratory in the US. Over many years his actually comes from air leakage, which is air quality and it has to be done right.
research has informed our fundamental why those homes did not have designed
understanding of building energy ventilation systems. With such air leakage SM: When you “ventilate right,” is
efficiency and occupant health. Sherman also comes uncomfortable draughts, there a basic formula to do that?
was keynote speaker at the AIRAH moisture intrusion, and degradation of MS: One of the points in my talk was
Building Physics Virtual Forum last year, insulation. It’s a real risk to a building that there are many steps along the way
and Sean Maxwell, AM.AIRAH, took and its occupants. to ventilating right. What is acceptable
the opportunity to ask Sherman about as a code minimum might be different
a range of issues relating to ventilation SM: So “building tight” must not
than what is considered best practice.
and building envelope physics. be that hard if most US state codes
What is best for a project will depend
require some airtightness and
Sean Maxwell: Max, your talk on how much influence you have on
even blower door testing. Right?
the design and what kind of budget is
at the AIRAH Building Physics
MS: After you know how to do it, it’s available. But there are almost always
Forum was titled “Build Tight
not hard, but getting that knowledge steps you can take along the way.
and Ventilate Right. It’ll Be
is not easy. The industry has a
Okay.” It sounds like you trying SM: For our codes and standards,
sometimes‑painful learning curve to
to reassure your Aussie friends? should acceptable solutions to
change practices to where it is easy.
Max Sherman: Australia can learn from requirements for ventilation
the American experience by informing SM: But can there be a vary by climate zone?
regulation with building science. The US tightness problem too?
MS: Minimum requirements for
certainly hasn’t done everything right, MS: Absolutely. Too tight can be ventilation (as in m3/hr) do not
but Australia can learn from what we did unhealthy and also cause building vary by climate. You always need
right and where we fell short. management problems that one proper ventilation. But the optimal
must be aware of. You can’t assume, solutions may change by climate,
SM: Why has so much of your
for example, that people are going to for example, because of infiltration
research been devoted to
just be there to open windows when impacts or costs to condition outdoor
understanding and addressing air
it makes sense to; some contaminants air. Furthermore, one needs to
leakage? Why is it such a problem?
aren’t properly diluted when it’s look at moisture as a contaminant,
MS: Well, energy is what gets one’s too tight. Ventilation systems can have which means that a moisture design
attention first, because a third to a half trouble working correctly in ultra-tight may be necessary in some climates.
22 F EB R UA RY– M A RCH 2021 • ECO L I B R I U M
Max Sherman
Sean Maxwell, AM.AIRAH
SM: California requires SM: Can something as simple
ASHRAE 62.2. What are as a continuously running
some of the basic features exhaust actually achieve
of that standard? adequate indoor air quality?
MS: One requirement is that local MS: Yes, and it’s the most
exhaust such as kitchens and baths common method we use.
must go directly outside. Intermittent In my talk I spoke about a recent
ventilation is good for singular events study of new homes in California
like cooking or a shower, but it’s not all and compared it to a similar
you need. ASHRAE 62.2 also requires study from 10 years prior.
continuous whole-dwelling ventilation,
One important thing to note
which is useful for extracting pollutants
that build or release gradually over was that when we started the
time, such as VOCs and moisture and study, most houses in the
emissions from occupants and their group had their ventilation
activities. systems disabled because of
noise, or not knowing what the
SM: Are there inexpensive ways random switch on the wall was
to provide adequate ventilation? controlling. We had to make sure
all the houses had their systems
MS: Yes, the cheapest solution is the running, then we measured
“double duty” bath fan. If you don’t contaminants.
already require exhaust fans in the
wet rooms, you should, because these Most of the houses were of
are the rooms that are responsible for moderate air tightness of
the most internal moisture generation 3 to 6 [m3hr-1m-2]. But with
and often other contaminants as well. their simple systems running,
California requires ASHRAE Standard basic markers of IAQ like
62.2. A continuously running Energy formaldehyde and particulates
Star bath fan, which has an added were improved compared to
cost of maybe AU$100 over standard the homes built 10 years prior
construction, can meet the minimum. with no continuous ventilation.
Again, a basic ventilation system is It didn’t take much to make
better than no system at all. a positive change.
FE ATURE
SM: Regarding building tight, If you had exactly the right amount step of the way. The reason for this is that
would you have any idea where of infiltration to serve as ventilation, no one, traditionally, does a moisture
we could set benchmarks to start you’d have the exact same amount of design. Things worked historically and
requiring ventilation? I’ve heard moist outdoor air entering through when only minor tweaks were made,
the number 7 ACH50 as a limit infiltration as you would indoor air they kept on working. If big changes
below which you need mechanical leaving through exfiltration. The exit are contemplated, one must design
ventilation. Is that a good limit? point with exhaust ventilation is for moisture as well.
obviously the fan itself. But if the
MS: The 7 ACH50 tightness limit was We did have huge problems in our
infiltration coming in is not a problem in
a rule of thumb from a bygone age. hot‑humid climates, which are not gone
a leaky house with no ventilation, it won’t
We now have the tools to do it better yet. One of the biggest contributors we
be a problem with exhaust ventilation.
and it’s not that hard. If you separate had was vinyl wallpaper. It was popular
the tightness requirement from the You could have a problem if you had for many years but is impermeable and
ventilation requirement, the job a poorly constructed building and so moisture condensed on the back side
becomes much easier. The tightness system in the first place, of course. and rotted the plasterboard. Of course,
requirement may start out lenient but any vapour retarder on the inside of
then tighten as the industry learns to SM: So the indoor air that was the plasterboard can be problematic if
do it easily. The limit itself may depend leaving through the building moist air can get behind it.
on the energy benefit of tightening and envelope is now leaving through
thus on climate and energy costs. the exhaust fan. But the outdoor SM: How do you handle
air infiltrating through the that mix of problems?
Underlying all these points is that envelope was doing that already?
there needs to be a way of determining MS: The right thing to do in an air
minimum acceptable ventilation. MS: Right, the moisture impact of conditioned home in such a climate
Having that allows the determination adding continuous mechanical exhaust is to have an air barrier and a vapour
of optimal strategies for airtightness, is quite small. In fact, tightening and barrier on the outside of the insulation.
energy, moisture, etc. I am obviously adding a fan will generally be an Even if there is no insulation, the
partial to ASHRAE, but both the improvement on the moisture front, barrier needs to be on the outside so
Europeans and east Asians have some but one can always think up pathological the condensed water can drain in a
of their own ventilation standards. cases. The time to look carefully is controllable manner. This is especially
I don’t think those apply as well to when one is adding ventilation to a true when the outdoor dew point spends
Australia given that those two were severely under-ventilated building. a lot of the time above maybe 20°C.
optimised for their climates and Now sometimes in the field we SM: This goes back to the
construction types (e.g., the European occasionally do see a problem, but when question about make-up air for
one requires room-by-room ventilation).
we look into those, the exhaust fan was a simple exhaust-only ventilation
There are lots of changes to consider minor contributor. The more important systems, and whether it’s a
at once, but treating the building as a contributor is usually the improper problem in hot humid climates.
system is really the only way to make sure use of vapour-impermeable layers. If you concentrate on sealing the
you don’t have unintended consequences People do sometimes blame the last straw, exterior weather‑resistive barrier
that could doom any advancement. which is often the ventilation. in the house, can you reduce the
risk of leaks carrying moisture
SM: The cheap and simple double- SM: What design and construction through insulated wall assemblies?
duty bath fan solution sounds ideal guidance is there to help
for homes in colder or mild climates. with these problems? MS: A tight envelope helps reduce
the moisture problems, but only if
MS: It’s the most common system that MS: A lot of problems that were at first the tightness is on the outside of
meets ASHRAE 62.2 for those climates, attributed to ventilation were really any moisture-sensitive materials:
and our research shows that it’s effective problems in designing the building plasterboard, untreated wood, etc.
enough to be a reasonable code or envelope. That does not happen much If you caulk and seal all the plasterboard
standard requirement. anymore, but it was a big deal for a while. to make it really tight, but still let
Building Science Corporation has a lot outdoor air get to its (cold) backside,
SM: Is there any danger to using of very good publications on how do the the plasterboard will rot from the
that solution in a hot humid building envelope correctly. If you have wall cavity inward – maybe the studs
climate like Louisiana (negative not had a chance to look at their stuff you too. Moisture retarders need to be
pressure, pulling humid air through might want to. Spring for one of their on the warm side of the wall and if
cooled building assemblies)? builder guides (e.g., hot‑humid climates) the air‑tightness layer is the same,
MS: I have to laugh. Not because it and it might be worth your while. then it must be as well.
isn’t a good question, but because a lot SM: OK, so the industry has SM: Can an air conditioning
of us worried about this exact problem
been seeking solutions to these system help with controlling
for a long time. In fact, for a few years
problems for some time already? the humidity indoors?
there was a prohibition in using that
system in those climates. But skipping MS: Moisture problems in humid MS: Yes, that can help because it
ahead to the answer – no it’s not a real climates have dogged the transition to supplies “free” moisture removal.
problem. Here’s why … high-performance buildings at every One important thing to note, however,
24 F EB R UA RY– M A RCH 2021 • ECO L I B R I U M
FE ATURE
is that as air conditioner size gets smaller our political example. We do, however,
with improved buildings, the amount have climates and building types quite
of latent extraction we used to count similar to Australia and have been
on to keep the inside dry goes away. working the technology side a bit longer.
This both extends the moisture problems My advice would be to steal our best
to less severe climates and also increases ideas and then use them better.
the need to do a dedicated moisture
design … which could necessitate SM: A few of your slides in your
additional or different equipment. talk referred to good intentions
but missed opportunities.
SM: Can modern AC systems “Beware the Bleeding Edge”
with a “dry mode” reliably help it said. Any advice to take away?
remove humidity, or is it not wise
to count on those as a strategy? MS: Going too far too fast often leads
to unintended consequences. It is really
MS: We have a variety of good to have a segment of the industry
air conditioning equipment that that wants to do this as long as everyone
has enhanced dehumidification understands there are going to be
capabilities. Depending on the setbacks. The more setbacks there are,
technology it can be complete or the faster we all learn how to progress –
partial. Such equipment can be but of course that means someone may
helpful in reducing indoor humidity, get the bad end of the stick.
but one must determine if the latent
capacity is sufficient and one must Another issue is optimisation versus
have a suitable control strategy. resilience. Many over-optimised
systems do not survive contact with
Energy-recovery ventilators can reduce real life. For example, if a key filter that
the latent capacity needed from such is supposed to be changed annually
equipment and save operating costs, gets clogged with insects every month
but they are not a solution by themselves. during the summer, the system won’t
work half the year. This is a real story.
SM: In the US you have the
Home Ventilating Institute (HVI) For the majority of buildings,
to give respectable performance it is probably a good idea to favour
ratings for ventilation systems. simple and robust compared to
We don’t have that here. complicated and brittle.
Any advice for us? Is it worth To say it another way, Australia may
setting up some performance make progress faster by taking baby
rating capacity here for ourselves, steps quickly than by taking big leaps
or should Aussie products be it might regret. ❚
sent to America for testing?
MS: Certified rating of equipment is a
good way to make sure all manufacturers Would you like
can have a level playing field to compete to know more?
in. HVI principally serves North
American customers, but manufacturers Access to Max Sherman’s keynote
from around the world have their presentation and other recordings from
products rated by them. The testing does the Building Physics Virtual Forum are
not have to be done in the US. available to registered delegates or can be
purchased via bit.ly/3mpVkS9.
I would encourage you to discuss
setting up what Australia needs with Sean Maxwell, AM.AIRAH, is technical
HVI, before striking out on your own. manager for Pro Clima Australia and
You would get a lot more product one of the members of AIRAH’s
availability if the manufacturers Building Physics Special Technical Group.
could use their usual test methods. If you would like to find out more about
the STG and get involved, go to
SM: From an American www.airah.org.au/buildingphysics
perspective, what advice would
For guidance on designing building
you give to Australia’s evolving
envelopes, Sherman recommends the
energy-efficiency landscape?
Building Science Corporation.
MS: The US as a whole is not very good Go to buildingscience.com/
at restraining its carbon footprint, book‑categories/builders-guides
so I would not recommend followingYou can also read
