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A note on humidity:
Many people try to cool themselves with
lower room temperatures when in fact the problem could
be humidity. By lowering the humidity in the space and
increasing air velocity with ceiling fans (or similar)
many occupants can sense and perceive cooling without
running traditional A/C systems.
For additional cooling techniques see:
Humidity: An Important Nexus in Indoor
Environmental Engineering
Thermal comfort analysis with elevated
air speed and moderate humidity, SI and IP screen shots.
For help in understanding this tool see
any one of our 2016 thermal comfort slide presentations;
or sign up for our 2017 programs as they become
available.
Why do homes need Tinkerbell and pixie
dust?
Because the design and build process happened in Never
Neverland.
Image source:
https://en.wikipedia.org |
Overheating Doesn’t Happen in Never Neverland:
Peter Pan is in The House
Copyright © 2016, Robert Bean, R.E.T.,
P.L. (Eng.) All world rights reserved. Originally in HPAC
Magazine Canada Fall 2016
Recently Wilma Leung, my new favorite sage and
her progressive team from BC Housing / HPO hosted 340 builders
and affiliated service providers from British Columbia to
participate in my class on
thermal comfort for vulnerable populations. The program
included passive cooling and solar control techniques for
mitigating overheating risks during climate changes of
consequence.
Now you don’t have to be a climatologist or
frontline healthcare worker to understand aggressive thermal
changes affect vulnerable populations as heat related morbidity
and mortality reports increase across the planet.
Concern is specifically for infants, infirm, injured, and
elderly and others in palliative care environments. This
group spends more time indoors, are unable to find refuge from
the heat, and are unable to or find it difficult to adapt.
It would be naïve for residential HVAC designers
to observe this specific combination of demographics and climate
change from the perspective of a spectator. Why? Because your
clients or their relations have a high probability of entering
one or more dimensions in the vulnerable category. Statistically
most will want to navigate that experience in familiar
surroundings of home rather than the confines of an institution.
But consider the significance in choosing the indoor
environmental quality and vigor of a modern healthcare facility
versus that of a
code built house. The former designed to promote healing and
built to function during environmental stress and/or failures in
infrastructure. The latter by default a satellite extension of
the former but only designed to reduce the probability of
illness.
Arguably code built homes are designed for
aesthetics rather than resiliency. Thing is…the last time I
checked there wasn’t any doctrine to say we can’t covet for our
homes the quality environmental systems found in modern
healthcare.
Peter Pan in HVAC Land
Setting aside the exceptions, residential HVAC
designers employed in the distribution chain generally practice
in a Peter Pan world where clients never become old, injured or
sick. For the most part the residents potential palliative needs
or needs due to changes in the earth’s environment isn’t even on
the radar screen for discussion as builders frequently isolate
the purchaser from the design technician. The majority of
practitioners simply by choice or direction take a passive
position within a segregated construction system; only a few
become active participants on
integrated design teams.
In the first scenario the design business follows
the status quo, resigned to have zero influence on property
schemes, architectural, interior and building systems. There is
no motivation or there is prevention to reach out to the client
to actually ask what they want in mitigating problems in the
event the home and/or occupants become at risk due changes in
health and/or the environment.
In this scenario the designer accepts the
construction drawings as delivered to the office, load
calculations are performed as per minimum requirements, and
schematics developed based on first cost rather than life cycle
cost. Then specifications and equipment lists are prepared and
sent out for tendering where typically the
lowest cost prevails.
It’s unfortunate but this represents the majority of residential
design practices across North America and ultimately creates for
some occupants a need for Tinkerbell and her pixie dust.
At the other end of the spectrum is the
forward thinking educated client who lives outside of Never
Neverland and is fully engaged in reality. They are looking
for that integrated HVAC designer to influence the architecture,
building orientation and enclosure; and materials and methods of
construction. It is here in the latter approach where
significant differences can be made for and on behalf of the
family.
Since we’re talking about rising temperatures and
staying cool, let’s look at some strategies and tactics that can
be used to control overheating at times when people need
comforting care environments during failures of one form or
another.
Strategies and Tactics
First let’s state the obvious - the number one
reason for overheating is a radiant problem. That being the
shortwave high intensity solar energy absorbed by and
transmitted through into the building. Poor to no shading,
inadequate enclosures and high window to wall ratios are the
major culprits here. Second to this is internal loads created by
the conversion of shortwave to longwave energy vis a vis
absorption and
conduction; plus
radiant (and
convective) loads from people, lights, appliances and
IT/entertainment equipment.
All of the above raises the
mean radiant temperature (MRT) which retards the shedding of
body heat. Why we try to solve predominantly MRT problems with
convective solutions is a testament to habits gone bad.
To solve the radiant problem you’re going to need
to understand how earth revolves around that big lamp in the sky
and how that path affects the real estate being developed. There
are many free online solar path tools but my favorites are those
developed by Dr. Andrew Marsh. Shown in Figure 1 is how that
solar load on a random spot in western Canada on June 21 looks
like from outer space.
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Figure 1 is how that solar load on a random spot
in western Canada on June 21st looks like from outer space. |
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Figure 2 is the sun path for
Figure 1 at the same location. |
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Figure 2 (above) is the sun path details for the same location
and Figure 3 (below) illustrates the exposure and shading at the
same location (mid morning) for a hypothetical city populated by
various building geometries. |
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Figure 3 Mid-morning solar exposure on the east
and south facades and shading result for a hypothetical city in
western Canada. |
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Figure 3 Mid-morning solar exposure on the east and south
facades and shading result for a hypothetical city in western
Canada. Now consider the sun paths and the solar irradiance at
the Earth's atmosphere of about 1.36 kW/m2 of which
approximately 1000 W/m2 makes it to the earth surface when
measured perpendicular to the sun. Even when measured through
various filters (i.e. clouds, particulate) at various altitudes
throughout the year it is still a lot of energy that gets
absorbed and transmitted into the building (see figure 4).
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Figure 4 This is a 750 Watt heat lamp. Imagine
one of these on every m2 of solar exposed surface. |
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Figure 4 is a 750 Watt heat lamp. Imagine one of these on every
m2 of solar exposed surface. That is a lot of energy that gets
absorbed and transmitted into the building which if not
controlled leads to an increase in the MRT leading to
overheating of the occupants. So the first non-mechanical
strategy to prevent overheating when, “designing for failure” is
to stop the shortwave radiation from hitting the building.
Unfortunately 100% success would be equivalent to living inside
a shaded bubble which is neither desirable nor practical.
So here’s some practical tactics that you the
HVAC designer should be discussing with your clients to
eliminate or reduce mechanical cooling or at the very least to
enable peak performance from cooling equipment when absolutely
needed.
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1.
Building orientation and shading
Fixed overhead shading (includes roof overhangs) doesn’t work on
eastern and western exposures but can work on the south during
cooling season; use windows inset into the structure with
adjustable exterior horizontal or vertical shading on the east
and west exposures (see Figure 5). When possible orientate the
building on an east-west axis. |
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Figure 5 fixed overhead shading (includes roof
overhangs) doesn’t work on eastern and western exposures but can
work on the south during cooling season. Note how with this
aspect ratio how a north-south orientation exposes more surface
area to solar loads known for their shading difficulties.
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2. Use exterior window shading as discussed below
and as shown in Figure 6 (types a through j). |
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Type a: A wall is the perfect window for HVAC
engineers. It has a 100% shading effectiveness and if built
properly it is secure, doesn’t leak, has a low thermal load and
is effective at reducing air and sound pollution. It’s only
flaws are, you can’t see out of it and it lets no light in.
What? Is this a problem? Type b: This is the project budget
window; large, clear, no shades, cheap and lots of them. |
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Type c: This is the bare minimum window, double
pane, single coating, no exterior or interior shades. Solar
coatings are good but both b. and c. still allow short wave
energy to hit 100% of the glass surface area thus the 0% shading
effectiveness. Type d: Getting clients to use exterior fins or
inset windows puts you into a new league – congrats! They work
on southern exposures and can work on some north east and north
west exposures. Up to 70% shading effectiveness. |
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Type e: Brise solei will put you in with the
architectural crowd but only if you pronounce it correctly.
Works on southern exposures but not the east or west. Works with
windows which swing both ways (hoppers, awnings, casement).(4) Up
to 70% shading effectiveness. Type f: Horizontal louvers work on
all exposures when adjustable. They don’t work with windows
which swing outward - only inward, sliders and double hung. More
stable in windy climates, but can accumulate snow, bird
droppings etc. Up to 90% shading effectiveness. |
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Type g: Vertical louvers as above
except no snow, bird droppings etc. Type h: Roller shades, as
above but better efficacy depending on openness factor. Up to
95% shading effectiveness. |
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Type i: Venetian blinds as per
type f
and type g but better rangeability. Some types not suited for windy
climates. They also don’t work with windows which swing outward
only inward, double hung and sliders. Up to 95% shading
effectiveness. Type j: Shutters - still and always the classic
solution. They don’t work with windows which swing outward only
inward, double hung and sliders. Up to 100% shading
effectiveness.
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Figure 6 Exterior window shading
options (above) can significantly reduce solar loads and when
used with other tactics can in some case eliminate the need for
mechanical cooling. |
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3. Window Orientation and Wind Patterns |
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Figure 7 For climates where night
time ventilation cooling is possible, try to integrate window
orientation and types with prevailing summer wind patterns
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4. Use window types which enable natural
ventilation and can be fitted with exterior shades; i.e. swing
inward, sliders, double hung etc. (see figure 7).
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5. Complex building geometry equals increases in
surface area, thermal bridging, infiltration and moisture
penetration all which lead to increases
in interior temperatures and moisture. Simple building shapes using advanced
framing techniques are better.
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Figure 7a
Complex geometry (L) increases the risk of the outside getting
inside, is more expensive to frame and finish and the primary
location for first and repetitive failures. Simple shapes (R)
make much better buildings from a performance perspective. Some
architects get simple, others do not. Those that like complex
don't have to live with the results of complexity. The same is
true with mechanical and electrical systems. Words of advice:
every original work of complex art remains long after the artist
has died at which point the artist no longer cares what you
bought.
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6. Use high performance light coloured enclosures
reducing U values, bridging and solar gains through exterior
shading and windows
orientated for natural ventilation (see figure 8). The
enclosure
specification for cold climates work for the summer; use R60
roofs, R40 above grade walls and cantilevered floors, R20 below
grade walls, R10 slabs and min. R4 windows with <.25 SHGC. The
width of overhangs should be a minimum of 50% of the height of
the window or glass doors. Window to wall ratios should be kept
below 40% (see figure 9).
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Figure 8 Use high performance light coloured
enclosures reducing U values, bridging and solar gains through
exterior shades and windows orientated for natural ventilation. |
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Figure 9 Window to wall ratios of less than 40%
are good, 30% is better. Even I draw the line at 20% everybody
needs some natural light. Over 40%
promotes excessive daylighting, destroys thermal comfort,
enables fabric fading and VOC emissions and significantly
increases HVAC loads
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7. Use architecture to create solar refuge zones;
north side, below grade, heavy mass enclosure. Think north side
courtyards either enclosed or open. If they can take advantage
of cool night time ventilation air all the better, if not use
fans (see figure 10). |
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Figure 10. Use architecture to create solar
refuge zones; north side, below grade, heavy mass enclosure.
Think northside courtyards either enclosed or open. If they can
take advantage of cool night time ventilation all the better. If
not use fans. |
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8. Use property shading starting with soft and
hardscapes such as trees and Pergolas.
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9. If you do the above and still need active
mechanical cooling then size your system based on night time
loads. Run the system to cool the building mass during the
evening, seal the house up during the day and ventilate only as
necessary (see Figure 11). This strategy works exceptionally
well with high mass enclosures with embedded cooling pipes.
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Figure 11 Cool the building in the dark hours
when sensible loads are lowest.
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10. If outdoor humidity is a problem forget the
natural ventilation. Seal up the home and use a
dedicated dehumidification system. By lowering the absolute
moisture in the space, increasing air velocity with fans and
encouraging or enabling reductions in clothing you can solve the
majority of overheating problems – but only if you get the
building and mechanical ventilation system right! |
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Short Stokes
These shading and ventilation strategies and tactics work on all
building types using air based systems, hydronic systems or
hybrid systems. Take advantage of your local climate. Below
grade soil temperatures are always cooler than above grade air
temperatures. Use trees. Put window shades on the outside. Work
window locations and selections into wind patterns. Night time
naturally ventilated cooling works in many parts of the country.
Low humidity, high air velocity and near naked works…just stay
hydrated. If necessary, consider using non compressor cooling
methods as discussed in the May 2016 Radiative Cooling article.
If you still can’t convince your clients and their architects to
employ these sustainable principles, we have been teaching our
students to use the CBE Thermal Comfort Tool and its new solar
calculator to shame them into submission (see figure 12). When
it comes to architectural choices leading to overheating, guilt
and reality are two gifts that keep on giving. |
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Figure 12 CBE Thermal Comfort Tool - shortwave
radiation calculator. The best tool to get clients and
architects to evaluate fenestration performance and WWR’s |
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References: |
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1. Hales, S., Edwards,
S.J., Kovats, R.S.. Impacts on health of climate extremes, World
Health Organization < http://www.who.int/globalchange/publications/climatechangechap5.pdf
> accessed August 2016
2. A Human Health Perspective On
Climate Change A Report Outlining the Research Needs on the
Human Health Effects of Climate Change. Environmental Health
Perspectives and the National Institute of Environmental Health
Sciences. April 22, 2010
accessed August 2016
3. Marsh, A. Sun Path Tools < http://andrewmarsh.com/software/>
accessed August 2016
4.
Engineers don’t judge we’re only
concerned about performance.
5. Canadians deliberately put the
letter “u” in the word mould and louvers. When Americans spell
these words without the “u” it changes the entire meaning. |
