Four Rules Of Thumb That Could Lead You
Astray
Rules of thumb can point you in the wrong direction. Here’s what you need to know so you don’t
get misled.
Copyright ©2013 Sefaira Ltd.
W H I T E PA P E R
Introduction
For many decades, rules of thumb have been the go-to resource for
architects seeking to design high-performance buildings. Rules of thumb
provide a convenient shorthand for capturing general responses to climatic conditions, and for illustrating the fundamentals of how energy is
captured, lost, and used in a building.
However, rules of thumb have their limitations. They generally do not take
into account the specifics of a project’s site, context, usage, or building
shape. They do not apply well to “edge cases” that fall outside the bounds
of normal expectations — but neither do they help designers identify
which designs are edge cases and which are not. Furthermore, they do
not identify which design elements have the biggest impact on performance. As a result, rules of thumb end up being prescriptive rather than
flexible — dictating design rather than empowering architects to understand tradeoffs and meet performance goals creatively.
This paper explores four common rules of thumb related to the building
envelope, including:
building orientation
shading depth
glazing ratios
operable area for natural ventilation.
In each case, normal variations in site, usage, and building design can
cause the rules to lead to less-than-optimal designs. At times they point
in the wrong direction entirely.
Today, designers have a number of alternatives to using rules of thumb.
Fast, intuitive sustainability analysis can provide real data to drive design
decisions in the right direction from a project’s inception.
This paper represents an initial exploration into the effectiveness of rules
of thumb. We invite anyone who has comments, suggestions, or different
perspectives to share them with us at info@sefaira.com.
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W H I T E PA P E R
Rule 1: East-West Orientation
The Rule
One of the most common rules of thumb related to sustainable design is that the
ideal alignment for most buildings is for its long axis to run east-west. This allows the
building to have a majority of its glazing on the north and south, where sunlight can
be most easily harvested and controlled for daylight and passive solar gain.
N
W
N
E
W
E
WHEN IT WORKS
A simple rectangular building
(optimal orientation is within
10° of due south)
S
S
N
W
N
E
W
E
WHEN IT MISSES
S
S
A building with self-shading
or overshading (here, optimal
orientation is 42° east of
south)
When it Works
This rule generally works for buildings with a simple rectangular shape, relatively
symmetrical glazing, and no significant obstructions to sunlight, such as neighboring
buildings or trees. Our example is a roughly rectangular office building located in
Pittsburgh, PA. In this case, analysis revealed that the ideal orientation is 10 degrees
east of south — not precisely what the rule of thumb suggested, but relatively close.
When it Misses
For sites with some amount of shading and/or non-rectangular shapes — particularly
forms with some amount of self-shading like the L-shape building shown above — this
rule can fall apart entirely. In the case we studied, the orientation that minimized
energy use was nearly 45 degrees east of south.
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W H I T E PA P E R
What’s Happening?
The reasons for these failures is that the right balance between heating and cooling
— and therefore the right amount of solar exposure — varies depending upon the
specific design of the building. For buildings with self-shading or obstructions, finding
this balance can become complex, because the majority of sunlight is not necessarily
coming from the south.
Rule 2: Shading Depth
The Rule
There are numerous rules of thumb for sizing shading devices. They typically follow
this form: shading should be 1/4 the height of the opening in southern latitudes (36°L)
and 1/2 the height of the opening in northern latitudes (44°L). The intent is to block
sunlight in the summer months, but allow solar gain in the winter, when the heat is
beneficial.
When it Works
This rule works well for externally-loaded building types (e.g., residential design),
buildings oriented due south, and buildings whose envelope is built to “typical”
specifications — meaning average levelsShading
of insulation,
air tightness, etc. Our example
- Low Insulation
building is a multi-family residence in Barcelona,
Spain. Parametric analysis revealed an
Shading - Low Insulation
optimal shading length of 0.8 meters (2.6 ft.), which is in line with the rule of thumb.
Shading - Low Insulation
WHEN IT WORKS
Shading - Low Insulation
Standard envelope construction (optimal shading is
expected depth)
Shading - High Insulation
Shading - High Insulation
Shading - High Insulation
WHEN IT MISSES
High-performance construction (optimal shading length
is longer than rule of thumb
suggests)
Shading - High Insulation
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W H I T E PA P E R
When it Misses
There are several cases in which this rule fails to yield optimal results:
High-Performance Construction: Improving the envelope of our example
building changes the balance between heating and cooling requirements. As
the envelope improves, heating is required for fewer months out of the year —
meaning that solar gain is beneficial for less of the year. In our example building,
the high performance case benefits from 4 fewer months of solar gain than the
typical case. This means that additional shading is beneficial — or that we might
want to explore a retractable shading option that could provide solar exposure
only when it’s needed.
Non-South Orientation: Often contextual factors preclude a precise southern
orientation (or, as we saw above, due south may not be optimal). For our example
building, rotating it 45 degrees increased the optimal shading length by 20%.
Office Building: This building type has higher internal loads, and therefore higher
internal heat gain. It requires less heating, and benefits less from solar gain than
a residence. For our example building, the optimal shading length was 1.5 meters
(4.9 ft.) — twice what the rule of thumb would recommend. In this case, we
would want to consider additional strategies to reduce solar heat gains, such as a
brise soleil or better glass.
What’s Happening?
In all cases, shading becomes more or less necessary depending on the moves that
the architect has made elsewhere. Elements like shading devices cannot be optimized
in isolation: they are an integral part of the building’s environmental response. The
goal is to design an envelope that strikes the right balance between letting heat in
and allowing it to escape — and that balance depends upon all factors of the design,
including not only shading, but also building geometry, envelope properties, and the
amount of glazing.
Energy Footprint
Energy
(kWh)
Footprint (kWh)
5000
10000
0000
0
20000
kWh
000
15000
25000
Equipment
Hot Water
Cooling
Lighting
5000
15000
0
10000
Jan Feb Mar Apr May Jun
JanJul
FebAug
MarSep
AprOct
May
Nov
JunDec
Jul Aug Sep Oct Nov Dec
5000
0
Month
5000
0
Month
Typical construction
Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
Footprint (kWh)
Gas Heating
Equipment
Hot Water
Cooling
Lighting
20000
25000
20000
15000
kWh
20000
Energy
25000
25000
Gas Heating
Gas Heating
20000
Equipment
Equipment
Hot Water
Hot Water
15000
Cooling
Cooling
Lighting
Lighting
Gas Heating 10000
kWh
0000
Energy Footprint (kWh)
kWh
25000
kWh
5000
Energy Footprint
Energy
(kWh)
Footprint (kWh)
15000
10000
Gas Heating
Equipment
Hot Water
Cooling
Lighting
Gas Heating
Equipment
Hot Water
Cooling
Lighting
5000
0
10000
Jan Feb Mar Apr May Jun
JanJul
FebAug
MarSep
AprOct
May
Nov
JunDec
Jul Aug Sep Oct Nov Dec
5000
Month
0
High-performance construction
Month
Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
Month
Month
Improving the building envelope reduces the number of months when heating is required.
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W H I T E PA P E R
Rule 3: Glazing Ratios
Glazing Ratio - Triple Glazing
The Rule
Like shading designs, there are numerous rules of thumb for finding the right amount of
glazing. Some versions provide detailed recommendations based upon climate zone or
latitude. In its simplest form, the rule is often stated like this:
In cold climates, provide 0.19 to 0.38 sq.m. (2 to 4 sq.ft.) of south glazing per
sq.m. (10.8 sq.ft.) of floor area.
In temperate climates, provide 0.11 to 0.25 sq.m. (1.2 to 2.7 sq.ft.) of south glazing
per sq.m. (10.8 sq.ft.) of floor area.
Glazing Ratio
- Triplecan
Glazing
The thinking is that higher amounts of south-facing
glazing
provide beneficial solar
gain in cold climates.
Glazing Ratio - Double Glazing
WHEN IT WORKS
Glazing Ratio - Triple Glazing
Standard double glazing
(optimal south glazing ratio is
22%, within expected range)
Glazing Ratio - Triple Glazing
Glazing Ratio - Double Glazing
WHEN IT MISSES
High performance glazing
(optimal south glazing ratio is
56%, above expected range)
Glazing Ratio - Double Glazing
Glazing Ratio - Double Glazing
When it Works
Like shading, this rule works well for south-facing, externally-loaded buildings with
typical construction. For a simple single-family residence in Paris, France, shown
here, the rule of thumb suggested a south glazing ratio between 20% and 46%. The
optimal glazing ratio fell within this range when we specified standard double glazing
in combination with shading. It should be noted, however, that the large range makes it
difficult for a designer to find the best design by rule of thumb alone.
When it Misses
We fell outside of the suggested glazing ratios in a number of cases: for instance, when
we used high-performance glazing or more extensive shading strategies. With better
glass, the optimal south glazing ratio increased to 55%.
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