Four Rules Of Thumb That Could Lead You...
Transcript of Four Rules Of Thumb That Could Lead You...
Copyright ©2013 Sefaira Ltd.
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.
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IntroductionFor 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 cli-
matic 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 perfor-
mance. As a result, rules of thumb end up being prescriptive rather than
flexible — dictating design rather than empowering architects to under-
stand 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 [email protected].
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Rule 1: East-West Orientation
The RuleOne 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.
WHEN IT WORKS
A simple rectangular building (optimal orientation is within 10° of due south)
WHEN IT MISSES
A building with self-shading or overshading (here, optimal orientation is 42° east of south)
When it WorksThis 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 MissesFor 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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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 DepthThe RuleThere 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 WorksThis 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 levels of insulation, air tightness, etc. Our example building is a multi-family residence in Barcelona, Spain. Parametric analysis revealed an optimal shading length of 0.8 meters (2.6 ft.), which is in line with the rule of thumb.
WHEN IT WORKS
Standard envelope construc-tion (optimal shading is expected depth)
WHEN IT MISSES
High-performance construc-tion (optimal shading length is longer than rule of thumb suggests)
Shading - Low Insulation
Shading - High Insulation
Shading - Low Insulation
Shading - High Insulation
Shading - Low Insulation
Shading - High Insulation
Shading - Low Insulation
Shading - High Insulation
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When it MissesThere 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.
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Improving the building envelope reduces the number of months when heating is required.
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The RuleLike 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:
The thinking is that higher amounts of south-facing glazing can provide beneficial solar gain in cold climates.
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.
Rule 3: Glazing Ratios
Glazing Ratio - Triple Glazing
Glazing Ratio - Double Glazing
Glazing Ratio - Triple Glazing
Glazing Ratio - Double Glazing
WHEN IT MISSES
High performance glazing (optimal south glazing ratio is 56%, above expected range)
WHEN IT WORKS
Standard double glazing (optimal south glazing ratio is 22%, within expected range)Glazing Ratio - Triple Glazing
Glazing Ratio - Double Glazing
Glazing Ratio - Triple Glazing
Glazing Ratio - Double Glazing
When it WorksLike 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 MissesWe 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%.