Four Rules Of Thumb That Could Lead You Astray

Transcription

1 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.

2 2 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.

3 3 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 E S WHEN IT WORKS A simple rectangular building (optimal orientation is within 10 of due south) N W E S WHEN IT MISSES 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.

4 4 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 levels of insulation, air tightness, etc. Our example building is a multi-family residence in London. 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 construction (optimal shading is expected depth) WHEN IT MISSES High-performance construction (optimal shading length is longer than rule of thumb suggests)

5 5 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 kwh kwh Jan Feb Mar Apr May Jun Jul AugSep Oct NovDec Jan Feb Mar Apr May Jun Jul AugSep Oct NovDec Gas Heating Equipment Hot Water Cooling Lighting Typical construction High-performance construction Improving the building envelope reduces the number of months when heating is required.

6 6 Rule 3: Glazing Ratios 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. The thinking is that higher amounts of south-facing glazing can provide beneficial solar gain in cold climates. WHEN IT WORKS Standard double glazing (optimal south glazing ratio is 22%, within expected range) WHEN IT MISSES High performance glazing (optimal south glazing ratio is 56%, above expected range) 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%.

7 7 What s Happening? Glass is often the weakest link in the building s envelope responsible for the most conduction losses in winter months. At the same time, it can provide beneficial solar gain during those months. Finding the right amount of glazing is about finding the right balance between gains and losses a balance that depends heavily on the properties of the glass. If you use high-performance glazing, you can afford to have more of it that is, you can take advantage of solar gain without paying the penalty of conduction loss. As with the other rules, finding the right balance is critical but that point is difficult to find by rules of thumb alone. Holistic analysis of the building is required. Rule 4: Operable Area for Natural Ventilation The Rule When designing for natural ventilation, a common rule of thumb is to provide operable area approximately equal to 5% of the total floor area that will be naturally ventilated. The goal of natural ventilation is typically to maintain occupant thermal comfort in the absence of mechanical cooling. In some climates, well-designed passive systems can eliminate the need for mechanical cooling entirely Occupied Hours < >28 ( C) < >82 ( F) Temperature WHEN IT WORKS High insulation and high thermal mass construction (5% operable area can create comfortable conditions) Occupied Hours < >28 ( C) < >82 ( F) Temperature WHEN IT MISSES Standard construction (5% operable area yields significant overheating)

8 8 When it Works We return to our small house, this time located in Sydney, Australia. In addition to the 5% operable glazing area suggested by the rule of thumb, we specified high thermal mass construction, high insulation values, and plentiful shading. With these parameters, the house can stay comfortable year-round without a cooling system. When it Misses However, when we adjust the properties of the construction and envelope, things change. With lower levels of insulation and thermal mass, the house heats up substantially, and 5% operable area is not sufficient to bring it within tolerable temperatures. What s Happening? Once again, our example is all about the balance between heating and cooling. In this case, thermal mass, insulation, and shading mitigated the cooling requirement, allowing natural ventilation to make up the difference. But change any of those variables and the others also shift. As before, the largest benefit is found in a combination of strategies that together deliver excellent performance. What Architects Can Do Buildings are complex objects that are not easily amenable to rule-of-thumb decisionmaking. But architects can cut through the complexity with fast, easy-to-use analysis that can answer fundamental questions about orientation, glazing ratios, natural ventilation and more in the earliest stages of design. Here s what you can do: Understand the energy profile of your building. Is it heating or cooling driven? What elements of the building are most responsible for these heating and cooling loads? This understanding can help you determine which strategies to investigate further. Look at the relative impact of different sustainable strategies. How responsive is your design to different passive measures? For instance, many of our example projects were not particularly sensitive to orientation; however, glazing amounts and locations often had a significant impact. Rules of thumb can provide a good starting point by suggesting strategies that may make sense, given your climate and building type. But these hypotheses must be tested against the realities of your design. Find the optimal parameters for the most effective strategies. Parametric analysis can help you find optimal parameters for key passive design strategies: glazing area, orientation, construction properties, etc. Understanding these parameters early in design allows you to incorporate them as a fundamental part of your design concepts.

9 9 Sefaira allows architects to quickly study the impacts of sustainable strategies, both alone and in combination. Understand how different design strategies affect one another. Which are synergistic, and which are detrimental to one another? Are there combinations of strategies that work well? Combining synergistic strategies can often yield significant savings. Armed with a holistic understanding and backed by real data, designers are better able to meet design objectives creatively, rather than relying on prescriptive rules of thumb that all too often lead in the wrong direction.

10 10 How Sefaira Can Help Sefaira makes sustainability analysis a seamless, integral part of the design process. This enables architects to discover the most important sustainable strategies early in design, and evaluate design options before the design is locked down. Sefaira is specifically tailored to help architects investigate the factors at play in the early stages of design, including the passive design strategies explored above. Sefaira is: Fast: The cloud-based engine provides real-time feedback on design decisions unlike traditional applications. Easy to use: Sefaira was built for architects to use from the first pen stroke. Parametric: Quickly explore the design space by evaluating a wide range of options. Comparative: Study multiple design options side-by-side to quickly hone in on factors that have the biggest impact. Collaborative: Cloud-based collaboration helps all stakeholders understand the design from the outset. Technology neutral: Sefaira supports passive and active design strategies, water and renewables. AEC DNA: Our team of engineers and architects brings a deep understanding of the problems designers are trying to solve in conceptual design. Decisions that architects make in the early stages of design can have a significant impact on building performance. Early, frequent feedback is critical for making informed design decisions, identifying strategies that have the most impact, and finding synergies between performance, aesthetics, and other project requirements. Sefaira allows architects to perform more analysis, more frequently, and at lower cost, helping to set projects on the right trajectory and ultimately delivering better-performing, more sustainable buildings.

11 11 About Sefaira Sefaira was founded in 2009 with a mission to promote more sustainable buildings by helping the building industry design, build, operate, maintain and transform all facets of the built environment. Sefaira s proprietary cloud-based technology, built upon deep building physics expertise, offers an integrated approach to sustainable design analysis, knowledge management, and decision support. Sefaira helps designers analyze and compare sustainable building strategies for new build or retrofit projects in a fraction of the time and cost previously required. By providing sophisticated analysis via an intuitive web interface, Sefaira helps users define, quantify and optimize the energy, water, carbon and financial benefits of relevant design strategies. Sefaira s offices in London and New York have supported projects across the US, Europe, the Middle East and Asia. CONTACT SEFAIRA SEFAIRA (US) +44 (0) (UK) info@sefaira.com FOLLOW SEFAIRA linkedin.com/company/sefaira twitter.com/sefaira Copyright 2013 Sefaira Ltd. SCO200C-01