Why Rules of Thumb Don’t Work for Passive Design

If you’re like many architects, you’ll often begin a passive or sustainable design by using rules of thumb to decide the ideal orientation of glass, window-to-wall ratios, shading strategies, and more. While these rules can provide a convenient shorthand for early design decisions, they turn out to be unreliable at best. In some cases, they can point in the wrong direction entirely.  We took a close look at three common rules of thumb and how they can go wrong. Here’s what we found.

Window to Wall Ratio 

How much south-facing glazing is ideal? This rule provides guidance based on the idea that buildings in cold climates can benefit more from solar gain and therefore from higher glazing ratios.

The Rule Size south glazing based on a percentage of the building’s floor area. In cold climates, provide south glazing equal to 16% to 20% of the floor area; in temperate climates, provide south glazing equal to 10% to 13% of the floor area. (Percentages vary with latitude.)

When it Works The rule worked for a mid-rise residential building in Milwaukee, Wisconsin, with high-performance glazing and optimized shading. In this case, the optimal south glazing ratio was within the range suggested by the rule.

When it Misses

• Code minimum: If the glazing properties are adjusted to code minimum values, heat losses exceed beneficial heat gains, and the optimal glazing ratio drops to zero: all glazing is a liability.
• Passive House windows: Conversely, if the glazing is improved to U-0.15 or better, the optimum glazing ratio increases beyond what the rule suggests.
• Passive House design: If, however, the entire building (including the entire envelope and mechanical systems) is upgraded to Passive House standards, the optimal ratio is significantly less than suggested by the rule — and higher ratios result in serious overheating risk.

What Happened The optimum glazing ratio in cold Milwaukee depends on how good the glass is. Better glass loses less heat, allowing larger expanses of glass. But the optimum also depends on the overall balance of heating and cooling loads in the building. With a Passive House design, the total heating load is quite small, meaning that the building doesn’t have as much need for solar gain. And because the building retains heat so effectively, too much solar gain can cause overheating, necessitating shading or larger, more complex HVAC systems. This demonstrates a key risk of rules of thumb: they can create problems that are difficult or expensive to reconcile later.

Shading Depth 

At first this rule sounds like common sense: size shading to block summer sun, but to let in the winter sun. Reality, however, turns out to be more complex.

The Rule Shading should be 1/4 the height of the opening in southern latitudes (30°L) and 1/2 the height of the opening in northern latitudes (44°L).

When it Works The rule worked for a linear residential building with a high-performance envelope, and came close for the same building with a code-minimum envelope.

When it Misses

• Different form: Changing just the building’s form to something with more glazing and more exposed envelope increased heating loads and made all shading a liability.
• Cost or peak loads: Looking at cost instead of energy use resulted in an optimal shading ratio of 0.89–much higher than suggested by the rule. The same was true when we looked at peak cooling loads.
• More efficient HVAC system: Changing the HVAC system to a more efficient Ground Source Heat Pump resulted in a different optimum: a 1-to-1 ratio.
• Passive House design: A super-efficient building required less shading than the rule suggested.

What Happened In all cases, shading becomes more or less necessary depending on moves made elsewhere. Elements like shading devices can’t be optimized in isolation. Changing the form, envelope, or systems changes the balance of heating and cooling — which also changes the amount of solar gain that is beneficial. In the Passive House example, an extremely tight envelope made the building extremely sensitive to changes in internal loads. With very efficient lighting and appliances, the cooling load was small and the optimal shading was less than the rule of thumb suggested. But standard lighting and appliances drove up cooling loads to such an extent that the optimal shading was higher than the rule suggested. The optimum also depends on what you’re trying to optimize. In many non-residential buildings, shading has a relatively small impact on overall energy use, but can have big impacts on peak cooling loads, HVAC system sizing, and thermal comfort — meaning it may be more important to size shading for those factors.

Insulation Values 

Is more insulation always better in “heating dominated” climates? Not necessarily. Sometimes it can even make performance worse.

The Rule The more Heating Degree Days (HDD) your location has, the more energy (and energy cost) your building can save with insulation. In colder climates (>5400 HDD@65F), more insulation is always better.

When it Works The rule worked for a 53,000 s.f. office building in Dayton, Ohio (~5500 HDD). Here, adding more insulation continues to reduce energy use, and to a lesser extent energy cost as well (see the proviso below).

When it Misses

• Large office: In the case of a 140,000 s.f. office with a deep floor plate, energy costs actually increasedwhen the roof R-value exceeded 19.
• Energy cost: This isn’t a “miss” exactly — but even for our base case, energy cost show steeply diminishing returns. While more insulation does continue to reduce cost slightly, at a certain point the money is clearly better spent elsewhere.

What Happened Despite being in a “heating-dominated climate,” the large boxy office building was cooling-dominated, thanks to high internal loads (lighting and appliances) and a relatively small amount of exterior surface area. Increasing the roof insulation decreased the heating energy use, but increased the cooling energy use. Beyond R-19 the increased costs of cooling outweighed the reduced costs of heating. And given the diminishing returns of energy and cost in the baseline case, beyond a certain point the client’s dollars are better spent elsewhere — on more efficient systems, improved equipment, or better lighting. The problem is that “certain point” varies by design.

Toward Performance-Based Design

If you’re sensing a pattern in these examples, you’re right. What’s common among all of these studies is that individual elements of a building can’t be optimized in isolation from the rest of the design. The best option depends on the combination of the building’s envelope, usage, and mechanical systems — as well as on the project’s goals. The good news, of course, is that early-stage analysis tools like Sefaira make rule-of-thumb guesswork unnecessary. Designers can quickly test hypotheses, study options, and arrive at a design that demonstrably achieves the client’s goals. This performance-based paradigm provides more creative freedom for the design team (performance becomes a design exercise) at the same time that it leads to better performance.