Things You Can Do RIGHT NOW: Passive Design
Authors: Bill Burke & Elena Nansen
Things You Can Do RIGHT NOW: Passive Design
This short article on Passive Design is next in the series, What you can do RIGHT Now. This article is specifically for a practitioner who is looking for pragmatic guidance on immediate steps to embrace contemporary passive design strategies.
What is Passive Building Design?
Contemporary passive design relies on understanding climate and taking advantage of siting, form, detailing, and construction assemblies to create buildings that achieve design excellence while reducing the need for energy consuming equipment to provide comfort and health. Reducing the need for energy makes it possible to downsize HVAC equipment, shorten operating times and seasons, shorten duct runs and, in some cases, eliminate equipment entirely. Passive design can mean shifting first cost from equipment to improvements to the building enclosure. Passive design requires focusing on the architecture first, before supplementing with active systems.
Passive design was an essential aspect of all building design until well into the 20th century as there were few alternatives to provide a more comfortable environment. With limited energy sources and technologies, architects and builders needed to understand the local climate and the thermal properties of regionally available building materials.
Climate & Daylight
Passive design starts with climate. Climate will influence orientation, shading, air movement, siting, and materials, among other things. Providing controlled daylight and enabling users some control over natural ventilation, when conditions and building type permit, are almost always preferred design strategies because they create pleasing spaces that connect building occupants to nature. Access to daylight is recognized as an essential element of human health and regulation of circadian rhythms.[i] While LEDs have reduced energy loads from electric lights, controlled daylight is always desirable and electric lights that are dimmed or switched off in response to daylight still save energy.
The 1970s saw the growth of passive solar design in response to resource shortages and other environmental concerns. Passive solar emphasized direct solar gain through expansive, seasonally-shaded, south-facing glass with internal thermal mass used for heating and cooling. Improvements in technology, insulation levels, quality of construction, and better understanding of the importance of air infiltration and air sealing have resulted in significant rethinking of passive solar design strategies. A building enclosure designed, detailed and built to deeply minimize thermal bridging and infiltration, with moderate amounts of glazed wall area, can achieve excellent energy performance even with a suboptimal site or orientation. Major improvements to the enclosure permit greater variation from the bioclimatic ideal. It is important to understand that well-insulated, air-sealed buildings do require mechanical ventilation! This is confusing to those who assume every aspect of performance is to be passive, including the ventilation. Buildings can and should open to the exterior, operating in different modes in response to outdoor conditions.[ii] Well-insulated, air-sealed buildings can open to the exterior during moderate weather, but can achieve deep operational energy reductions while supplying filtered ventilation air when outdoor conditions are inhospitable.
Contemporary passive design strives to mechanically heat or cool the building with the smallest possible energy use when you can’t achieve comfort with operable windows, thermal mass, or stack ventilation. With the impact of climate change, which has led to higher temperatures and poor air quality from wildfires, this is more relevant and appropriate than ever. Passive design means designing buildings that are very ‘tight’, minimizing interior sources of poor air quality such as combustion appliances and off-gassing materials, and providing filtered ventilation with outside air. Improved building enclosures permit greater aesthetic freedom, while still achieving excellent energy performance.
Today, the Passive House movement represents the leading edge of Passive Building Design. Passive House Institute[iii] was founded in Germany by Wolfgang Feist, inspired by super-insulated homes built in North America in the late 1970s.[iv] In North America, the Passive House movement split into two groups in 2014 with modest variations in approach and requirements. The North American Passive House Network (NAPHN)[v] remains affiliated with the German Passive House Institute. Passive House Institute US (PHIUS)[vi] instituted climate-specific requirements developed in cooperation with the US Department of Energy and Building Science Corporation. The two Passive House standards in North America both call for a super tight enclosure and mechanical ventilation, among other requirements. The Passive House standards apply to both residential and nonresidential buildings and are best thought of as Passive Building Standards. Both standards require a series of blower door tests throughout the construction process to document that the targeted level of air sealing is actually achieved.
The principles of climate based design still apply. Elongating a building axis in an east/west direction makes it easier to control sunlight and daylight and supports occupant well-being. South facing roofs can shade windows and maximize effectiveness of installed solar electric systems, especially with inclusion of battery storage. Modest amounts of thermal mass in nonresidential buildings, on the interior side of insulation and protected from direct sun, can increase comfort by absorbing heat over the course of a day.
Things You Can Do Right Now to Use Passive Design in Architectural Practice
1. Follow bioclimatic design principles to ‘design with climate’. Implement pre-design climate research in order to better understand which bioclimatic design principles are applicable to your project.
2. Conduct early Daylight Analysis, to optimize (not maximize!) natural light coming into the space, while minimizing any unwanted heat gain by strategic window to wall ratios and shading devices. As a starting point, an analysis for Spatial Daylight Autonomy and Annual Sunlight Exposure[vii] can be performed with many commonly used architectural drawing tools, sometimes requiring a ‘plug-in’, or with a variety of lighting design tools.
3. Improve your knowledge of required control layers and their role in enclosure performance. Achieving continuity of control layers will not only improve energy performance, but aid in management of moisture and better durability[viii],[ix],[x] while making it possible to downsize HVAC equipment.
4. Reduce thermal bridging through improved detailing of wall assemblies. During the design Quality Assurance/Quality Control process check for thermal bridging within project details.
5. Improve the air tightness of your buildings. To save energy and provide good indoor air quality, require careful air-sealing of all your projects. New buildings with ventilation cannot be too tight. In existing building renovation, short of gut-rehab, the goal is to reduce air infiltration. In project documents for new construction, set air-tightness targets. Require blower door tests during construction in all projects to confirm new construction targets are achieved and to assess percentage reduction in renovation of existing buildings. [xi] [xii]
6. Separate mechanical heating and cooling from ventilation.[xiii] Even when you have eliminated the need for heating or cooling by passively providing thermal comfort, occupants need fresh air for ventilation.
7. Incorporate building electrification, eliminating combustion appliances such as gas stoves, water heaters, and furnaces, to reduce greenhouse gas emissions while improving indoor air quality. Limit products and finishes with high VOC content, to reduce off gassing.
8. Include a drawing explaining your design intent and the basis of your design. Describe how enclosure design contributes to load reduction and proper HVAC system sizing.
9. Window locations, shading devices, and roof overhangs used for occupant comfort and included in compliance documentation should be specifically noted so they are not ‘value engineered’ out as the project advances or during construction.
10. Energy Modeling: Optimizing design for energy is most easily achieved when performance modeling occurs throughout the design process. There is no single approach to modeling that is applicable to all firms. The important task is to find a way to incorporate energy analysis into the design process for your firm and design team. Some design firms may have skilled staff who can perform selected energy modeling analyses in-house. Others may have relationships with engineering consultants who can deliver energy analysis at key points in design. The situation for small firms and individual practitioners obviously varies. Some may have the skills and interest to learn software and perform select energy analyses, while others will not. At minimum, ask the Title 24 Part 6 compliance modelers you use if they can provide additional analysis at key points in the process. If your compliance modeler cannot perform additional analysis find one who can. For more guidance see Architect’s Guide to Building Performance: Integrating simulation into the design process.