Passive design for commercial buildings - Part 1: Achieving passive design
September 28, 2021 By Anthony Broatch
What is passive design in buildings?
Passive design of buildings is a practice that utilises natural energy to heat, cool, ventilate, and light a building. Active design relies on mechanical heating, ventilation and air-conditioning (HVAC) systems and internal lighting to produce the same results.
Passive design of buildings should always be carried out prior to looking at active systems. It can reduce or remove the need for HVAC systems as well as reduce internal lighting operational hours. A reduced HVAC requirement leads to lower Capital Expenditure (CapEx) costs as a smaller system can be used. Passive design also results in lower Operating Expenditure costs (OPEX) as less and/or smaller equipment requires less energy to run, maintain, and in the case of both lighting and HVAC the equipment can operate less frequently. Passive design also facilitates the creation of/or transition to carbon zero buildings.
How Can Passive Design be Achieved?
Passive design of buildings can involve the following:
Site selection affects sun & wind availability.
Orientation of buildings in relation to the sun and wind.
Building insulation to manage heat transfer.
Prevent thermal bridging through continuous insulation providing seamless barriers to heat transfer.
Glazing placement, size, and type. Glazing refers to glass in windows, doors, and skylight.
Glazing shades to manage heat load from the sun (solar gain).
Natural ventilation to cool buildings, introduce fresh air, and remove stale air/odours/moisture.
The use and proper placement of thermal mass to store the sun's energy.
The image below illustrates the aspects of passive design described above.
Figure 1: Simplified New Zealand building showing passive design principles.
1. Summer Sun
The sun in summer is higher in the sky at midday than in winter. The higher the sun, the higher intensity of solar radiation which inturn leads to warmer weather. As a result, in summer buildings need cooling most of the time. Due to this, blocking direct summer sun from entering the building at peak times, by the use of solar shading, is essential to keep the building within comfortable temperatures. Refer also to item 5 (solar shading).
2. Winter Sun
In winter buildings should harness the sun’s energy to heat indoor spaces. Therefore, it is important to maximise the direct sunlight entering buildings in winter. This will reduce the amount of energy required to heat buildings (though HVAC or other sources).
3. North facing windows
During the day the majority of the heat from the sun enters the building through north facing windows. Due to this you will get the most benefit from maximising North facing window area and lesser benefit from East and West facing windows. Windows allow more heat loss than well insulated walls. Due to this they should only be used when required to provide heating from the sun and/or light into the building.
4. South facing walls
Windows on south facing walls should be avoided or minimised as much as possible. This is because no heat from the sun can enter through these windows in a Southern hemisphere building.
5. Solar shading
Solar shading should be installed, outside the building, in a way that blocks direct midday summer sun but allows direct midday winter sun. The solar load is often the largest heat load in the building. As a result, the heat load from the direct mid-day sun directly affects the max temperatures that a space will experience. By reducing solar loading in summer, the size of an additional active cooling system can be reduced.
6. High thermal mass flooring
Direct winter sun is useful for heating up high thermal mass building elements - an example being concrete floors. The heat stored in these elements will be released back into indoor spaces during cooler periods such as at night. This can remove the need for active cooling systems during colder parts of the day.
7. Floor plan shape
Building floor plans should be long along the north side and shallow on the east and west sides. This provides maximum area for north facing windows and also reduces the depth of the space - allowing natural light to reach the back of the space.
8. Insulated Skylights
Skylights should receive indirect sun. This provides daylight to underlit areas while avoiding glare and overheating. Skylights should be installed where natural lighting from north facing windows is insufficient to properly light the space.
9. Natural ventilation
Natural ventilation can remove moisture, stale air, and CO2 by introducing outdoor air into the building. It can also cool the building when the outdoor temperature is lower than the indoor temperature. Effective natural ventilation can be achieved by open windows on opposing walls.
10. Continuous insulation
Continuous insulation is the act of ensuring that all parts of the building have a high resistance to heat transfer. Areas/elements that allow heat to pass through easily are often referred to as thermal bridges. Examples of thermal bridges include studs in walls, wall-wall and wall-roof connections. DuPont Performance Building Solutions elaborate on the concept of continuous insulation in this video.
11. Airtight construction
Air entering or escaping through the building facade (infiltration/exfiltration) can be reduced through airtight construction. It is important that the air entering or leaving the building is controlled to heat, cool, and ventilate the space only when required. The healthy home design guide elaborate on the concept of airtightness here.
Passive Design For Commercial vs. Residential
High occupant density and /or city buildings face additional challenges when implementing passive design. Some of the challenges are listed below.
Ventilation Challenges
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Opening windows or louvres in commercial buildings should be automatically opened/closed for the following reasons.
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An understanding is required of how the natural ventilation affects the floor's temperature and interacts with any active systems.
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Human operated natural ventilation is not ideal to remove CO2, odours, and moisture as occupants often won't open windows due to cold external conditions and drafts. As a result spaces can become damp, and the air buildup of odours and stale air can occur, all of which creates an unhealthy space for the occupants. The effect of this is exacerbated by higher occupant densities.
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Outdoor air for building code compliance should be managed by heat recovery systems with filtration as opposed to opening windows for the following reasons.
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When it is cold outside, using natural ventilation to ventilate a building can over cool space and cause cold drafts. Heat recovery systems provide preheated outdoor air by recovering energy from exhaust air.
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In cities pollution and noise may prevent natural ventilation openings being used.
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Large end of trip facilities in commercial buildings, containing multiple showers, and even showers within the building, generate a lot of moisture that require active systems to properly manage.
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Heat and Cooling Challenges
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Commercial buildings predominantly require cooling as opposed to heating due to higher internal heat loads. Heat load sources include.
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Heat generating equipment such as computers, photocopiers, etc. as well as people heating up food and using hot water.
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A higher total heat load from people in offices. People give off heat and as a result the more people in a space the higher the heat load.
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higher lighting loads as the standard lighting levels required for an office is higher than a residential home.
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The building code allows commercial buildings to have larger glazing areas than residential buildings and this is often done. Glazing allows more heat gain/loss than walls therefore larger glazing areas increase heating and cooling loads.
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Internal blinds are important where occupants are seated near windows to prevent glare and overheating. However, blinds should only be used where occupants are near windows as the direct winter sun is useful for heating up high thermal mass building elements. Internal blinds do not replace the need for solar shading. This is because solar shading prevents the suns energy entering the building where internal blinds do not. The figure below shows the difference between internal blinds and external shading when it comes to heat gain in the building.
Figure 2: Solar radiation blocked by external shading vs internal blinds.
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There is an expectation in commercial buildings that the temperature remains constant throughout the year. In passive houses you can typically expect a temperature range between 20 and 25°C however in commercial buildings the normal temperature range that most people expect is 21 and 23°C. Due to this thermal modeling should be carried out to determine what the expected daily temperatures in a space will be. It is my opinion that a wider temperature range (20 and 25°C) should be accepted in office as people typically dress for the conditions in their own homes or outside resulting in most being over or underdressed for office conditions.
Glazing Challenges
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In commercial buildings It is not always possible to have windows on the face of the building you want for the following reasons.
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Due to segregation of tenancies and walls connected to neighbouring buildings.
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It is sometimes ideal, to add value to a building, to have windows overlooking a view on a non north facing wall.
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As placement of glazing has such a large impact on internal temperature thermal modeling should always be carried out to determine the effect of glazing type and placement.
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Commercial buildings often have larger floorplates and also site restrictions often lead to non optimal floor plan shapes. Due to this additional glazing is sometimes required to provide natural light. Natural light should be utilized wherever possible as it can greatly reduce the operational hours of artificial lighting.
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In cities you may not get direct sun due to shading from surrounding buildings.
All of these factors can alter the ability to implement certain passive design principles and also change the requirements of a building and the active systems serving them. Due to this it is important to review what design principles can be implemented and create a thermal model for the building to determine how it will operate.
The need for thermal modeling of commercial buildings
Thermal modeling should be carried out for all commercial passive buildings to determine the following.
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Improvements that can be made to the passive design.
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Daily temperature in each space.
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Where active systems need to be added to supplement passive designs.
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How large the active systems need to be.
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The best placement of natural ventilation openings and how they are controlled for optimal ventilation.
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What the optimal level in insulation is.
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A thermal model can be used to prove compliance with H1 of the New Zealand building code.
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A thermal model can estimated energy usage. This can also be used for a NABERSNZ or Greenstar performance pre-assessment.
Get in touch today if you would like a thermal model created for our building and assistance with building optimization.
Conclusion
For commercial buildings, once the first principles of passive design have been implemented and allowances have been made for the site shortfalls, thermal modeling should be carried out to determine how the building performs. Once the thermal modeling is complete the results should be reviewed to determine where modifications need to be made and where active systems need to be implemented to achieve an optimal building design.
If you're interested in learning more about active systems that complement passive design check out our second article in the serries Passive design for commercial buildings - Part 2: Active systems for passive design
Further Reading
Sustainability Certification
Sustainability certifications like Greenstar, Homestar, or NABERSNZ can provide you with a benchmark to see how your building performs against other similar buildings.
Government initiatives
Read the current government initiatives on climate change here.