Improving Performance in Multi-Unit Residential Buildings
In August, our monthly LAB Event focused on multi-unit residential buildings – or as they say in the States, multifamily housing. As we learned from Dr. Straube, multis can achieve higher levels of performance, but they do have some unique design considerations compared to single-family houses.
First, there’s the enclosure. With less enclosure area per unit, there should be reduced heat loss and gain through the “skin” of the building. However, many multifamily buildings squander this advantage through excessive use of glass or thermally-bridge-compromised enclosures. High window-to-wall ratios (WWRs) have a tremendous impact on heat loss and solar gain, an impact that is especially noticeable in better-insulated enclosures – for example, in an enclosure with a 30% WWR, U=0.32 windows, and R20 wall, almost 75% of heat loss and 90% of solar gain is due to the windows. As John stated, “the windows start to be the big leak”, so once you have R20 walls, it becomes more useful to focus on better and fewer windows, rather than adding insulation.
Combined performance of walls and windows.
Airtightness and thermal bridging are also important considerations for larger buildings. Balconies, steel studs, and projecting floor slabs can all create thermal bridges by allowing heat to get past insulation and leave the building. Unlike single-family housing, structural materials such as concrete, steel, and aluminum are widely used in multis and these materials transmit heat tens or even hundreds of times more easily than wood. Continuous exterior insulation is part of the solution and is implicit in some modern systems (e.g., EIFS, ICFs). Good details are needed to reduce projections through the insulation, and luckily there are many guides available (one that John mentioned is “RDH Technical Bulletin 11: Cladding Attachment Solutions for Exterior Insulated Commercial Walls”).
Beyond the enclosure, multi-unit residential buildings also present unique challenges for mechanical systems. In taller buildings in cold climates, stack effect can create a kind of negative feedback loop, where warm air is continuously drawn to the top units, released by overheated occupants opening windows, and replaced in the bottom units by chilly occupants plugging in space heaters. Compartmentalization, or the restriction of airflow between interior spaces, is an important strategy for managing stack effect, and will also help with sound control and fire control – which are clearly non-trivial considerations. Even where stack effect is not an issue, the size and multi-unit nature of the building means more thought must be put into reducing distribution losses; suite-by-suite heating and cooling should be considered. As far as ventilation, any system that delivers a controlled and steady amount of fresh air to each suite is acceptable. Unfortunately, the most approach used is a pressurized corridor system, which research has definitively shown does not meet the need. Although energy recovery is not always needed, in John’s words “it’s hard to imagine a market anymore where an ERV or HRV doesn’t make sense.”
Efficient appliances can make a big difference, especially for hot water heating, which is a major user of energy in multi-user residential buildings. Efficient lighting is becoming standard practice and should always be used.
Energy Use Profile of a Multi-Unit Residential Building in Vancouver, BC.
What about renewable energy? Photovoltaics and other renewable energy sources get a lot of media attention and are familiar to most people as a “green” strategy. They can make sense for some projects, but according to John, their impact varies and tall buildings in dense urban settings often don’t have a lot of options for collecting sufficient energy. Enclosure and HVAC improvements will provide more “bang for the buck” and should be prioritized.
Although the focus of this presentation was on energy use, John also touched on durability, and pointed out that it shouldn’t be forgotten in a rush to improve efficiency. There are critical durability issues for multi-unit residential enclosures – for example, because taller buildings are more exposed to wind, more rain is driven onto the enclosure. As always, the best way to approach design is from a whole building perspective, considering the building as a system.
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