Common Multifamily Building Envelope Design Mistakes
Originally published by: EEBA — February 7, 2020
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Although I'm an architect by training, I've worked for nine years as a sustainability consultant on low-rise multifamily projects. My designers and builders usually seek green building certification for their projects, whether ENERGY STAR or LEED for Homes. They want to be known for delivering high quality, high performance buildings.
Good intentions notwithstanding, I see the same mistakes again and again. This article covers the top four, all of which concern envelope detailing. Few projects have all these errors, but every project I see suffers from at least one of them.
These mistakes begin during design, which is why most of the live trainings I give on this topic are geared to architects. However, I also know that the most effective preventative measure is for educated architects and builders to help each other do good work. That benefits everyone.
The four top design mistakes are as follows:
1. Overcomplicated Geometry
Architects love to draw buildings with complex shapes. Maybe it's their training, or maybe it's because design competitions reward people who do something unique.
Either way, the belief that jogs, bump-outs, and cantilevers make a building more attractive is subjective, and tastes could change by this time next year. What's not subjective or temporary is nature: rain, wind, heat, sun, and gravity. These factors have way more impact on a building's durability and profitability than aesthetics, but people don't seem to spend enough time thinking about them.
A complex shape has more surface area than a simple cube. To build it you will need more materials and labor, and a longer construction schedule. Those jogs and bump-outs make it difficult to correctly install air and water barriers, which makes drafts and leaks more likely. They also complicate the insulation details needed to prevent thermal bridging. And with all that extra surface area, the mechanical system has to work harder to keep the building comfortable.
My point is not that you shouldn't incorporate interesting shapes into your design. However, you really need to be realistic about the costs of doing them right and the long-term risks to the building.
Although some architects love complexity, simple buildings have fewer problems and, as seen in the photo of these Brooklyn townhomes, can be quite attractive.
With that in mind, I want to make the case that simple buildings can be quite attractive if they're intelligently detailed. Just compare Figures 1 and 2, which show a complex apartment project and a row of condos in the Brooklyn's Park Slope neighborhood. The latter are simple, rectangular boxes but there's something very pleasant about the trim proportions and the regular window spacing.
2. Design Irregularities
This category includes a wide range of different errors. The big one is designing units in a wide variety of sizes and shapes, which unnecessarily makes life difficult for the rest of the project team, from the engineers and energy raters to the builders and their subs. All of this extra effort doesn’t come cheap, and it increases the chances for failure. Instead, consider offering a couple of floor plans and replicating them in a consistent fashion. Construction will be more predictable and it will be easier to correctly detail individual units.
Other problems include offering too many options in everything from light fixtures to mechanical systems. Keep it simple. Instead of spending time designing a good water barrier connection between three different types of cladding, just stick with one. Pick plumbing fixtures that are efficient, accessible, affordable and attractive, and use them as often as possible.
Of course, some architects can't help themselves and feel like they want to try a different design approach on every project. But the fact is that consistency is efficient and effective, and it can also make your designs more recognizable (and marketable).
To avoid unnecessary problems, costs and energy penalties, limit your offerings to a toolbox of proven design and construction details. If you want to show off your creativity, you can explore different ways of combining those details—as long as you don't overcomplicate the building in the process.
3. Inadequate Air Barriers
This is more likely in complex buildings, but I also see it in the most simple of projects. Air barrier detailing is often left to the builder, but builders generally build the plan, so if the air barrier is not clearly defined there's a good chance it won't be done correctly.
You have to draw and detail the air barrier on the plans, and also include a performance requirement in the specs.
Plans for a multifamily building should show the air barrier surrounding the individual apartments as well as the building as a whole. This is a classic belt-and-suspenders approach.
Some people ask whether the air barrier should enclose the building perimeter or individual units. My answer is that you need to do both. That means the plans will show a bunch of little bubbles surrounded by a big bubble, as in Figure 3.
Why seal each unit individually? For one thing, it's required by most green building programs. For another, airborne annoyances like sound and cooking smells travel more readily through leaks than through solid assemblies. Finally, compartmentalizing the building like this supplements the structural firestopping.
It's a classic "belt and suspenders" approach. The individually sealed apartments reduce air pressures on the building air barrier, which in turn reduces air leakage to the outside. The tighter the little bubbles, the less hard the big bubble has to work.
My company has written an air sealing guide that you can insert into your drawings or specs, but the best approach is to incorporate our details into your details. First, identify the location of the air barrier (for example, is it at the ceiling drywall or at the subfloor of the unit above?). Then zoom in on every connection, transition and penetration to anticipate how to stop air movement.
To ensure good results, some builders complete a few units ahead of time and have a consultant like myself run a blower door test on them. What if the don't meet the air leakage goal? "Oops," I tell them, "Remember all that stuff I told you do to? You're obviously not doing it." That can lead to some interesting conversations between the building owner or contractor and subs, but the problems usually end up getting fixed.
4. Thermal Bridging
This common slab-on-grade detail is a major thermal bridge. A better approach would be to put the insulation on the outside. Click to enlarge.
In case you haven’t heard it before, thermal bridging is where energy moves more quickly through the solid parts of an assembly. Basically it’s the path of least resistance between where energy is to where energy isn’t (inside to outside, or vice versa).
Preventing this means eliminating those paths, and the best approach is installing rigid foam on the outside of a roof, wall, floor or foundation. While this is simple in theory, not all thermal bridges are obvious, so in practice a lot of architects and builders miss them. These hidden bridges are more numerous on buildings with complex geometry, which is another reason to keep things simple wherever possible. (If you detect a theme here, you're right.)
However I also see a lot of thermal bridging on simple assemblies like slabs on grade, as you can see in Figure 4. The part of the assembly with the most heat transfer has the least insulation!
There are way too many potential thermal bridges to point out in a short article. The point is that you have to think through the insulation details on the plans and predict how heat might move through or around them. If you see any short circuits, then you need to add details to eliminate them.
The above errors aren't the only ones I see in multifamily projects. Other common ones include a lack of lighting controls, improperly sized HVAC, antiquated ventilation and oversized domestic hot water distribution. I cover these in my live trainings (including one at the 2019 EEBA Summit), but you can also find more information here.
One last point: Preventing these issues doesn’t just happen on paper; it requires collaboration and communication. In fact, I find that good communication—between the architect, consultants, builder, subs and the building owner—to be the major difference between a successful project and a failure. If everybody is on the same page and willing to learn from each other, your chance of creating a high quality and cost effective building increases exponentially. Who wouldn’t want that?