Load-bearing timber curtain wall are a surprisingly flexible way to dazzle an indoor space and bathe it with natural light.

But what are the implications around timber curtain wall when it comes to combustibility, thermal performance, structural integrity, and ease of installation – especially for glaziers used to working with traditional aluminum products?

Unicel Architectural’s Samuel Doyon Bissonnette recently sat down with Patrick Flannery of Glass Canada Magazine’s Glass Talk podcast to explain how timber curtain walls outperform aluminum.

This Q&A has been edited for length and clarity.

Patrick Flannery: Can you tell us a little about yourself and the company?
Samuel Doyon Bissonnette: I have an engineering background. I joined the industry as a project manager with Unicel, then became director of engineering. Since January I have been Co-President and shareholder, along with my brother Vincent, who was on the podcast a couple weeks ago. Unicel is a family business and we’ve been in business more than 60 years.

We have three main product lines: First is the specialty glazing, the Vision Control. We also have our own custom aluminum curtain wall and skylight system. And then there’s the newest edition, which is our timber curtain wall and skylight system. It comes from an acquisition we did in 2018, when we acquired a small company that was doing timber curtain wall. And we’ve grown that business pretty significantly in both the residential and the commercial markets in the last couple of years.

PF: There have been a number of changes to combustibility rules, along with increasing energy efficiency concerns, that have contributed to the recent popularity of timber curtain wall. Could you take us through those changes?
SDB: There have been two main shifts in the rules the last couple of years, one in terms of height for mass timber buildings, and another around thermal performance – both of which encourage the use of timber curtain wall.

Let’s start with the height limitations. The code used to be pretty strict on height for timber buildings and mass timber buildings. But in the last couple years, there’s been a lot of work done by organizations like FPInnovations and Cecobois to really showcase and test, including live demonstrations on fire resistance of mass timber structures. Because what needs to be understood is that, yes, timber burns – but when it burns it actually chars, and the char forms a natural protection against fire. It’s easy to calculate how much it chars per hour, so if you want to achieve a certain fire resistance, you oversize your beams and columns by a certain amount. It resists and keeps its structural properties for a certain amount of time as the wood chars.

So there’s now more height to mass timber buildings. And the codes are getting more open to that. Nowadays you can do a 13-storey building with exposed timber. And when you have a mass timber building, it just makes sense to have a timber curtain wall made from the same species as the mass timber. So it’s a perfect match.

The other element is thermal performance. All the new energy codes take thermal bridges into account, and timber is much better than steel at preventing thermal bridges. It’s a way less conductive material. All the new energy rules are also more stringent around the fenestration system, and our timber curtain wall systems are Passive House certified.

With the proper glazing it can go down to an overall U-value of 0.8, which is NRCan’s stated goal to reach by 2030, but we’re already there. We see this product as a future-proof product that way, in that we’re already where they want to go in a few years.

PF: What else is important to note in terms of timber curtain wall and combustibility?
SDB: Sometimes we also do fireproof joints between floors. And one other very important point is that on the exterior side, most of the time have an aluminum cap, an aluminum cover plate, so there’s no fire propagation on the exterior. It’s all on the interior side that you’ve got the wood, just like you would have wood furniture.

PF: The first thing people often react to when they hear about timber construction on a high rise, that would include the facade, is that it’s going to fall down. It won’t be strong enough. You’re an engineer – what’s the truth there?
SDB: It can be strong enough when you design it properly. With all this engineered wood and engineered glulam, you can reach those multi-storey heights and the structural part is really a non-issue.

What used to happen, many years ago before all these engineered wood and glulam products came on the market, is you’d have to do some basic calculations for wood. Since wood is a natural material it’s imperfect, there are imperfections, and the codes and the way you normally calculate for the strength of the wood structure reflect that. You’ve got all these factors, such as the number of knots, they might have cracks and this and that, which greatly reduces the usable strand of the wood.

But when you work with engineered wood there’s much more control and way fewer cracks. They’re tested and certified.

PF: What kind of other structural considerations come into play when designing a timber curtain wall? Do you need to come at things differently to get the strength and rigidity that you need?
SDB: There are some different considerations, but I see them as opportunities. The fact that we use timber as the back member of our mullion gives us a great deal of flexibility. We’re not fixed with preset numbers of dyes or extrusions, so we can size the mullion exactly to what we need for every specific elevation of every specific job, to perfectly optimize the mullion dimension.

And on top of that we work with wood, so we can make the facade load-bearing. So that’s really a paradigm shift in the curtain wall and facade industry: We are the building envelope, but we can also be part of the structure. So we often get involved earlier in the design process. And then we coordinate with the project engineers, the structural engineers. We can advise them they can remove that big column at the corner of the building, and the architect loves it because you can get some clear unobstructed views. We can fabricate curtain walls that span 30 feet without any structures in the back and on top of that they support the roof load.

PF: Another thing that comes to mind is timber’s flexibility. That could be an advantage in earthquake-prone areas, right?
SDB: The key, as you said, is flexibility. And the engineering term is ductility – that is what you need to withstand earthquakes, and where you need the most ductility is in the anchor and the connections. Timber is great for these kinds of applications. We’re actually in the process of getting it tested against various earthquake standards.

PF: Just to come back to the energy efficiency angle, you mentioned that you can avoid some of the thermal bridging with a timber curtain wall. How does that work?
SDB: Where you avoid the most of the thermal bridging is when you use a mass timber structure instead of a steel structure. But another aspect around thermal performance isn’t just the wood, it’s the whole engineering of the system. The biggest enemy of wood is water and condensation, right?

And so with our system, we absolutely want to avoid condensation on the mullion. So the gasketing system is designed to be super insulating on top of that, to prevent any risk of condensation. We have some really, really thick EPDM gaskets. We have some insulating foam within the air cavity to block any convection that could happen there. And we also have thick gaskets on the exterior. All of this makes it very well insulated. And on top of that, it’s a very, very flexible system. So we can incorporate any thickness of glass ranging from half an inch to four inches into the system – we can go with those triple glazing with laminates and all sorts of things, with the oversized glass that we do once in a while, too. We can accommodate all of that.

It’s actually a dry glazed system, so there’s no sealant in the system at all. Sealant is great until it’s not, and being able to avoid all sealant is great from a durability perspective. Another trend that’s popping up in the industry is deconstructability – what do you do with it in a couple years when it’s at its end of life. And it’s much easier to deconstruct and recycle when there’s no sealant.

There’s also much less embodied carbon in a timber curtain wall than a traditional curtain wall. It’s a net negative carbon product.

PF: Did you get an EPD or a lifecycle analysis done for this product, or anything like that?
SDB: It’s something we’re looking at. We have EPDs for the wood part of the product. We don’t have an EPD yet for the whole system, but we do have some for the wood parts.

PF: How about insulating behind the spandrel – any challenges there, or differences in how it’s handled?
SDB: No, it’s very similar to a traditional curtain wall. We can insert anything we want in the glazing pocket, just like in a traditional curtain wall. So doors, windows, commercial entry doors, commercial windows. We can do all of those.

PF: How about fastener systems?
SDB: It’s a stick-built system, so everything is machined in the shop and all the connectors are put in place in the shop, And it’s all hidden connectors that are machined in between the mullions. So they’re all hidden.

In terms of anchoring to the structure, that’s where we have to get creative. It requires a lot of engineering and is probably the area that differentiates that most from traditional aluminum curtain walls, where you have a sleeve. With a timber curtain wall there’s no way of putting a sleeve. But we’ve come up with various strategies – we have knife plates, we have rods that go into the mullions, we have bent steel plates. We’ve pretty much worked out every kind of connection detail.

PF: Any thought of doing a prefab unitized system?
SDB: It’s in the works, actually. We work with a German partner on this and it’s at the R&D stage.

PF: How about for installation – the mullions must be significantly lighter?
SDB: Actually, they’re a bit heavier than the aluminum. But lighter than those tall, steel reinforced mullions for oversized glass units. Much lighter than that.

PF: So you can put larger glass spans in this than in aluminum systems?
SDB: We just finished a job that’s being installed right now where every vertical mullion is 43 and a half feet long.

PF: Holy smokes. Wow. That is something else.
SDB: That’s the beauty and flexibility that you get when working with wood.

PF: Can you give us more information around the history of the product? You mentioned an acquisition earlier?
SDB: The product itself originated in Europe, mostly in Germany. They started doing timber curtain wall about 20 years ago. About 10 years ago, there was a company called IC2 Tech, just outside Quebec City, that started working with a German partner doing mostly high-end residential. These timber curtain wall turned out to be a great match for high-end residences, where you may have a nice open view towards the lake or a mountain, and you don’t want to stack a couple windows on top of one another. Architecturally, that’s not interesting because you need to add beams and stuff like that. So timber curtain wall were a perfect match for that.

Over the years their projects got bigger and bigger, and in 2018 we acquired the company and put the resources of Unicel Architectural behind the product. And we opened up the commercial market – we started doing it in some big commercial jobs, libraries, schools, a lot of university buildings. It has been a great success story. This division is growing exponentially, and it’s a really fun product.

As I said earlier, I see it as a future-proof product, both in terms of thermal performance, carbon content, design, flexibility, deconstructability. And it’s really interesting to be at the forefront of the building envelope industry.

PF: How do customers typically react to the product?
SDB: Architects love it. They usually say: ‘How can I put this on my building?’

And so where we have to do the most work is with the glaziers. This is something new, and they’ve been doing aluminum for years. They’re sometimes pretty uncomfortable with it at first, but we provide a lot of support. We go on site. We have some nice installation documentation. If you go on our website, we have some nice installation videos that we’ve done, and we do provide side support on every job that we sell.

We guide glaziers through the process, which is very similar to a traditional stick-built system. There are two or three different details. And the fact that it’s dry glazed makes it easier and faster to install when you’re used to it.