October 30 2020
Cladding Systems for Mass Timber Buildings
Paul Carter is an Associate and Timothy Wong is a Building Envelope Specialist at Entuitive. Recently, we sat down with them to learn more about cladding systems for mass timber buildings.
Thanks for sitting down with us today, guys. First question – are there any types of cladding that aren’t suitable for mass timber buildings?
Paul: Virtually any cladding that is used on a concrete or steel structure building can be used on a mass timber building. There are some differences in how these structures move vertically over time, including creep and seasonal humidity fluctuations that need to be considered in envelope design.
Timothy: Designing the envelope for rapid installation and enclosure to protect the wood superstructure from excessively absorbing moisture is an important consideration when selecting cladding strategies and evaluating your options.
Interesting – do you have any project examples that you can speak to?
Paul: A good example is a Hines T3 project in the Lower Junction area of Toronto called Sterling Road. This building is clad with window wall and metal panel slab edge covers that protect the project beyond the glazing. The original design had an exposed mass timber exoskeleton with structural penetrations through the envelope. Entuitive provided design support for these challenging envelope details, focusing on minimizing air leakage.
We used thermal modelling to assess the thermal performance of column and slab edge covers and found that timber beams and floor panels greatly improved the performance of these details, when compared to our experience with similar details on projects with concrete or steel structures.
Timothy: At 550 East Broadway, in Vancouver, the design team opted to explore the use of mass timber as a structural wall element which allowed for the project to benefit from the added thermal mass to improve effective R-values which traditionally would have been lower performing when going with either a conventional steel or wood stud framing assembly. In addition, the solid timber mass contributes to the envelope acoustics and structural capacity.
Fascinating. So we see there are definitely some options when considering cladding systems for mass timber buildings, but how do you deal with moisture management during the construction phase?
Paul: Well, for starters, it seems obvious that wood can be damaged by moisture, but the effects are actually quite staggering. For instance, for each 1% change in moisture content, there is a 0.25% change in dimension.
Timothy: It’s important to note that the long-term exposure to moisture is more harmful than exposure to one large rain event. If the timber has time to dry out, the water on the surface will not penetrate further but if it is continually wet, the water will seep deeper into the panel causing it to stay wet for longer and add to it’s ability to cause more damage and rot.
Paul: That’s a good point, Tim, and cyclical wetting and drying can also damage the timber since repeated swelling and contraction can open up joints and seams. This could allow water to penetrate past the surface and increase the amount of time it takes for the surface to dry out. This can delay the installation of other materials, causing significant scheduling impacts. Of course, it’s possible to dry out the timber using natural or mechanical means, but this can also cause delays in the schedule and add to the budget.
Wow, so how do you avoid this or minimize the impact?
Timothy: Well, we certainly try to get ahead of the curve by crafting a construction moisture management plan. This needs to be done early and the plan needs to consider how different design decisions can impact the needs for different types of construction protection whether in the factory or in the field. The earlier we can get started on the plan, the lower the cost and the higher the effectiveness of your moisture management strategies will be.
Do you have any examples of how you’ve dealt with moisture management during the construction phase in practice?
Paul: I can think of a 10-storey office building case study in Toronto with an NLT deck and unitized curtain wall enclosure. We were able to do a risk evaluation very early on and we knew that with nail-laminated timber floors there was very high potential for moisture entrapment. This meant we had to choose dry weather for the installation.
Once we knew when to schedule the work, we had to consider risk mitigation. To help minimize risk, we decided that construction should occur over the summer months to avoid rainfall. We also considered prefabricating as many structural components as possible to help speed up the frame erection. We also knew prefabricated wall assemblies would help to speed up the enclosure of the building and installing a roofing membrane as early as possible would be important.
The next step was factory and early site protection. NLT slabs are susceptible to moisture damage due to the gaps between dimension lumber laminates, which allow water to penetrate from the surface to deep in the slab. To prevent this, a peel-and-stick asphaltic membrane was factory applied to keep the slabs protected from the moment they were installed. Joints between slabs were taped along with any open penetrations that could allow water to leak to floors below.
We also specified that tenting the structure and drying by mechanical equipment be provided if the moisture content of the mass timber becomes excessive. Schedule slippage, unusually wet weather and inadequate protection can result in the wood unexpectedly becoming saturated, which can have significant cost a schedule impacts and should be planned for.
Removing water from the deck immediately after rain or snowfall is a good way to prevent water from penetrating into the NLTs, and this can be done by squeegee or shop vacuum.
Great, thanks guys, any final thoughts?
Timothy: Mass timber continues to be more and more relevant with the on-going discussions about carbon sequestering in the building industry. But a key feature is still developing resilience and durability while we explore these more advanced methods of engineered wood construction. Designing our building envelopes for long term service life is another key strategy to reduce our carbon footprint.
Paul: Protecting the structure from the elements during construction and throughout its lifetime maintains durability and the beautiful aesthetic of mass timber buildings. I enjoy being a part of making that happen.
Thanks for sitting down with us today, Paul and Timothy. It was awesome to learn a little more about mass timber.