Wood. The United States is the largest producer of the natural resource in the world. But yet we rarely see it in commercial, high-rise construction. So we asked a wood expert -- Rebecca Holt at Perkins+Will, an analyst for reThink Wood's recent Tall Wood Survey -- to tell us about its potential benefits.
AD: Why is wood a material architects should use in taller buildings?
There are lots of reasons to consider wood – first it has a lower environmental impact than other traditional choices like concrete and steel. Wood is the only major building material that is made the by sun and is completely renewable.
In North America, forest management is strongly regulated to ensure that forests are legally harvested and managed to meet society’s long-term demand for forest products. Wood products have less embodied energy, are responsible for lower air and water pollution, and have a lighter carbon footprint than other commonly used building materials.
Wood can significantly impact the quality of space – on the interior and exterior. People typically describe spaces that include exposed wood as warm and inviting. People tend to have strong, positive reactions and connections to spaces with wood. This was certainly emphasized by reThink Wood's Tall Wood Survey participants as an important reason for choosing to work with wood. Faster construction times - many of the mass timber products lend themselves well to prefabricated components, which make for quick installation and assembly which have a number of advantages – cost savings in a shorter construction schedule, advantageous for dense urban sites, with little space to store components – components can arrive on site ‘just in time’ and be erected/installed immediately. A timber structure offers a cleaner, drier, healthier working environment for the construction team. Finally, it’s important to consider timber where it make sense and use it appropriately (as it is with any material).
AD: What are the environmental benefits of using wood?
I think there are several issues to consider, certainly it’s a low carbon material. [...] In addition, one of the most consistent messages from Survey participants is that a wood solution is integral and complementary to goals of optimizing energy performance and creating high quality spaces for occupants. Wood was noted as beneficial in contributing to good envelope performance; as a poor conductor of heat, it minimizes thermal bridging, improving the effectiveness of the insulation compared to other materials. In addition, several Survey participants emphasized the complementary advantage of achieving good air tightness afforded by the precision cut and fit of prefabricated panel components, with less joints, gaps and penetrations to seal as compared to other materials/systems – aligning well with the Passive House standard. It must also be emphasized that wood contributes significantly to a healthy indoor environment and high quality spaces that foster a sense of well-being.
AD: How exactly does a wooden building sequester carbon?
Trees naturally absorb carbon dioxide from the atmosphere, incorporating it into their wood, leaves, needles, roots and surrounding soil (a carbon sink). That carbon is only released when the wood begins to breaks down. Wood products sequester stored carbon, keeping it out of the atmosphere for the lifetime of the structure — or longer if the wood is reclaimed, reused and manufactured into other products. It is important that forests are maintained responsibly to manage the balance between carbon sink and storage and of course, a healthy ecosystem.
AD: Cross-Laminated-Timber (CLT) and other innovations allowed buildings to become taller; what is the structural height limit or capability of wood?
While the tallest building included in the Survey (and the tallest completed project to date) is 10 levels – Forte in Melbourne, Australia – all participants were confident that the height limit of timber buildings could certainly be much taller. The Survey showed a temporal trend toward refined design and construction solutions, so as more techniques are applied and tested, I expect heights will grow. In addition, Survey respondents identified market acceptance as an important part of growing the market for taller wood buildings – there is work to do to change perceptions of fire risk and durability.
AD: One thing people assume about wood is that is less resistant to fire than a material like steel, which isn’t necessarily true. What steps do you take to fire-rate a tall building?
Structural or mass timber products like CLT or LVL have an inherent resistance to fire. Wood burns slowly and the char layer created on the surface as it burns helps protect and insulate unburnt wood below the charred layer. The unburnt portion of a thick member retains 85-90% of its strength. In addition, timber panel products can eliminate void spaces between walls and floors where fire tends to spread easily, making it more fire resistant than other construction material/methods. Fire protection strategies vary depending on the type of timber products used and the requirements of the jurisdiction. Among all the projects surveyed, timber elements were oversized to include a char layer, in addition to encapsulating timber elements with gypsum to some degree. Sprinkler systems and intumescent paint applied to exposed timber were also common fire protection strategies.
AD: Moisture damage can be an issue in wooden structures. How do you prevent moisture damage on a large scale project?
In all cases, any exposed structural wood elements are located either inside the building envelope, protected by an overhang or in the case of cantilevered panels, exposed only on the underside [...,] as with any building or material, the qualities and capabilities of wood must be respected, and buildings must be well maintained to ensure long term durability. During operation, moisture is addressed in most cases by a mechanical ventilation system, but Survey participants all emphasized the importance of educating building occupants on optimal operation strategies. To address the perception that moisture could be a concern, two of the buildings in the Survey have installed moisture sensors within the building envelope to monitor performance over the long term (Limnologen, Forte).
AD: What are the financial benefits of using wood in larger scale buildings?
The Survey results emphasized the benefit of shorter construction schedules where prefabricated elements were used. Survey participants noted that the quality of construction was superior in most cases, resulting a more durable building and better long term investment. Operational energy savings as identified in question 2 certainly contributes to building the financial case. At least two of the projects in the Survey set out to demonstrate that a timber structure could be construction cost competitive with a concrete structure and were successful.
AD: Is there a specific reason tall wooden structures are more prevalent in European countries than in the USA?
There is a strong regulatory grounding in Europe where governing policies in support of low carbon construction, energy efficiency, or renewable resources, directly or indirectly encourage tall wood and mass timber construction. As well, expertise across related sectors appears to have benefited from a greater blending of professional roles, creating a strong culture of collaboration between developers, designers, timber fabricators and researchers. These nuances appear to be significant for advancing strong and credible markets for tall wood construction, and are only just beginning to emerge in North America.
AD: Lastly, what are some striking examples of vertical wooden buildings?
The interesting thing about most of the buildings that participated in the Survey, is that most don’t appear from the exterior to be wooden buildings. In almost all cases the wood is hidden in the structure and only exposed on the interior – if at all. Where the wood is featured, is where it makes the most striking impact, and there are several beautiful examples in our Survey participant projects: The Tamedia building features the timber structure on the interior in a very modern way. The interior suites of 3XGrun and Forte are lovely, inviting and warm residential spaces. The LCT ONE office space is beautiful – having spent a day working in the building myself, I can attest to the sense of well-being afforded by the timber elements in the finished space. The feature timber stair at the Earth Sciences Building is certainly a striking signal of the timber elements within the structure of the academic wing.