Sustainable Building Effort Reaches New Heights With Wooden Skyscrapers
Kurt Kleiner, writing for Knowable Magazine and republished by Ars Technica, covers mass timber: manufactured wood elements such as glue-laminated beams (glulam) and cross-laminated panels (CLT) that replace steel and concrete in tall buildings. The technology is spreading, with the 25-story Ascent skyscraper in Milwaukee standing as the current record and dozens more mass timber towers built or proposed worldwide, most of them in Europe.
The appeal is carbon. Steel and concrete together account for close to 15 percent of global CO2 emissions, and wood is renewable. But the article is careful about the fire, moisture, and accounting problems that come with building this way, and about how much of the climate case still rests on assumptions researchers haven’t fully verified.
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- Mass timber elements can be stronger than steel by weight, but need more bulk to match that strength, so very tall buildings like Ascent still combine wood with steel and concrete.
- Fire codes historically limited wood construction to low-rise buildings; testing showing that char forms on the timber’s surface and insulates the interior helped convince regulators, and the International Building Code now allows mass timber up to 18 stories.
- Moisture, not fire, is described as the real technical challenge: wet wood invites fungus and insects, so builders manage humidity through transportation, construction, and the building’s ventilation design.
- Carbon accounting for mass timber assumes felled trees get replaced by new growth that absorbs equivalent CO2, an assumption environmental groups question, especially when old-growth trees are replaced by plantations that may never reach the same size.
- Not all carbon in a felled tree ends up locked in the finished building. Branches, roots, and mill waste can decompose or burn, and demolished timber sent to a landfill can release methane.
- Mass timber enables “integrated design,” where the parts are engineered and often pre-manufactured off-site before assembly, which Steffen Lehmann says can make construction up to 40 percent faster.