High-rises built with wood are showing up as sustainable building options

Typically, when you want to build a high rise, you need iron, steel, cement, concrete; materials that are super strong and durable but also come with a high carbon footprint. Now, some builders are looking at constructing these very tall buildings using a traditional material in a new way.

On the northern edge of downtown Milwaukee, one high rise is not like the others. The building is called the Ascent and it’s made of wood. Cross-laminated timber to be exact, also called CLT.

“It is 19 stories of mass timber over a six story concrete parking structure,” said architect Jason Korb of Korb Architecture.

The Ascent is currently the world’s tallest building that’s a hybrid of timber and concrete.

Fifteen years ago, it would have been hard to find a wood building that was more than four or five stories tall. But this building stretches up 25 floors thanks to extra strong CLT. It’s made by layering and gluing lumber in a crossing pattern to form massive strong beams.

Korb took the elevator up to show off a model apartment. Inside, the ceiling is made of warm exposed timber that connects to thick wooden support beams, framing a stunning view of Lake Michigan.

Originally, Korb said the building’s developers considered cross-laminated timber as an aesthetic choice.

“But the more you dive into it, the more you learn about its other benefits,” Korb said. “The largest one is the level of carbon sequestration in the timber.”

Trees take carbon out of the atmosphere. When you cut down a tree to turn it into cross-laminated timber, you’re storing a lot of carbon into a building for a long time, said John E. Fernández, a professor of architecture at Massachusetts Institute of Technology.

“When you look at a tree, the fact is that about half of the dry weight of that tree is carbon,” Fernández said.

The Ascent’s designers estimate the building offsets 7,200 metric tons of carbon dioxide. They say that’s the equivalent of taking 2,400 cars off the road for a year.

CLT doesn’t just lock carbon into a building, it also avoids emissions from steel and cement. Together those two materials produce almost 15% of planet warming greenhouse gas emissions worldwide.

But Fernández said there’s a caveat. The forest the wood comes from needs to be sustainably managed for this all to work out and the logging process does produce emissions.

Right now, CLT is still a relatively rare building material in the U.S.

“It’s really early days,” said Bill Parsons, the chief operating officer at WoodWorks Wood Products Council, which is a nonprofit funded by the forestry industry.

Parsons said changes in the international building code in the last few years opened the door for high-rises made of CLT.

And there’s some momentum around it. Walmart’s New Home Office in Arkansas includes mass timber and Microsoft announced it’s building two new data centers with CLT.

But even though there’s this long history of using wood in single family homes, it hasn’t caught on for bigger buildings.

“There’s very little wood utilized into commercial and institutional buildings,” he said.

Parsons said there are 15 tall CLT projects either built or under construction around the U.S. and more are in the works.

A lot in St. Louis’ Downtown West neighborhood doesn’t look like much right now. It’s gated off and covered in gravel.

But it will soon have a mass timber high rise that’s a bit taller than the one in Milwaukee.

Kyle Howerton is a principal at AHM group, the developer behind this project. He said city officials were shocked when they first heard about it.

“They didn’t realize that you could build something over 300 feet tall using wood,” Howerton said.

Now he wants this building to inspire similar projects in St. Louis and beyond.

“I hope that this is a catalyst for more structures being built using the same technologies,” Howerton said.

Howerton plans to break ground on the climate-friendly high-rise next year.