My first reaction upon being invited to Denver’s Populus, which claims to be the country’s first “carbon-positive” hotel, was, admittedly, one of skepticism. After all, the transport, construction, and hospitality industries are among the most damaging to the environment, and the 265-room hotel, designed by award-winning firm Studio Gang and developed by Urban Villages, covers all those bases. …Another immediately confusing design choice was the building material: concrete, one of most carbon-intensive materials on the planet. …The concrete used for Populus is the proprietary ECOPact low-carbon mix by Holcim, which claims to have 30 percent lower carbon emissions compared to standard concrete. But why use concrete at all—particularly when mass timber, for example, is more sustainable, and often more durable? …But the existing building code did not allow timber for a 13-floor structure and the city was ultimately not comfortable issuing a variance.