The Pacific Northwest is not built for heatwaves at all. It has a very mild and stable climate as such things go due to the geography. There simply isn't a need

The Pacific Northwest is not built for heatwaves at all. It has a very mild and stable climate as such things go due to the geography. There simply isn't a need to deal with much heat. A serious "heatwave" in Seattle is 30-35°C, and they pass as quickly as they come. Even if homes had a lot of thermal ballast by construction, long runs of high temperatures like now require air conditioning. Something like 25% of homes, mostly newer ones, have air conditioning. It simply has not been needed historically. If there are a few days of 30°C temperatures, you opened a window. This current weather system is running much hotter and much longer than traditional Seattle summer weather.