About a year ago, my husband and I moved into our new home in the Parisian suburbs, a 2019 build with many large French doors across every room and floor. Those big windows-slash-doors bring a level of luminosity we’d never experienced in our previous rental, especially during the gloomy winter months, but we soon discovered that they also had a potential disadvantage: insulation.

Even though all of these are double-glazed and seem well-insulated, they’re still glass, which will always be less efficient than a properly insulated wall. These French doors seemed like the weakest link in our new home’s thermal isolation and the biggest culprit in our high energy bills last January and February. But I couldn’t really prove it. As winter time rolls around again, I’ve been wanting to get to the

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