Insulation doesn’t make your house warm. It’s a thermal barrier that separates the inside of your house from the outside environment to reduce heat transfer.
So in the summer when it’s hot outside and cool inside your house with the AC on, the insulation will minimize the heat from outside trying to get in. And in the winter when you have the heat on, insulation will minimize the heat trying to escape.
A sort of unrelated note, if you live in an area that gets snow in the winter, you can tell who has poor ceiling insulation by looking at the snow on the roof. Poor insulation allows the heat from inside the house to escape, and melt the snow on the roof. So oftentimes when you see a house with little snow on their roof, while all the neighboring houses are covered in snow, they most likely have bad insulation. This leads to an increased cost of energy bills in an attempt to keep a constant indoor temperature while combating the heat lost to the environment.
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