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Does putting a duvet cover over a comforter make the comforter warmer?

Yes, but not to a degree that you will be able to notice. Duvet covers are not designed to keep heat, although an extra layer of silk will certainly help retain some extra heat inside. The purpose of duvet covers is two-fold: to protect the duvets it keeps inside itself from dust and other possible sources of pollution, as comforters are usually difficult, if not impossible, to wash normally; and to provide an aesthetically pleasing layer over the often plain and non-decorated comforters to match with the rest of the bedroom decor. Our silk duvet covers also provide protection against skin abrasions and help with thermal regulation, but in the end, you should not rely on duvet covers for warmth.