First of all, to achieve better sound insulation and thermal insulation effects, you must choose real insulating glass instead of ordinary double-layer glass. Insulated glass refers to two pieces of glass that are sealed with a certain distance between them. There is usually an inert gas in the middle, which requires machine processing. However, ordinary double-layer glass is simply to stick two pieces of glass together by hand. The interlayer will definitely leak. Performance varies greatly.
Because handmade double-layer glass will leak, a simple method is to check whether there is any freezing between the two layers of glass in winter, and whether there is fog before the two layers of glass in spring and summer.

In addition, real insulated glass will have an aluminum spacer between two pieces of glass, while double layered glass usually has a black sealing strip or sealant in the middle.