Multi-layer glass is a member called a spacer, which has a hollow layer between two glasses.
In the space kept with the spacer, there is "insulation" effect as a feature of Multi-layer glass by encapsulating dry air.

By having a space in between, it is possible to prevent heat transfer, such as deterioration of air conditioning effect and condensation. You can further enhance the function by using glass with functions such as thermal insulation and ultraviolet cutting, security, fire prevention.