Cavity wall insulation is the filling of the space between two walls with a few centimeters between them. As a result, less heat is lost and you have to burn less. But bats, a protected species, also like to be in that space and that is why the government considers ecological research so important.
Not every house has cavity walls. For example, very old houses have ‘single-brick walls’. According to a calculation by Milieu Centraal from Utrecht, you can save around 500 euros on your gas costs through cavity wall insulation. In addition, as a homeowner you can receive a subsidy for installing insulation. The amount of the subsidy depends on the size of the house.