A thermally expandable composition, including (a) at least one polymer P, cross-linkable by a free-radical initiator, and (b) at least one solid butyl rubber BRu, and (c) at least one free-radical initiator, and (d) at least one blowing agent, and (e) at least one tackifier TA, whereby the cured expanded composition has a volume increase compared to the uncured composition of less than 1300%. The thermally expandable composition is able to provide good expansion behaviour and good adhesion on metal surfaces and is especially suitable for baffle and/or reinforcement elements, e.g. in automotive manufacturing.