Expanding foams should not be used on any historic or Listed Building as once installed they can’t be removed without damage to the slate or tiles. They are, in my view, a complete waste of money.   The foams are sprayed on to the underside of the roof and when dry they provide a sealed space without any ventilation.

They were, and perhaps still are, marketed towards vulnerable homeowners who are not aware of the consequences of using the foam.   They are very thin and any possible thermal benefit provided by the foam will be minimal.  It could be argued that they reduce draughts, but ventilation of historic roof spaces are a good thing and not something to be viewed as a problem.   I am not aware of any long-term studies about the lifespan of there foam integrity.   By comparison, it does seem that similar foams used to fill cracks and gaps tend to last for about five years before suffering from degradation.

These foams should not be used on any historic or Listed Building.   It is an alteration that would require Listed Building Consent and a application for Consent would almost certainly be refused.