Soundproofing works according to the spring principle. Hard-soft-hard. The rigid foam boards, whether open-pored or not, do not exhibit any "soft" behavior. Sometimes you even find people on the internet claiming that it is actually a reinforcement. Personally, I cannot confirm that. I learned too late what the deal with rigid foam insulation is, but luckily it’s not a problem. However, I also attribute that to the local conditions! For example, I hear rain when it pours down heavily with a slight background pattering/rustling. I have no problem with that because I really like it and it is part of what I enjoy about a top-floor apartment. Traffic noise is hardly transmitted to me. The house is 3m away from the street and about 6m to the eaves. The traffic noise is too weak to be transmitted loudly to the roof. But those who want to be on the safe side regarding sound insulation should not use rigid foam, which I ultimately took away from hours of internet research and talks with the expert. My rigid foam boards have 4 or 5 cm of mineral wool applied for soundproofing. But honestly, that little bit doesn’t make much of a difference under appropriate conditions. When such a board lies in front of you and you see it, you understand why. Those who value sound insulation should definitely choose something else, better safe than sorry. What exactly is better, for example wood wool or mineral wool or something else, I cannot say. From everything I read afterwards, as a layman I would probably opt for wood wool. Because of insulation values and soundproofing. Although I believe it doesn’t make much difference whether it’s stone wool, wood wool, or some other wool. When I read the partial horror stories about rigid foam online, I very quickly made an appointment with the roofer and expressed my concerns accordingly. He reassured me and showed me that the residential area/the street has no more than 60db of traffic noise (during the day). The rigid foam boards, or rather the roof construction, dampen ~40-48db. So 20-12db remain that I would and do hear as traffic noise. I have to be quiet, as if I were sleeping, to hear the bus (which is very quiet and only runs during the day) and that only applies to the rooms facing the street. My bedroom is on the other side. Of course, other factors come into play such as masonry and/or windows. But as already mentioned, those who place value on sound damping and really want peace under the roof must inform themselves beforehand about what is done. My roofer presented everything with sound calculations etc. and clearly showed how it behaves for me, and I have an old building. In a new building, this must surely be considered somewhere and also made transparent. If not, I would have it done. By the way, my rigid foam boards dampen summer heat quite well. The last few days were very warm and the apartment never had more than 23° C all day. When I think back to earlier times, with insulation from the ’70s, it would already be 28° C now. From what I have gathered, wood wool should be the better alternative here. But of course, it also always depends on how thick the insulation layer is, etc. In conclusion, talk to an expert and preferably get the expected values in writing. It may also not hurt to contact an independent expert with the documents and situation. @ Sirhc: I consider it an absolute myth to describe a sound insulation value of 50db or an outdoor noise exposure of under 50db in the apartment as difficult. I really cannot imagine that this should be so hard to achieve.
