I would recommend an Architect sooner than an Engineer when it comes to insulation. As for replacing it yourself, the builder is 100% responisble for having the correct insulation (in accordance with building regulations). If you end up replacing it yourself then at least make a reasonable effort to get the builder to do it, otherwise the builder can simple claim you didnt allow him the oppertunity to replace it. Once you have replaced it, sue him for the cost plus expenses.