Yes, you can. The standard is called EnerPhit and the targets you need to achieve are slightly more relaxed than those for new-build Passivhaus standards. It is quite a challenge to retrofit existing buildings to this standard, mainly because they are so poorly built from an insulation and airtightness point of view. But, it is possible and people are doing it. We have a selection of materials to help you with this, ranging from the UdiSTEAM airtightness products to the Unger-Diffutherm wood fibre insulation systems to help you achieve your required thermal targets. Contact us if you'd like to know more.