Interstitial condensation

Ceiling joists should be covered with insulation board, also insulated in between joists, If they used pir insulation. Because if joists are not covered then it's a cold bridge. Or maybe membrane or felt damaged when installation done and there is a leak. It's hard to say when can't see it. Or there is bad ventilation between joists and roof, and condensation builds up and salks through the timber and that's why your joints are wet. Should be minimum gap 50mm between joists and roof tiles. And vent grills installed in the soffit.

Assuming the cross ventilation is working as it should, as the builder claims, it sounds like the vapour control layer is either absent or has failed which is allowing moisture to pass through the insulation to the cold surface above and condensation is happening and you are seeing the effects below. First check if there is a VCL and if there isn't that's what needs to be addressed. If there is and it has failed, then the VCL will need to be replaced. Have you downlights in the ceiling? This would require breeching the VCL and unless you can get sufficient insulation and an un-breeched VCL above the lights you will not eliminate the problem. Another cause of failure of the VCL is the screws that fix the plasterboard to the joists above. Purists use butyl tape under where the plasterboard screws will go so it self heals around the screw penetration. See a video here https://www.youtube.com/watch?v=zqw0gu4px7Y Most builders don't do this and many have no issues because moisture through the screws will be limited and comfortably managed by cross ventilation that is working properly. Without seeing it though I suspect downlights may be the culprit. One last possibility is that there isn't enough insulation or a VCL over the steel ridge beam. Hope this sets you on the right track. Good luck!

Hi , I can definitely help with the interstate condensation.to give a definite answer I will need to have a site visit to investigate in details. Kind regards Etmond Konesha Ebk Building Services

Condensation can significantly reduce the effectiveness of insulation, and result in damage to the building fabric. A Condensation Risk Analysis evaluates the likelihood of interstitial condensation in your roof or wall construction. These calculations are regularly required by building control to demonstrate compliance with building regulation requirements. To have such an analysis done there are several companies which can do this, this analysis will expose the problem after which informed action can be take.