Insulation / Vapor Barrier Top Floor Ceiling / Collar Beam, Open Ceiling

Hello, our house (built in the 60s) is supposed to get a new roof with insulation up to the ridge in the next few years. But today we need to replace the floorboards in the attic (roof slopes slightly insulated and clad with wood) and the question arises whether a vapor barrier should be installed. 180 mm mineral wool is planned between the collar beams. Current ceiling structure from below: wallpaper, plaster, straw, collar beams. From above, the ceiling is open. Because a new vapor barrier cannot be connected airtight to the old roof slopes from below, I am unsure what the point of the vapor barrier would be at all. Would be grateful for tips and opinions. Looking forward to it. Best regards, Karsten

Hello Karsten, yes, connecting it is not that easy... But in any case, I would insert a vapor barrier with an Sd value of about 2m into your collar beam ceiling. And that as far as the new vapor barrier, which has to be installed in the roof slopes during the renovation, can be connected to it. Whether such a foil always makes sense... In the wet rooms definitely, because damp insulation is not insulation. So usually you make one across the entire surface. P.s.: you can lay the foil over the collar beams, but nice and tight, preferably with slats, so that the wool lies properly in the cavities.

Hello Homi,

thank you for your answer. I just wonder what the purpose of the vapor barrier in the top floor ceiling is if I later insulate the entire roof up to the ridge. Isn't that double vapor-barriered then?

Regards, Karsten

Hello Karsten, yes connecting it is not that easy... But in any case, I would insert a vapor barrier with an Sd value of about 2m in your collar beam ceiling. And as far as possible, so that the new vapor barrier, which has to be installed in the roof slopes during the renovation, can be connected to it. Whether such a foil always makes sense... In wet rooms definitely, because damp insulation is no insulation. So usually you make one over the entire area. P.S.: you can lay the foil over the collar beams, but nice and tight, preferably with slats, so that the wool lies properly in the compartments.

Hello Karsten, the risk that too much moisture (wet rooms) penetrates unchecked into the insulation is too great. If then a floor is installed on top (usually made of OSB), the moisture can no longer escape. I have already seen hundreds of OSB boards mold from underneath...

Hello Homi, I understand. Please don’t laugh, but now the following question arises for me. Until now, the warm air or the humidity could go everywhere, namely outside. If I now lay the foil, the moisture will remain on the 1st floor. More precisely, the collar beams also belong to the 1st floor, because the foil is laid afterwards from above. Can moisture damage, for example, occur on the collar beams? Regards, Karsten

Hello Karsten, now the proper ventilation naturally comes into play... You will have to change your ventilation behavior, because as you already mentioned, until now your Swiss cheese construction has taken care of it alone. The air exchange took place by itself (along with a lot of expensive heated air). If you ventilate properly in the future (shock ventilation is the keyword, preferably 3 times a day), there should be no moisture damage at all.