ᐅ Insulation / Vapor Retarder for Top Floor Ceiling / Collar Beams, Open Ceiling

Hello, our house (built in the 1960s) is planned to get a new roof with insulation all the way up to the ridge in the next few years. But today, we need to replace the floorboards in the attic (the sloping roof sections are lightly insulated and clad with wood), and here the question arises whether a vapor retarder should be installed. 180mm (7 inches) mineral wool will be placed between the collar beams. The current ceiling construction from below is: wallpaper, plaster, straw, collar beams. From above, the ceiling is open. Since a new vapor retarder cannot be sealed airtight at the bottom to the old sloping roof, I’m unsure what purpose the vapor retarder would serve in this case. I would appreciate any tips and opinions. Looking forward to your input. Best regards, Karsten

Hello Karsten,
yes, connecting it is not that simple... In any case, I would install a vapor retarder with an Sd value of about 2 m (6.6 ft) in your collar beam ceiling. It should be placed far enough so that the new vapor retarder, which must be installed in the sloped roof areas during the renovation, can be connected to it.
Whether such a membrane always makes sense... In wet rooms definitely, because insulation that becomes damp is not insulation. So it’s usually installed across the entire surface.
P.S.: You can lay the membrane over the collar beams, but keep it taut, preferably fixed with battens, so that the insulation wool sits properly within the cavities.

Hello Homi,

thank you for your answer. I’m just wondering what the purpose of the vapor retarder is in the top floor ceiling if I’m going to insulate the entire roof all the way up to the ridge later. Wouldn’t that mean there are two vapor retarders installed?

Regards, Karsten

Homi schrieb:
Hello Karsten,
yes, connecting it is not that easy... But in any case, I would install a vapor retarder with an Sd value of about 2m (6.6 ft) in your ceiling joist layer. It should extend far enough so that the new vapor retarder, which will have 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 isn’t insulation. So usually, it is installed over the entire area.
P.S.: You can lay the foil over the ceiling joists, but keep it tight, preferably fixed with battens, so that the insulation fits properly between the joists.

Hello Karsten,
the risk of too much moisture (wet rooms) entering the insulation uncontrolled is too high. If a floor is installed on top (usually made of OSB), the moisture cannot escape at all. I have already seen hundreds of OSB boards with mold damage from below...

Hello Homi,
I understand. Please don’t laugh, but now I have the following question. Until now, the warm air and moisture could escape everywhere, meaning outside. If I now install the vapor barrier, then the moisture will stay in the first floor. More precisely, the collar beams are also part of the first floor, since the vapor barrier will be installed later from above. Could moisture damage occur, for example, to the collar beams?

Regards, Karsten

Hello Karsten,
now proper ventilation comes into play... You will need to change your ventilation habits because, as you already mentioned, your Swiss cheese-style building alone previously took care of that. The air exchange happened naturally (along with a lot of costly heated air). If you ventilate properly in the future (the key is shock ventilation, ideally three times a day), there should be no risk of moisture damage.