ᐅ Insulation of trapezoidal metal sheet flat roof with friction-fit mineral wool and OSB boards

Hello everyone,

First, let me introduce myself:
My name is Markus, I’m 35 years old, from Upper Franconia, and after 15 years in the beautiful Rosenheim area, I have returned home to renovate and prepare my house and property here.

I have a specific question for which I would appreciate some different opinions:

I plan to convert a former tractor garage into a (hobby) workshop.
The floor area is 10 x 7 meters (33 x 23 feet), with brick walls all around, a roof with about a 5° slope made of trapezoidal metal sheets, and three large doors.

I want to insulate the roof.
Although I will only heat the hall occasionally (probably using a warm air fan heater), I want some level of efficiency and to avoid losing all the heat directly through the metal roof.

Currently, the roof structure is as follows:
- trapezoidal metal sheets
- 70/40 mm (3 x 1.5 inches) battens
- 130/90 mm (5 x 3.5 inches) rafters spaced about 90 cm (35 inches) apart

I would like to use 120 mm (5 inches) mineral wool insulation and visible OSB boards.
I have read that OSB 3/4 can act as a vapor barrier if the joints are sealed. How should the wall connection be executed in this case?

Would that be sufficient?
If so, I could save the cost of a separate vapor barrier, additional battens, and the taping and painting of drywall.

Thanks in advance and best regards,
Markus

Hello

Somehow, I’m not comfortable with the construction method you have in mind.
What happens to any condensation that might occur under the metal sheets? That would then drip into the insulation.
Have you done any calculations for this setup?

My instinct would be to remove the metal sheets and battens, install an External Wall Insulation Board (DWD board), then counter battens, battens, and finally the metal sheets.

Olli

Hello Olli,
thank you for your reply.

I was actually hoping to avoid taking off the roof...
Attached is the calculation from the U-value calculator.
I hope I entered everything correctly.

Do you think there will be enough condensation to cause dripping?