ᐅ Timber Frame Construction – Prone to Mold?

Hello,
last year we renovated our upper floor using a timber frame construction. The wall assembly from inside to outside is as follows:
2 layers of gypsum board
Counter battens / installation space
18mm (0.7 inch) OSB board, tightly glued
200mm (8 inches) mineral wool with a thermal conductivity of 0.035 W/mK between the studs
18mm (0.7 inch) OSB board

My question is whether the outer OSB board might be a potential weak point regarding mold growth—possibly because it is too airtight? If so, how could this issue be addressed?
Perhaps additional external insulation?
Or should it simply be removed again?

I would appreciate any constructive advice.

Hello,
a building component (in your case the exterior wall) should be designed so that the vapor permeability decreases from the inside to the outside. The OSB board acts as a vapor retarder, meaning it is relatively vapor-tight. Therefore, the assembly is physically incorrect in terms of building physics. However, there are software programs available that can calculate the dew point condensation risk for different constructions.
One alternative would definitely be to install a vapor-permeable wood fiber board on the outside.

Regards

Thank you for your response.
So, does the outer OSB board need to be removed again??

Or are there alternative solutions??

After the blower door test, there were no leaks detected from the inside, but if the tape deteriorates after a few years, could warm air then escape outside??
And would the moisture not be able to dry out quickly enough??

With wood fiber insulation boards, the vapor diffusion is supposedly much better??

I was thinking about a simple roof boarding plus a vapor-permeable underlay membrane??

I had hoped to move the dew point outward by adding insulation on the outside, but that would probably require about 20cm (8 inches) of insulation???

Hello,
of course there are alternative solutions. However, you have to consider the financial resources available to you.

With your current construction, condensation definitely occurs at the 5th layer (second OSB board). You can counteract this by either removing the board and replacing it with a vapor-permeable layer or by adding exterior insulation to raise the temperature at that layer.
From a building physics perspective, I would replace the OSB board with a wood fiber board or an appropriate membrane, even though the blower door test was positive.
In terms of cost, replacing the OSB board is probably cheaper, but you would have to calculate that.
Theoretically, it also works with additional external insulation.

Best regards

Thanks first of all,
I ran some calculations using a software program, but no matter what I applied to the OSB board, the dew point was always inside the board... Damn... well, I guess we'll have to tear it down again...
How much soft fiberboard, about 40mm (1.6 inches) thick, do you generally need per square meter?

The carpenter in charge didn't really seem comfortable with it during construction... but if the architect calculated it that way, it should be fine... it just turned out really badly — maybe we should have done more research beforehand :-(
Now it's going to get more expensive again :-(
Best regards and thanks

Better have a professional calculate this for you, as the OSB panels also serve a structural function for the wall assembly. Possibly, a vapor barrier foil between the OSB and the drywall on the inside and 100 mm (4 inches) Neopor/expanded polystyrene insulation on the outside will be sufficient.