ᐅ Sloped Ceilings, Double-Layer OSB and Gypsum Board for Thermal Insulation

I want to improve the summer thermal insulation during the roof conversion and therefore plan to install double layers of OSB and gypsum board. A vapor retarder, Isover KM UV with a variable Sd value, was used.

Now I am concerned that the vapor retarder might not function as intended due to the diffusion-tight OSB panels, meaning it might not remain vapor-permeable in the summer as expected. Above this, there is 240 035 glass wool and a vapor-permeable underlay membrane installed.

I see the following alternatives:
1. OSB and gypsum board as planned
2. Double-layer gypsum board
3. Single-layer gypsum board as usual.

Well, basically you wrote it yourself: "as usual"!

I have another option.

Diffusion-open wood fiber insulation boards instead of OSB. Then gypsum board on top of that. Has anyone ever used something like this?

OSB is indeed a vapor barrier. However, I don’t see this as a problem. OSB has mass, and mass helps with heat protection.
I have not used wood fiberboard yet. But it could be a possible solution.

Joedreck schrieb:

OSB is indeed a vapor barrier. However, I don’t see this as a problem. OSB has mass, and mass helps with heat protection.
I haven’t installed wood fiber boards yet, but they could be a possible solution.

On one hand, OSB could reduce moisture diffusion into the insulation during winter, but on the other hand, it might also impair drying back towards the interior in summer.

That is why the underlay membrane is vapor permeable