ᐅ Roofer intends to install without a vapor retarder.

Unfortunately, I need to watch my budget carefully and build as cheaply as possible. I have now received a quote from a roofer for roof insulation. He suggested installing 12 cm (5 inches) of mineral wool between the rafters and adding 6 cm (2.5 inches) of wood fiber insulation on top of the roof. He believes a vapor barrier is unnecessary because the attic is already fully finished, and the layers of drywall, cement board, and wood-cement panels already act like a vapor retarder. If moisture should condense on the insulation, it would dry out quickly.

Is this correct? Can I really skip a vapor retarder?

If well planned and properly executed, a vapor retarder can be omitted, but it is essential to ensure that the vapor permeability decreases toward the outside. This means that any vapor entering the insulation layer is directed outward.

Drywall (gypsum boards, etc.) does have a vapor-retarding effect; however, this is not the only concern. It is also very important to prevent warm air from entering the insulation layer through gaps, where it can condense and cause wet insulation.

Therefore, the gypsum boards must have airtight joints—especially at connections to walls, beams, chimneys, and similar elements. It might be a good idea to have this checked.

We had the same problem and I sought several opinions. They varied, and another roofer said he would not do without the vapor barrier. Now I’m a bit unsure, and maybe there are more experiences here.

Hello,

I think this decision should be left to the roofer, as they are responsible for it. As far as I understand, the roof must be completely watertight. In that case, a vapor barrier can be omitted.

At this point, I have a different opinion and would not do without a vapor retarder. I also don’t believe that any moisture entering there would dry out immediately.

On the contrary, this is especially true when condensation occurs.

This is quite a lively discussion with different opinions. However, I am happy to forgo a vapor retarder. The joints of the gypsum boards must be sealed airtight. If this is done, I can do without it.