ᐅ Insulation between and under rafters: Does this require a double vapor barrier?

For a construction with 220-240mm (8.7-9.4 inches) pitched roof insulation between rafters and 40-60mm (1.6-2.4 inches) insulation below the rafters, the vapor barrier is usually installed between these two insulation layers. But why is that? I understand that it cannot be omitted, but why is an additional vapor barrier not installed on the room side?

Common setup:
1) Roof covering
2) Underlay membrane
3) Rafters + insulation between rafters
4) Vapor barrier
5) Substructure + insulation below rafters
6) Plasterboard

Why not?
1) Roof covering
2) Underlay membrane
3) Rafters + insulation between rafters
4) Vapor barrier
5) Substructure + insulation below rafters
6) Vapor barrier
7) Plasterboard

Vapor retarders/vapor barriers are a building physics necessity resulting from modern airtight construction standards. It is advisable to limit their use to a minimum, especially when the vapor retarder/barrier is a foil product whose long-term durability and robustness during onsite handling cannot be fully guaranteed. It is preferable for more suitable building materials with corresponding sd-values to perform the function of vapor control—in roofs, typically OSB boards with taped joints and sealed connections to adjacent components on the interior, and the wood-based underlay panel on the exterior as an underlay deck.

The downside of foil barriers is that they can never be installed completely airtight (due to faulty connections, damage, or missing sealing). Warm, moist indoor air can penetrate the structure by convection and vapor pressure, condense at cold spots, and possibly need to dry back into the interior during summer, in the phase of reverse diffusion. Foil barriers hinder this drying process. They also prevent capillary drying, which is much more effective but requires capillary-active materials—mineral wool, for example, does not qualify.

In conclusion: use as much vapor retarder/barrier as necessary, but as little as possible. An exterior-ventilated construction is always significantly more advantageous than an airtight one that must be sealed on the inside. The overall assembly must be calculated based on sd-values ("more airtight on the inside than outside"). The choice of materials should not be left to the contractor but must be proven to be physically sound by them.

I would definitely favor a wood-based underlay panel over an underlay membrane/roofing membrane on the outside.

You didn’t really answer his question, but rather gave your opinion about the membranes.

This is somewhat contrary to many other opinions...

Grym, you should check the U-value calculator – no critical condensation occurs with the thin insulation layers, especially because the actual rafter insulation is behind them anyway.

I chose to skip that because it requires a lot of work, has little effect, and I already have an additional exterior roof insulation.

To put it simply: if two vapor barriers are installed, a "plastic bag effect" occurs. If moisture gets in there (and unfortunately, despite all precautions, this can never be guaranteed 100%), it creates a greenhouse effect inside, because nothing can evaporate towards the outside. That’s why the rule is "always use a more vapor-permeable layer on the outside."

By the way, I found the first post very fitting, as it clearly explains that two vapor barriers are simply too much.

But would a fully vapor-permeable construction be better? Can this achieve a U-value of 0.14? This is the requirement for KfW55 according to reference values and also, based on precise calculations, a good value to just meet KfW55. Unfortunately, insulation materials that make a vapor barrier unnecessary usually have poorer values, and the roof structure is already quite "solid" with mineral wool WLG032 at 280–300mm (11–12 inches) of insulation.

Why use "fully vapor-open"? A vapor retarder is sufficient; two layers defeat their purpose. Use only as much vapor retarder as necessary, not as much as possible (e.g., vapor barrier). By the way, there are also partly vapor-open or one-sided "open" membranes that allow any moisture trapped behind the membrane to escape back into the interior.