ᐅ Exposed Roof Structure – Where Should the Vapor Retarder Be Installed?

It’s me again…..

A brief recap: we are building a KFW 55 house through individual contracts with a construction supervisor.

We have exclusively chosen local suppliers, some of which have been in business for three generations. I mention this because it’s reasonable to expect they know what they’re doing and have solid experience.

We have an exposed timber roof frame insulated with wood fiber insulation boards from Gutex. The roof is installed but not yet covered with the final roofing material.

Yesterday, the building inspector came and asked us about the roof structure. We explained it to him, and he suggested we check with the Central Association of German Carpenters to determine the required performance class (I assume it’s class 3 since the standard roof pitch would have been 28 degrees, and ours is 19 degrees), and of course, the roof is part of the living space.

We looked into how a roof like this should be constructed and read that a vapor retarder and a nail sealing tape must be used.

Here is our build-up:

On top of our spruce multilayer panels, we installed an Alujet Professional JKE underlay membrane, then the 20 mm (0.8 inch) wood fiber boards, followed by a hardwood multiplex plywood board, and then, overlapped, another layer of Alujet Professional JKE, and finally the counter battens.

We are annoyed that we didn’t receive any nail sealing tape, but maybe this is acceptable due to the use of the Multiplex plywood. What I’m more concerned about is: where exactly is the vapor retarder? The Alujet Professional JKE doesn’t seem to have that function.

We have now sent an email asking the carpenter for clarification, but I’m afraid, as often happens, we’ll just be reassured without real answers…..

Unfortunately, the roof construction is specified exactly like this in the contract…..but as a layperson, you wouldn’t know that it might be wrong…..

As expected, the carpenter believes that a vapor retarder is not necessary.

We supposedly don’t know the difference between a vapor retarder and airtightness.

... however, on all the images from the wood fiberboard manufacturers, I find references to:

vapor retarder / air barrier

Does that mean you need one or the other, or both?

Does anyone know about this?

From the outside, the insulation must be covered with a windproof membrane to prevent it from cooling down. This is done using the underlayment membrane. From the inside, it must be protected against moisture. For this purpose, a vapor retarder is used. There is a rule stating that the vapor retarder must have a diffusion resistance factor X times higher than that of the underlayment membrane; otherwise, condensation will form within the insulation. I believe X is 7 or 10, but in any case, it can never be the same product for both applications. If this rule is not followed, proof of safety against condensation must be provided through calculations, which is likely impossible in this case.

According to the manufacturer, the underlay membrane is approved for airtightness, with an Sd value of 0.02, so the 3-layer board can then act as the vapor retarder, since it has an Sd value greater than one.

I don’t understand why this can’t be simply explained to a layperson, because then it would be clear...

Could you please sketch your structure?

The structure is as follows:

Rafters
Spruce 3-layer panels, 20mm (0.8 inches) thick, used as visible sheathing
Alujet jke Professional breathable membrane, glued for airtightness
20cm (7.9 inches) wood fiber insulation from Gutex
3cm (1.2 inches) Gutex Multiplex top layer
Another layer of Alujet jke Professional breathable membrane, overlapped and glued

As a non-expert, I would have expected a vapor retarder after the visible sheathing, not a highly vapor-permeable breathable membrane.