ᐅ Insulation on Upper Floor Concrete Slab / Roof as DIY Work – Vapor Retarder?

Hello dear members,

I want to insulate my upper floor ceiling / attic space as a DIY project.

Construction: 2 layers of 10 cm (4 inches) EPS insulation with a thermal conductivity of 0.035 W/(m·K) and on top of that 1.5 cm (0.6 inch) OSB boards for walking on.

Around the outer edge, I will leave out the first row of OSB boards.

Now the question: Do I need a vapor barrier?

I keep reading very different opinions and recommendations.

Hello,

At the end of July, we insulated our upper floor ceiling and constructed the cold roof together with the local master carpenter (KfW70 house).

First, the roof frame was raised. Part of the vapor retarder was already installed under the wall plates. Then, the vapor barrier was carefully laid over the entire upper floor ceiling area. The joints were properly sealed. At the posts of the ridge beam and around the chimney, everything was also neatly sealed.

On top of this, we installed two layers of 12 cm (5 inches) EPS insulation boards, staggered, covering the entire surface up to the wall plates. Behind the wall plates (at the eaves), where the boards could not reach, we filled the space with glass wool. We made sure not to leave any gaps in the insulation!

Afterward, we immediately installed the roof decking, roofing underlay, and counter battens to prevent moisture from reaching the insulation and to make the roof weather-tight.

I hope this helps.

Edit: I just noticed that I wrote vapor retarder once and vapor barrier once. However, I mean vapor barrier throughout. This was the recommendation we received from the master carpenter.

That sounds very much like you have a wooden ceiling – but Harald asked about a concrete ceiling. In this case, I am quite sure that a vapor barrier is not necessary because concrete is naturally almost vapor-tight.

K1300S schrieb:
That sounds like you have a wooden ceiling – but Harald asked about a concrete ceiling. I am quite sure that a vapor retarder is not needed here, since concrete is naturally almost vapor-tight.

No, we also have a concrete ceiling, and it is naturally somewhat vapor-permeable – unless the ceiling is made of waterproof concrete.

I assume the vapor barrier prevents moisture rising from the interior rooms through the concrete ceiling into the insulation, which would then reduce the insulation’s effectiveness, because wet insulation insulates less well. (Note – non-expert opinion).

Hello,

stefanh schrieb:

No, we also have a concrete ceiling, and it is naturally somewhat vapor permeable.

Since when?

stefanh schrieb:

- unless the ceiling is made of waterproof concrete

WU stands for WaterProof; this property is generally inherent in all concrete by default.

Regards, Bauexperte