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In construction in Central Europe are in use vapour barriers, in special cases more precise as vapour retarders. These enable the drying of building materials such as insulation, if moisture has coming in. On the other hand, these prevent as second barrier the penetration of water from the outside, such as dripping water by rain. The effect is analogeous to some outdoor clothing. A common installation in Central Europe is, from outside to inside: roof tiles - laths - air layer (2...4 cm) - vapor retarder - insulation.

Nowadays, predominantly is used for that polyethylene or polypropylene textile with a plastic coating. The problem is that the resistance of that plastics decreases with increasing temperature. The vapor barriers may decompose on the time.

There are materials available with a significantly higher stability such as glass fibers and PTFE. These materials are already used in building construction. But, to my knowledge, not as a vapor barrier.

My question is therefore: Are glass textiles with a slightly open-pore PTFE coating also used as vapor retarders in buildings?

If not, why not? Or what are other suitable alternatives?

  • why not is off-topic here, try physics – Jasen May 10 at 9:34
  • I do not know of anything like that it would probably be expensive and the large area of a roof this would impact building costs so not be used like normal barriers like tarpaper / felt. – Ed Beal May 10 at 17:18
  • @Ed Beal, you are right. It is not cheap with a price of about 16 €/m². But the thought is the alternative to open the roof in maybe 20 or 30 years to replace the crumbled conventional plastic layer. This may be also expensive. – gotwo May 11 at 17:58
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One reason could be the important general insulation/anti-mold rule for moderate climate zones like Central Europe:

A vapor barrier should be close to the warmer inside layers. The insulation layer should be close to the cold outdoor side. And the inner window glass should be the coldest surface in a room, enabling condensation on a well defined suited surface.

That way the dew point most likely is not inside the wall or on the wall paper/plaster which has organic material to feed fungi.

To get precise information, this web site may help:

Ubakus.de

It contains many construction/insulation material, foil layers can be inserted everywhere to see the difference.

Possible problems will be shown in a second.

Experimenting/researching is easy by switching on/off of layers.

R-value can be shown by clicking on the spanner/tool icon next to the U-value.

It is free for private use.

Fastest way is to choose an adequate example (menu) and to edit the layers.

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  • Yes, it is correct, there should be a vapor barrier on the inside of the insulation. And additional a vapor retarder is often required on the outside. If with any reason any moisture has penetrated into insulation, the moisture can always leave out via vapor barrier and the subsequent air layer. The available special retard plastic layers for that has a life period of approx. 20 ... 30(?) years. However, I would like to use a material that is stable as long as the other materials like wood, tiles and glass wool. – gotwo May 11 at 17:19
  • My 1930 home has tar paper on the exterior of the home iris still in good shape we modified a doorway, the roof has probably been replaced 3 or 4 times so who knows how old the felt was but it was intact with 2 layers of shingles (I put down new felt though). – Ed Beal May 11 at 18:08
  • "And additional a vapor retarder is often required on the outside". Why? This layer would delay the drying time of the mineral wool. The approved reliable layer sequence from outside to inside is: rain and weather protection (e.g. Eternit, plastic sheets etc.), open air layer (circulation), mineral wool, foil, solid wall like bricks, concrete etc. That is also named "rain curtain", which is one of the best wall constructions. Many of these walls in Central Europe have no problems even after 50 years or more - except for damaged paint at the outside sheet, which is easily solved by painting. – xeeka May 11 at 19:22
  • @gotwo I added a link to a side where the effect of foils can be easily simulated via the finite element method. It contains many insulation products, especially those available in Central Europe. – xeeka May 12 at 5:52
  • @Ed Beal, could you give your response as full answer? Please with information about installation of the insulation. E.g. between rafters? Are wooden boards, sheets or air over and under the insulation? – gotwo May 12 at 7:22

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