De l'utilité d'un pare-vapeur sur un support béton

[olivir]

Bonjour,

Je tente mon explication en français, le sujet me paraissant complexe. Je m'essaierai à l'anglais si besoin

J'ai un mur en ossature bois qui repose sur un support béton isolé par l'extérieur (avec un relevé d'étanchéité, j'ai utilisé la membrane V13). Ce support bénéficiera également de l'isolation intérieure complémentaire (nue sans pare-vapeur) qui est prévue pour le reste du mur.

support beton pour MOB.png (52.25 KiB) Viewed 13616 times

Un calcul statique type Glaser m'annonce alors une très forte condensation dans la laine intérieure au contact du béton. Au contraire, WUFI voit lui un comportement beaucoup plus sain sans le pare-vapeur en amont de l'isolant intérieur !

WUFI support beton pour MOB.png (151.46 KiB) Viewed 13616 times

Qu'en pensez-vous ?

Je comprends que l'absence de pare-vapeur permet un meilleur séchage mais mon modèle est-il suffisamment valide ? Et s'il y a des remontées capillaires ?

[olivir]

OK, no help, I try in English

I am astonished enough by the WUFI results for a concrete support for a timber frame construction (without vapour barrier film).

Whereas a Glaser calculation announce a lot of condensation without the vapour barrier film, WUFI says exactly the opposite : the healthiest wall is without the vapour barrier !

I understand that no vapor barrier provides better drying but is my model sufficiently valid?

[olivir]

OMG, even, Christian Bludeau has no opinion ?!

[Christian Bludau]

Hello Olivir,
sorry, did not read your post.
It is possible here, that Glaser shows dew water, while WUFI does not, as Glaser works with much stronger boundary conditions and most of the effects happening in the assembly are not taken in account.
As in your case the case without retarder is better than with retarder, you should think of the initial moisture of your concrete. It is possible, that the moisture can dry out without the retarder while with retarder it is blocked.
You could further do an assessment of the layer, in that Glaser shows the dew-water. I guess it would be the connection mineral wool - concrete. Duplicate the mineral wool and split of 1-5mm close to the concrete. Assess the water content of this small layer and transform it to g/m². In this case to amount of dew water should not be higher than 200 (EN13788) to 500 (DIN 4108) g/m² to avoid the water running off the wall and out off the insulation.
Christian

[olivir]

Thank you, Christian !

Waiting for your answer , I noticed that concrete humidity was determinant : with 2,5 % initial moisture, retarder is a bad option, with 8 % (typical construction level given by your database) retarder or nothing are both bad options, a variable Sd value (intello) is needed.

I am going to check if there is a risk of running water as you suggest. I did not do it yet because I thought that the concrete porosity was big enough not to be worried by that.

In the WUFI help, there's only the DIN4108 recommendation, is the EN13788 only for concrete ? What should I think about a result between 200 and 500 g/m² ?

Finally, I have a better comportment with Sd variable retarder but it began with a big variation and ends with low variation (196 g/m²), what do you think ?



Subsidiary question : my method for measuring the increase in humidity is correct ?!

[olivir]

As I think about it, it would not hurt to see these recommendations appear somewhere in the charts...

[Christian Bludau]

Dear Olivir,
yes, thats the way you can access the dew water in a layer. The DIN 4108 is the glaser which is still in use in Germany and many other countries. EN 13788 is the so called EURO-Glaser, its the old glaser with a view changes to adapt it to the climate etc. It is not handling a special material. For the effect of water running down the 500g/m² might be quite high. So the value of the EN 13788 may be more realisitc. But that always also depends on the material, in which water is running.
There is a further Standard handling the dew water in constructions again with other values: BSI 5250 (BSI 5250:2011: Control of condensation in buildings. BSI - British Standards Institution, London, 2011) which gives the effects of condensate on impermeable surfaces:

• < 30g/m² - A fine mist which does not run or drip
• 30 – 50g/m² - Droplets form and begin to run down vertical
  surfaces
• 51 – 250g/m² - Large drops form and begin to run down sloping
  surfaces:
  • 70 g/m2 will run down a 45° slope
  • 150 g/m2 will run down a 23° slope
• > 250g/m² - Drops form and drip from horizontal surfaces

Thats all for tight surfaces. You have to choose something which suites to your material. As the concrete is not completely tight and will "take up" a small amount of water I guess I would use the 200g/m² to be on the save side. If there is no problem if some drops of water can run down probably 500 g/m² is usable as well.

Christian

[olivir]

Oh thank you, that's a very comprehensive answer !