On the water vapour transmission of cellular glass

logo_smallCellular glass according to EN 13167 “Thermal insulation products for buildings – Factory made cellular glass (CG) products – Specification” needs to have a water vapor diffusion resistance factor µ of at least 40000. This property has to be measured according to EN 12086:2013 “Determination of water vapour transmission properties”. This is a difficult way to state that cellular glass according to EN13167 has to have 100% closed cells.
cup

The µ-value is expressed as the ratio to the water vapor transport properties in air. µ=40000 for a material means that the water vapour is diffusing 40000 times slower in the material than in air. Like shown in the following, this is hard to measure for two reasons:

  • The seal between the cup has to “vapor tight” but not one flexibe material has a µ-value like cellular glass. This induces always an error.
  • The weight changes of the samples are extremely low, which means that the weight balance has to be stable and reproducible.
  • The EN standard also request that the system is in a stationary state.

Indeed, for a sample of 40 mm thickness and a diameter of 100 mm we get a transmission of 0.004 mg/h if the dry cup is placed in 50% humidity. To have a certain accuracy (10%), the total weight difference needs to be about 10mg (we measure with 1mg error) which means we need to wait 10/0.004 = 2500 hours or about 100 days. It is clear that this test takes a long time and cannot be used for daily quality control.

Therefore, we suggest to measure daily the amount of closed cells with a pycnometer, like described in a previous post.

 

 

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