Might contact the WUFI team and see what they have: http://www.wufi.de/index_e.html

You simply need the sorption isotherm for the material you have! That will make it possible to translate RH into moisture content. If you want to be precise, you can include the temperature dependency of the sorption isotherm. If you want to even more precise you need to consider the "moisture history", i.e. whether your material was drier or wetter before, and include the scanning curves between the desorption and the absorption isotherm.

I you haven't got the actual sorption isotherm for your particular material, I would suggest to use any sorption isotherm for a wooden based material, with the moisture content expressed in percent by weight; they are almost identical.

You can use i-button which is produced by Maxim integrated. they provide the software online for setting up the i-button and take the reading.

Hi, I found this link to a really useful table:

http://www.tis-gdv.de/tis_e/misc/klima.htm

Try this

AH (g/m3)=((0.000002*T^4)+(0.0002*T^3)+(0.0095*T^2)+(0.337*T)+4.9034)*RH

where AH is absolute Humidity, RH is relative humidity and T is Temp in degC.

If your goal is to access water redistribution between wooden insulation and the surrounding materials, measurement of relative humidities inside of the materials is a promising approach because relative humidity is directly related to chemical potential of water that is the fundamental driving force of water irrespective of its phase and medium.

For a more accurate measurement, dew point measurement with a chilled-mirror type equipment is recommended. Confirm the equipment can be traceable to the national standard humidity generator.

Finally, sorption isotherms of the materials are needed as suggested by Prof. Nilsson.

@ all of you: thanks a lot for your tips. It' amazing how this platform can save hours!

I already thought, that there're different ways of getting an answer depending on the level of accuracy one needs. We made some calculations with WUFI but the program's materials and constructions didn't fit to our construction under research (we're building a prefabricated roof element with integrated PVT-collectors without any backside ventilation. The thermal absorber of the PVT cools down the PV module and changes the boundary condition of the insulation etc. In WUFI we just had to make to much simplifications. Therefore we decided to build a model in 1:1...)

So in a first steep I will wirk with the sorption isotherm for our material. If someone has an such a sorption isotherm for a wooden insulation (we use pavatex with a density of 240kg/m3 and an inaugural humidity of ca. 8%) it would be nice to get it. The "stress tests" for the construction can be based on rough estimations and afterwords we can decide if it's worthwhile getting into more details.

@shinsaku tada: If I understand the principle of chilled mirrors right, they still measure the due point of a gas, so in our case of the air inside the chamber. Our sensors are precise enough (0.1°C, and 0.1% RH, which for our scale is very good), so I can just calculate the due point. But I would still have the question of the the material's humidity or not?

Would it be possible to place the entire assembly on a scale? The change in mass would be easy to measure - and would not miss and heterogeneity in the moisture distribution.

I agree - all of sudden this site seems very useful! Thank you!

@tim

the whole mockup is around 200kg. First of all, the differences will not be very big compared to the weight of the whole construction. Additionally the installation is on top of a building (to have good solar exposure...), there is no crane or anything to hold it up. But we can take the insulation out and measure it. It would just be much simpler to work with the sensors and some calculation.

Hi Nicklaus, I'm working on heat and mass transfer for insulation materials. See on my page, I hope you can find interesting informations in publications . If you have questions send me a mail. Good luck,

Dear Niklaus,

A very good estimate of the sorption isotherm for any wooden based material is a sorption isotherm for (any) wood, expressed in terms of moisture content in % by weight (or kg/kg). With your density you can then translate such an isotherm to a sorption isotherm for your material, expressed in moisture content in kg/m3, which you need for a calculation!

I am still confused with your expression "Absolute humidity"! Do you mean vapour content of the air volume (g/m3) or moisture content of the wooden material (kg/m3 or kg/kg).

When you measure the humidity condition in an enclosed air volume in a layer of a material you have to make sure that you do not get any temperature differences between your sensor and the material. Such a temperature difference gives an error of some 5 % RH per degree C.

Niklaus - from what I understand from colleagues involved in measuring moisture in straw bale and cellulose constructions the moisture content of timber samples embedded within a wall can be measured using timber moisture meters which effectively measure the electrical resistivity. These probes have a diameter of around 10mm and are inserted into the wall at a measured depth. The electrical resistivity is converted into a moisture content using calibration data for different material types. This technique is invasive in that it leaves a hole in the surface and a void in the material though. This is a similar process to that used by researchers investigating the moisture content of strawbales and other cellulose based insulation. So contacting one of the researchers active in this field (e.g Prof Pete Walker or Dr.Mike Lawrence at Bath University or Prof Steve Goodhew or Dr Jim Carfrae at Plymouth University) might yield some useful insights for you.

You can use sorption isotherms to convert the ambient RH to absolute humidity within a material. One way of obtaining these isotherms is dynamic vapour sorption method. You can have a look at one of our papers titled "Environmental performance assessment using monitoring and DVS testing".