Automatic net generation
Institute for Buildings and Energy (BYGDTU)
Technical University of Denmark
BYG DTU
November 2001
Moisture calculations in constructions with multiple material and/or moisture resistance
An automatic generation of the calculation grid in connection with moisture calculations takes place when more materials are located near the internal surface and/or when a vapour barrier or layer of paint occurs before or between the material layers.
- By moving from one material to another in the order of material layers, the automatic subdivision of control volumes continues, but with a new development in the layer thickness. When the border zone between layer I and II is analysed, and if sI is the thickness the first control volume after the layer border would have had if material I had continued, the thickness of the first control volume in material II is:
\[
S_{II} = \frac{s_{36.7 \%, II}}{s_{36.7 \%, I}} \cdot s_I \tag{1}
\]
where s36.7 %, II and s36.7 %, I are the effective moisture penetration depths of materials II and I. For example, mineral wool behind a gypsum board will have much larger control volume thicknesses than normal.
- For materials behind a "single moisture resistance", e.g. layers of paint on the surface facing the indoor environment or a vapour barrier between two material layers in the construction, the control volumes do not need to be as slender as otherwise. The following layer thickness is used for the first control volume behind a single moisture resistance:
\[ s_{II} = first \cdot Z_{VR} \cdot \delta_{II} \tag{2} \]
- where sII is the layer thickness of the first control volume after the moisture resistance.
- first is the part of the effective moisture penetration depth normally attached to the first control volume according to the previous definitions in the automatic net generation algorithm.
- ZVR is the moisture resistance (of paints, vapour barriers or similar), Pa·m²·s/kg
- δII is the vaopur permability of the layer behind the moisture resistance, kg/(m·s·Pa)
The thickness of the control volume can thus be scaled up as usual according to the selected "Factor".
sII can be equal to the maximum calculated size or the value found in paragraph 1 (if there is another material in front of the vapour barrier).
Finally, the thickness of the control volume must be equal to the minimum of the above values and the value normally used in tsbi5 according to the desired "Layer thickness" on the Options tab of tsbi5.