Wall Assembly with multiple metal layers - Initial RH question
Posted: Wed Apr 29, 2026 5:11 am -1100
Hi All,
I'm trying to study a wall assembly that includes a metal faced SIP panel but would appreciate some guidance on the initial conditions. The wall is for an offsite fabricated system, constructed in a factory. The wall assembly is (from inside to outside):
15mm Plasterboard
12mm Plywood
VCL (Sd=50)
90mm mineral wool (with metal stud in the actual condition) - (various options breaking this down into 3 layers of 1/88/1mm for example)
1mm steel sheet (inner face of SIP panel - actual thickness is 0.5mm)
78mm PIR @ 0.023 W/m.K (various options include breaking this down into 3 layers of 1/76/1mm for example) [SIP insulation]
1mm steel sheet. [outer face of SIP panel actual thickness is 0.5mm]
Vented rainscreen cavity
Timber / other rainscreen outer layer
With 80% RH Initial conditions
I have tried various configurations of the geometry, including omitting the outer timber and cavity and replacing the outermost steel layer with an sd-value (and turning off the rain). I've also tried the same approach to omit the VCL layer internally. However, For a 5 year simulation, there are always convergence errors and B1 and B2 values that are not sufficiently similar. Eg -0.32 and -0.419 kg/m2 respectively.
With 70% RH initial conditions:
There are zero convergence failures and B1 / B2 values of -0.118 / -0.119 kg/m2 respectively. This would appear to be a satisfactory numerical result.
The question therefore, is how do I evaluate whether it is acceptable to start with the 70% RH initial conditions throughout. In both cases, the total water content in the assembly is lower at the end of the first five years than at the start, suggesting that the initial RH is not too low:
70% RH starts at 1.68 kg/m2, ends at 1.58kg/m2 - reduced by 0.1kg/m2
80% RH starts at 1.99kg/m2, ends at 1.68 kg/m2 - reduced by 0.32kg/m2.
All comments and observations are gratefully received. Thank you.
I'm trying to study a wall assembly that includes a metal faced SIP panel but would appreciate some guidance on the initial conditions. The wall is for an offsite fabricated system, constructed in a factory. The wall assembly is (from inside to outside):
15mm Plasterboard
12mm Plywood
VCL (Sd=50)
90mm mineral wool (with metal stud in the actual condition) - (various options breaking this down into 3 layers of 1/88/1mm for example)
1mm steel sheet (inner face of SIP panel - actual thickness is 0.5mm)
78mm PIR @ 0.023 W/m.K (various options include breaking this down into 3 layers of 1/76/1mm for example) [SIP insulation]
1mm steel sheet. [outer face of SIP panel actual thickness is 0.5mm]
Vented rainscreen cavity
Timber / other rainscreen outer layer
With 80% RH Initial conditions
I have tried various configurations of the geometry, including omitting the outer timber and cavity and replacing the outermost steel layer with an sd-value (and turning off the rain). I've also tried the same approach to omit the VCL layer internally. However, For a 5 year simulation, there are always convergence errors and B1 and B2 values that are not sufficiently similar. Eg -0.32 and -0.419 kg/m2 respectively.
With 70% RH initial conditions:
There are zero convergence failures and B1 / B2 values of -0.118 / -0.119 kg/m2 respectively. This would appear to be a satisfactory numerical result.
The question therefore, is how do I evaluate whether it is acceptable to start with the 70% RH initial conditions throughout. In both cases, the total water content in the assembly is lower at the end of the first five years than at the start, suggesting that the initial RH is not too low:
70% RH starts at 1.68 kg/m2, ends at 1.58kg/m2 - reduced by 0.1kg/m2
80% RH starts at 1.99kg/m2, ends at 1.68 kg/m2 - reduced by 0.32kg/m2.
All comments and observations are gratefully received. Thank you.