Consistent increase in water content of walls

[World2Build]

I am running my first couple walls through WUFI, but I am seeing a consistent trend of increasing water content over time on wall assemblies that regularly perform without problems in practice. It leads me to believe that I have input a setting or parameter wrong, but I am unable to determine what could be causing this same result in all the wall constructions I analyze. Would it be possible for someone to validate that the construction I have listed below should be increasing in water content based on my parameters? Or could someone provide a sample wall construction and related parameters that has validated results for me to recreate. This would greatly boost my confidence in the results provided by WUFI.

Wall Construction: From North American Generic Database
Red Matt Clay Brick – 3-5/8”
Air Space 40mm – 1-1/2”
Extruded Polystyrene Insulation – 2-1/2”
Vapor Retarder (.1 perm) – standard thickness from database
Concrete Brick – 5-5/8”

Orientation:
North-East because of primary driving rain direction
90 degree inclination
Short building, height up to 33ft

Surface Transfer Coeff:
Wind Dependent
Red Brick for Short-wave radiation absorptivity
Interior permeance of 20 for high perm paint

Initial Conditions:
Not changed

Calculation period:
3 years

Numerics:
Not changed

Climate:
Exterior (Left side) is set to Chicago, IL
Interior conditions (Right side) is from ASHRAE 160 with air-conditioning, 1 bedroom, and standard construction, building volume of 112,000

[Daniel]

What time period did you use for the simulation?

In this type of wall with wind driven rain load you can reach an high equilibrium water content but there shouldn't be any more increase after few years.

What is the final moisture level at the end of the simulation in the Brick - and to which RH is it corresponding? Initial moisture is 80 % as i assume?

Daniel

[World2Build]

Daniel,

Thank you for replying to the posting. You are right in assuming the initial Relative Humidity is set to 80%. Also, the initial moisture contents of the materials have not been changed from their default values.

The simulation was run for both 3 years and 10 years, but in both cases a dynamic steady state was not achieved for the total moisture content of the assembly. Instead there is a cyclical increase each year.

3 year results for moisture content
Total: Start=1.57, End=1.98 (Increased)
Exterior brick: Start=.14, End=.1 (Decreased)
Interior Concrete Block: Start=3.2, End=4.13 (Increased)

10 year results for moisture content
Total: Start=1.57, End=2.36 (Increased)
Exterior brick: Start=.14, End=.1 (Decreased)
Interior Concrete Block: Start=3.2, End=4.96 (Increased)

Since the exterior material dries, but the interior does not then it appears that my interior conditions are the problem. Since this is just a sample wall without know values from a mechanical engineer, I have used the ASHRAE Stnd 160 values.

Use left climate is selected. My AC type automatically populates the set points and float. I have reduced the bedrooms to the minimum of 1 since it would commercial. And selected standard construction with a bldg. volume of 112,000.

Experience tells me that I would not have 100% RH at the inside face of the wall from November to May and that the Concrete block would not continue to rise in MC. Is there any modifications you could suggest to the interior climate that would make it more accurate for my climate?

[Daniel]

The problem could be the vapor retarding polystyrene outside the concrete brick which doesn't allow a dry out of the moisture to the outside... but i don't understand the supersaturated water contents...

Can you please send me the WUFI file without results (due to the file size) to support@wufi.com ? I think it is the fastest way to check the file.

best regards
Daniel

[Axelarris]

Was there a resolution to this? I have gotten similar results using XPS on wood framed wall assemblies in Detroit. The issue goes away when I remove the ASHRAE 160 1% driving rain at the inner-most elements of the XPS.