Why do I get completely different results when using separate layers for exterior and interior paint coats (5 perm each with thickness of .003") compared to setting coatings of 5 perm under the Surface Transfer Coefficient section?
I get excellent results (no water accumulation) when the separate layers are added to represent the latex paint coats and setting "no coating" under the Surface Transfer Coefficient Section; however, I get serious water accumulation when using surface coatings under the Surface Transfer Coefficient Section.
Here is my wall configuration from left to right:
1. vapor retarder (5 perm) 0.003" thick used to represent several coats of latex primer and paint needed to protect the composite wood siding.
2. Composite wood siding (7/16" thick)
3. #15 Felt
4. OSB (7/16" thick)
5. Extruded Polystyrene Insulation (1" thick installed on interior wall between 2X4 wall studs, caulked around perimeter)
6. Roxul Insulation R-15 3.5" thick
7. PE-Membrane (Poly; 0.07 Perm) 0.006" thick
8. Gypsum Board (5/8" thick)
9. Vapor retarder (5 perm) used to represent latex primer.
Climate setting is Cleveland, Ohio (USA)
Indoor Conditions set to Sine Curves with High Moisture Load
Again, if I remove layers 1 and 9, and set the coatings to 5 perm under the Surface Transfer Coefficient Section, I get water accumulation in the OSB and Composite Wood Siding exceeding 10 lb/ft^3, which is close to complete saturation.
With layers 1 and 9 in place, the water accumulation in the OSB and Composite Wood Siding drops to 4 lb/ft^3
Which simulation is more accurate?
I'm using the ORNL Version 5.1, so I can't try changing many parameters.
Transfer Coefficients vs Separate Layers
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Don Anthan
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Christian Bludau
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Re: Transfer Coefficients vs Separate Layers
Christian, can you please share your response on the forum? I would like to know the appropriate application of surface transfer coefficients.
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Christian Bludau
- WUFI SupportTeam IBP
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Re: Transfer Coefficients vs Separate Layers
Hello Axelarris,
there should be only small differences by using the external sd-value instead of a layer. We had this discussion here:
http://www.wufi-forum.com/WUFIForum/vie ... f=15&t=794
In this case i do not remember the solution any more, but a mistake often made is to use the sd-value in the boundary conditions together with rain. This sd-value does not affect the rain wicking, if surface material allows. So what happens if you use this sd-value in the boundary conditions with rain is, that the rain can hit the wall and is sucked in. The drying is working only by diffusion and for that the sd-value is a barrier. So you construction will suck up.
Contrary to that the layer (maybe water-retarder) has no coefficients for suction, so rain will not be absorbed.
Christian
there should be only small differences by using the external sd-value instead of a layer. We had this discussion here:
http://www.wufi-forum.com/WUFIForum/vie ... f=15&t=794
In this case i do not remember the solution any more, but a mistake often made is to use the sd-value in the boundary conditions together with rain. This sd-value does not affect the rain wicking, if surface material allows. So what happens if you use this sd-value in the boundary conditions with rain is, that the rain can hit the wall and is sucked in. The drying is working only by diffusion and for that the sd-value is a barrier. So you construction will suck up.
Contrary to that the layer (maybe water-retarder) has no coefficients for suction, so rain will not be absorbed.
Christian