Dear Thomas and the User Community,
I am modeling wall sections in North America and my company IBACOS would like to compare the performance of a latex vs. an oil based paint applied to interior drywall, though oil based paint is not in the library database nor as a "drop-down" selection in interior/exterior wall coating.
I ask, how do you model an interior oil base paint (~2-4mils)? I can find neither the values to enter a custom material type nor am I confident that a film should be entered as would drywall or sheating. Doesn't a film work differently?
As a temporary solution, I have taken the values for latext paint and modified the perm value from .156 perm inch (latex) to 0.56 perm inch (oil based). Is a higher permeability consistent with your experience?
Kindly,
Parker Snyder
IBACOS
tel: 001.412.325.1827
Paint Coatings
-
parkersnyder
- WUFI User
- Posts: 2
- Joined: Fri Jul 28, 2006 2:13 am -1100
RE: Paint Coatings
I came up with these values:
Latex: permeance= 3.24, perm-inches=0.162, thickness=0.05 (2 mils)
Oil: permeance= 0.56, perm-inches=0.028, thickness=0.05 (2mils)
The conclusion is that oil based paint is less permeable (more like pastic than paper).
Concur?
Kindly,
Parker Snyder
IBACOS
001.412.325.1827
Latex: permeance= 3.24, perm-inches=0.162, thickness=0.05 (2 mils)
Oil: permeance= 0.56, perm-inches=0.028, thickness=0.05 (2mils)
The conclusion is that oil based paint is less permeable (more like pastic than paper).
Concur?
Kindly,
Parker Snyder
IBACOS
001.412.325.1827
Re: Paint Coatings
Dear Mr. Snyder,
I cannot currently comment on any specific values for different types of paint, as I am on vacation and my resources at home are very limited. I would expect any oil-based paint to be much less permeable than latex paint, but I am no expert on this, and I can't look it up right now.
As regards your numbers: if the "mil" is one thousandth of an inch, then the conversion from permeance to permeability for 2 mils thick paint layers should be done like this (assuming your values for the permeances):
Latex: permeance = 3.24 perm; permeability = 3.24 perm *0.002 inch = 0.0065 perm inch
Oil: permeance = 0.56 perm; permeability = 0.56 perm * 0.002 inch = 0.0011 perm inch.
Entering the paint layer as an explicit layer in the construction or just entering its permeance in the "surface transfer coefficients" dialog should give the same results, as long as the layer's vapor diffusion resistance is its only property that matters in the given situation. For your paint layers this condition is met, because the thermal resistance and the thermal capacity of thin paint layers are negligible, and in an indoor environment there is no rain or condensation, so that the liquid transport coefficients and the moisture capacity of the material are irrelevant, too
Disadvantages of using explicit layers are
-that you need to have at least the basic material data, including thermal conductivity, thermal capacity, bulk density, porosity (entering any values not too wrong will work as they do not matter in this situation - see above - but it's still some work to type in some numbers),
-and that very thin layers imply very small numerical grid elements, so that neighboring layers also need many small grid elements to avoid large jumps in the thicknesses of adjacent element widths at the layer interfaces, increasing computation time.
So in general, it's best to include such thin layers only in the form of their permeance in the "surface transfer coefficients" dialog.
Regards,
Thomas
I cannot currently comment on any specific values for different types of paint, as I am on vacation and my resources at home are very limited. I would expect any oil-based paint to be much less permeable than latex paint, but I am no expert on this, and I can't look it up right now.
As regards your numbers: if the "mil" is one thousandth of an inch, then the conversion from permeance to permeability for 2 mils thick paint layers should be done like this (assuming your values for the permeances):
Latex: permeance = 3.24 perm; permeability = 3.24 perm *0.002 inch = 0.0065 perm inch
Oil: permeance = 0.56 perm; permeability = 0.56 perm * 0.002 inch = 0.0011 perm inch.
Entering the paint layer as an explicit layer in the construction or just entering its permeance in the "surface transfer coefficients" dialog should give the same results, as long as the layer's vapor diffusion resistance is its only property that matters in the given situation. For your paint layers this condition is met, because the thermal resistance and the thermal capacity of thin paint layers are negligible, and in an indoor environment there is no rain or condensation, so that the liquid transport coefficients and the moisture capacity of the material are irrelevant, too
Disadvantages of using explicit layers are
-that you need to have at least the basic material data, including thermal conductivity, thermal capacity, bulk density, porosity (entering any values not too wrong will work as they do not matter in this situation - see above - but it's still some work to type in some numbers),
-and that very thin layers imply very small numerical grid elements, so that neighboring layers also need many small grid elements to avoid large jumps in the thicknesses of adjacent element widths at the layer interfaces, increasing computation time.
So in general, it's best to include such thin layers only in the form of their permeance in the "surface transfer coefficients" dialog.
Regards,
Thomas