I read the previous topics on paint coatings and I have a parallel question.
First, can a paint coating on the interior 8" wythe (4" CMU, 1" air, 2" insulation, & 8" CMU assembly) greatly effect the water or vapor transport capabilities of the wall section?
Second, I am not sure, at this time, of the exact characteristics of the filler and paint that was used to caot the interior 8" wythe but if the paint's perm rating was extremely low (not allowing moisture transfer) what effect would this have on the 4" exterior unit? (Would it not dry as rapidly or possible retain moisture?)
C. Furtaw
Paint Coatings on Interior Wall
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C. Furtaw
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Paint Coatings on Interior Wall
Charles E. Furtaw, P.E.
Re: Paint Coatings on Interior Wall
Dear Mr. Furtaw,
the effect a paint coat can have on a wall depends on the diffusion properties of the particular type of paint. Ordinary mineral paints have a diffusion resistance which corresponds to a few millimeters of gypsum plaster. Usually, the performance of a wall will not be compromised if the plaster is applied a few millimeters thicker than planned, so such a permeable paint coat will not have any effect either.
On the other hand, impermeable paint coats, such as oil paint, may indeed have an effect, but it is difficult to give a general assessment.
An oil paint coat may be beneficial if it prevents condensation in the wall that might otherwise be caused by water vapor migrating into the wall by diffusion. An oil paint coat may be detrimental if it prevents moisture in the wall (caused driving rain, rising damp, condensation, built-in construction moisture...) from drying out.
So it depends on whether in the specific situation there is some possible harmful moisture load acting on the wall that can be either caused or prevented by an impermeable paint coat.
If your wall in question contains a ventilated air gap, the drying potential of the wall is probably less likely to be adversely affected by an impermeable interior paint coat, but one could certainly think up scenarios where such a paint coat can make a difference (what if moisture happens to be ventilated into the air gap for some reason?).
Any effect on the exterior wall leaf should be very small under almost all circumstances, however.
Regards,
Thomas
the effect a paint coat can have on a wall depends on the diffusion properties of the particular type of paint. Ordinary mineral paints have a diffusion resistance which corresponds to a few millimeters of gypsum plaster. Usually, the performance of a wall will not be compromised if the plaster is applied a few millimeters thicker than planned, so such a permeable paint coat will not have any effect either.
On the other hand, impermeable paint coats, such as oil paint, may indeed have an effect, but it is difficult to give a general assessment.
An oil paint coat may be beneficial if it prevents condensation in the wall that might otherwise be caused by water vapor migrating into the wall by diffusion. An oil paint coat may be detrimental if it prevents moisture in the wall (caused driving rain, rising damp, condensation, built-in construction moisture...) from drying out.
So it depends on whether in the specific situation there is some possible harmful moisture load acting on the wall that can be either caused or prevented by an impermeable paint coat.
If your wall in question contains a ventilated air gap, the drying potential of the wall is probably less likely to be adversely affected by an impermeable interior paint coat, but one could certainly think up scenarios where such a paint coat can make a difference (what if moisture happens to be ventilated into the air gap for some reason?).
Any effect on the exterior wall leaf should be very small under almost all circumstances, however.
Regards,
Thomas
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C. Furtaw
- WUFI User
- Posts: 85
- Joined: Tue Jul 19, 2005 2:04 am -1100
- Location: Malvern, Pennsylvania
- Contact:
Thomas,
I received this response from the paint manufacturer:
" A 2 coat system 10 mils dft (total dft) perm rating is 24.00 +/- 1.53 perms. "
The structure is built on or next to a very wet area. From your response could water be drawn into the 1 1/2"ventilated air gap and settle into the outer wythe?
For some reason the exterior face unit of the wall assembly is retaining water and you may have suggested one scenario in your response.
Do you have any further ideas/suggestions with the new data above? The building is more than 5 years old.
Charles
I received this response from the paint manufacturer:
" A 2 coat system 10 mils dft (total dft) perm rating is 24.00 +/- 1.53 perms. "
The structure is built on or next to a very wet area. From your response could water be drawn into the 1 1/2"ventilated air gap and settle into the outer wythe?
For some reason the exterior face unit of the wall assembly is retaining water and you may have suggested one scenario in your response.
Do you have any further ideas/suggestions with the new data above? The building is more than 5 years old.
Charles
Charles E. Furtaw, P.E.
24 perms are approximately the perm rating of an ordinary layer of gypsum plaster, and thus completely harmless under almost all circumstances. So it seems the paint coat is not to blame.C. Furtaw wrote:" A 2 coat system 10 mils dft (total dft) perm rating is 24.00 +/- 1.53 perms. "
Possible causes for increased moisture in your wall will depend on the local surroundings, the local weather, the construction details and the workmanship quality of the wall, so remote diagnosis will be difficult. Maybe rain is intruding where it shouldn't, maybe it's a case of rising damp, maybe the wall is in a warm and humid climate and air-conditioned indoor conditions make the damp outdoor air condense in the air gap, etc. You'll have to consider all of the individual local circumstances to find the cause for the problem.
Regards,
Thomas