Is emulsion paint waterproof?
henryparr
| Posted in General Questions on
We have peeling paint on a chimney breast in a bathroom. It’s ordinary emulsion paint. Some peole say I need special waterproof paint to cure the problem I think I need special breathable paint to let the moisture flow in and out of the wall without knocking the paint off. I can’t find a clear answer as to whether normal emulsion paint is vapour proof. I imagine it depends a bit on the type. We used vinyl silk as its a bathroom, I imagine this is less permeable than matt. I get the general impression that emulsion paint is a bit permeable, not fully ‘breathable’ but not fully ‘vapour proof’. (a perm value of around 1!) Is this right?
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Replies
Henry,
No one has yet invented a waterproof paint.
All paints are vapor-permeable. However, the permeance of dried paint varies depending on the type of paint and the number of coats.
According to one source, one coat of primer plus two coats of latex paint (on gypsum drywall) has a permeance of about 19 perms.
Another source measured a brand of primer that had a permeance of 8 perms.
Vapor-retarder paint (on gypsum drywall) has a permeance of 0.6 to 0.9 perm.
When installed on rough surfaces, the permeance of a coat of vapor-retarder paint will be much higher than when the same paint is installed on gypsum drywall.
Paint failure in a bathroom is almost always due to condensation on the paint surface.
The solution for that is to insulate the problem area. XPS foam works well and looks good painted , but fire codes require it to be covered with drywall.
An exterior grade paint will survive longer if you don't insulate it.