I see a black wall, and I want it painted white

October 20, 2008 2:23 PM

Can I seal the inside surface of an exterior brick wall in my house without trapping moisture in the brick or in the insulation?

I own a 1920s brick house in philly w/ some fire damage. The sellers didn't disclose the damage, so I didn't learn of it until the dog days of summer when the place started to stink of smoke. The fire started in the first floor kitchen and smoked the two rooms immediately above it. I tore down most of the plaster and lath in the two rooms and sealed the joists and studs with bin shellac.

How do I treat the exterior brick wall? The brick is blackened and has a faint char smell. A local restoration contractor said he would paint over the brick, frame out and insulate the walls, and omit a vapor a barrier on the warm side of the wall. That way, the paint would double as a vapor barrier. I'm worried condensation would collect against the painted brick during the winter and grow mold. Is this a valid concern? Instead, could I leave the brick untouched, and try sealing the odor by shellac'in the drywall? That way, the vapor barrier would be on the correct side of the insulation and any odor would be trapped within the wall cavity. OR, should I just pay to have the brick soda-blasted?

Just FYI, I sent the sellers a demand letter and am waiting their response before taking them to small claims court.

posted by malp to Home & Garden (4 answers total)

Best answer: The vapor barrier should always, always, go right behind the sheetrock. If you omit it, what happens is that vapor from inside travels through the wall, through the insulation, until it hits a point where the temperature is cool enough for it to condense. That could be on the brick surface, but it could also be in the middle of the insulation. In any case, you'd get water in there, which would cause mold, etc. If the odor is pretty faint, I think you're safe to just frame out the wall, insulate, staple a continuous plastic sheet as vapor barrier, then sheetrock. You could, before building the wall, spray paint the wall for good measure. Normal paint will not act as vapor barrier, so that would cause no issues. But with a continuous plastic sheet behind the sheetrock you should have no problem anyway. To get a pro's opinion, you might check with a fire remediation outfit.
posted by beagle at 2:40 PM on October 20, 2008

I think you're right. The vapour barrier is on the warm side to stop moisture in the air from migrating to the cold wall and condensing, causing mold.

I wouldn't hold out hope that you can lock an odor into the wall cavity because pressure differentials that could occur for any reason are pretty much irresistible, there's no way to completely stop air from moving when it needs to.

I have seen cathedral roof cavities and basements insulated with expanding spray foam that bonds to the inside of an exterior wall (or roof) and it's my understanding that you don't need a vapour barrier in this case. You might want to wait for warm enough weather that you can keep the winows open until it cures.
posted by bonobothegreat at 2:43 PM on October 20, 2008

Beagle's advice is solid. I would take exception to one thing in there though: "Normal paint will not act as vapor barrier, so that would cause no issues." Standard latex paint is actually considered a semi-permeable vapor barrier. In more moderate climates (like here in Oregon) it is all you need in some situations. (not so in Philly, but still -- be aware that a layer of paint at the back of the brick WILL trap some vapor, which might lead to problems when combined with a "real" vapor barrier elsewhere in the same wall assembly)
posted by misterbrandt at 10:14 PM on October 20, 2008

Response by poster: I paid $450 to have the brick soda blasted clean. The blasting left powdered baking soda all over the house but seemingly cleaned the brick perfectly. I spoke to several fire restoration contractors. Every one said they would paint over the brick if hired.
posted by malp at 6:27 AM on February 24, 2009