Brick veneer needs space for drainage

Estimator said they couldn't do much with the wall cavities because of the existing insulation, but could drill holes in the mortar joints and fill the cavity between the back of the brick and the sheathing (black Celotex fiberboard) with a foam insulation.

Could this create any sort of moisture problem by filling this air space?

-- J.W., Evansville, Ind.

Answer: There should be a 1/2- to 3/4-inch space between the backside of the brick veneer and the exterior side of the home's sheathing. The main purpose of the space is to allow for the flow of any water that might penetrate the brick veneer.

During severe weather, rainwater or snowmelt can enter through cracks in the bricks' mortar joints and through gaps at window and door openings. If the water is trapped behind the brick, it can lead to decay damage of the home's wood framing. At the base of a brick-veneered wall there should be a metal flashing system designed to divert the captured water to weep holes in the brick veneer.

A weep hole is a gap strategically located between two bricks and should be located just above the finished grade of the yard or landscaping. An additional set of flashings and weep holes should be located above each window and door that is exposed to the weather.

If you fill the drainage plane with an expanding foam, the flow of water would be impeded or totally blocked. Accumulated water will cause decay damage and can support mold growth inside the wall cavities.

If it were my home, I would not fill the void with foam.