Q. I think I have efflorescence on the concrete walls of my basement. My house is 90 years old with a poured-concrete foundation. The surface is blistering and peeling, and I see small white deposits. I’ve been scraping the loose concrete (sometimes going as much as ¼ of an inch deep) and skim coating with mortar or thin set, but then the blistering migrates to the perimeter of the repaired area. I should mention that the previous owners painted all of the walls in the basement 10 to 15 years ago. Is the paint causing the blistering?
A. You have blistering because moisture in the concrete is causing the paint to lose adhesion. Efflor- escence is a crystalline or powdery salt deposit that forms when water is present on masonry surfaces. Rain, snowmelt, and ground water are the primary sources of moisture. It is often a seasonal problem; humidity will affect whether soluble salts appear.
I’ve removed the powder using muriatic acid in a mild solution (one part acid to 12 parts water), but it doesn’t solve the problem. The only way to get rid of it permanently is to remove the moisture in the walls. Look at your exterior grade slope, ensure that your gutters are not clogged and that they are draining well away from your foundation walls, and check for leaking or sweating pipes in the area.
Q. In an article on icicle prevention (Ask the Carpenter: Advice on freezing pipes and icicle prevention’’ March 18), you talk about leaving gable vents in place. Does this apply even if you have a ridge vent? Our home was built in the late ’70s. I had a ridge vent installed in the ’90s when the roof was reshingled. At that time the contractor said we needed to block off the gable vents so air would flow up and out through the ridge vent. He simply covered the inside of the gable vents with thick plastic. We have installed metal soffit vents in the eaves between every other rafter, as well as polystyrene foam vents between every rafter. The attic has 6 inches of insulation. We still get ice dams several times each winter. Would adding more ventilation in the eaves and removing the covering on the gable vents help?
JIM, Londonderry, N.H.
A. I’ve always thought that the combination of a ridge vent and continuous soffit vents gives you good air flow from low to high, which helps reduce heat and moisture buildup that can lead to ice damming, roof deterioration, and mold infestation. The whole idea is to keep the attic as close to the outside temperature as possible and get any accumulated warm air out. In the end, more venting is usually better than less. Adding to or increasing the size of your soffit vents is a great idea, as is taking the plastic off your gable vents.
You definitely need more insulation than 6 inches, but first I would seal all of the air leaks into your attic. Many people think that insulation alone does the job effectively, but in order to improve the energy-efficiency of your insulation, you need to seal air leaks. Insulation works best when air is not moving through or around it.
Sealing air leaks is not hard; you just need to know where to look. Focus on all penetrations from the attic to the floors below. These include, but are not limited to, wiring, pipes, and flues; chimneys, soffits, and other blocked framing areas; stairway slants; and between studs on balloon-framed homes.