Bubbles: check how deep they go
Sherwin-Williams' guide to blistering gives a simple way to read a blister. If the bubble goes all the way down to bare masonry, the likely cause is moisture escaping from the wall, and it lists damp basements and water seeping in through exterior walls as typical sources. If the bubble only goes down to an older coat of paint, the problem is more likely adhesion between layers, such as painting over a glossy or dirty coat.
You often don't need to break a blister to find out: some have already popped and you can see what's underneath. In a house built before 1978, don't scrape or sand to look (see the lead section below).
Powder: salt, alkali, or worn-out paint
- Efflorescence. Sherwin-Williams' efflorescence page says the salts come from the mortar, block or concrete itself, and the moisture behind them is usually a building defect. Crystals keep growing as long as both salt and water are present. On brick or block, it tends to show up in bands along the mortar joints.
- Alkali burn. The alkali guide explains that masonry and mortar are strongly alkaline, and moisture getting into cured walls can reactivate that alkalinity, causing efflorescence, blistering and peeling. It says alkyd or oil paint on a "hot" surface can fail within six months.
- Chalking. A fine, even powder that rubs off like chalk, with no crust or crystals, is more likely the paint wearing out. The chalking page links it to low-grade, heavily pigmented paint, over-thinned paint, and porous surfaces that weren't sealed.
The difference matters because chalking is a paint problem, while efflorescence and alkali damage are water problems. If you're not sure whether the white growth is salt or mold, our guide on efflorescence versus mold on masonry walks through the differences.
A foil test: is the water coming from the wall or the air?
The National Park Service's Preservation Brief 39 describes a quick check for damp basements. Tape a square of aluminum foil to the masonry, on a sound spot rather than flaking paint, and look at it after a day or two. Moisture on the wall side of the foil means water is coming through the masonry. Condensation on the room side means the moisture is coming from the air.
If it's the air, humidity is the lever. EPA's mold and moisture guide recommends keeping indoor relative humidity below 60 percent, ideally 30 to 50 percent, and says a humidity meter costs about $10 to $50 at hardware stores.
Fix the water before the paint
Preservation Brief 39 says handling roof runoff is one of the most important ways to keep ground moisture out, and estimates a heavy 2-inch-per-hour rain can send 200 gallons out of a house's downspouts in an hour. Before you price waterproofing, check that gutters are clear, downspouts aren't leaking or dumping water against the foundation, and the ground slopes away from the house. The brief recommends those steps before excavation or exterior dampproofing.
In Philadelphia, the Water Department's Rain Check program installs a rain barrel at a home for free and a metal downspout planter for $100 after a one-hour workshop. That can help when one downspout empties right beside a basement wall.
Be wary of a quote that only seals the inside. The NPS brief warns that waterproof coatings such as cement parging applied to the inside of a damp wall can push the moisture, and the damage, higher up the wall. Sherwin-Williams also says to delay painting while efflorescence keeps coming back. If water is actively entering, documenting it before you get repair quotes helps you compare what each contractor proposes.
Two safety checks before anyone scrapes
EPA says a home built before 1978 may have lead-based paint, and its lead-safe renovation page recommends hiring a lead-safe certified contractor for repair or painting work in those homes. Basement walls in older houses often have many layers. Read what to check before sanding or scraping paint in a pre-1978 home before any prep starts.
If you see mold, EPA's guide says not to paint or caulk over it: clean it up and dry the surface first, because paint over mold is likely to peel.
What to ask a contractor
- Did the blisters go to bare masonry or to an older coat, and what does that tell you?
- Where do you think the water is coming from, and how did you check?
- What will you fix outside, at the gutters, downspouts or grading, before any coating goes on?
- Is this house pre-1978, and is your crew lead-safe certified?
- What primer and paint will you use on masonry, and how long will you wait for the wall to dry?