Someone sealed the front of my house in East Rock in the mid-nineties, before I bought it. I know this because there is a crisp line four courses above grade where the treatment stopped, and above that line the brick faces have been coming off in flakes for thirty years while the untreated brick below is fine.
Why a wall that cannot dry fails faster
A solid masonry wall manages water by absorbing it and releasing it. It gets wet when it rains and dries out over the following days, mostly through the mortar joints, which are the most permeable path.
Water still gets in whatever you put on the outside. It comes up from grade, in through hairline cracks, around windows, down from a blocked gutter, and as vapor from inside the house in winter. A coating that keeps water from leaving does not keep water from arriving.
So the wall stays wetter for longer. Higher saturation means more freeze-thaw damage in a climate with 40 to 60 cycles a winter, and it means salt crystallization happens below the coating rather than harmlessly on the surface — which is why the failure mode is the brick face lifting off in sheets rather than a bit of white powder you could brush away. The mechanism is the same one described in reading a salt bloom, just driven inward.
The coatings, from worst to least bad
- Elastomeric wall coatings. Thick, film-forming, marketed as permanent waterproofing. On old brick they are the most damaging option available, and removing them is expensive and often impossible without damaging the face.
- Masonry paint. Slightly better because it is thinner, but it still forms a film, and painting previously unpainted masonry is irreversible in practice and usually refused in historic districts.
- Acrylic sealers. Form a film, yellow with UV, and trap moisture. Common on driveways, wrong on walls.
- Siloxane and silane water repellents. Penetrating, breathable, do not form a film. These are the only category with legitimate uses on masonry walls, and even then narrowly.
When a repellent is actually justified
Three cases, and all of them assume the wall is otherwise sound and dry:
Driving rain exposure on a solid wall. A west-facing wall in an exposed position that saturates repeatedly can benefit from a breathable repellent, applied after any pointing is complete and cured.
Horizontal surfaces. Copings, sills and cap stones take standing water and are worth treating, because water sitting on a horizontal surface is a different problem from water hitting a vertical one.
Below-grade exterior faces, where the coating is a proper damp-proofing membrane behind backfill, not a surface treatment on a visible wall.
What all three have in common: the wall’s actual water sources have been fixed first. A repellent applied to a wall that is wet from a leaking gutter simply traps that water. The research literature on moisture in masonry assemblies published through NIST is the technical grounding for most of this if you want to read further.
If it has already been coated
Assess how much damage is happening. A wall that was coated and is not visibly spalling can be left, with the water sources managed and the coating allowed to weather off over a decade or two. A wall that is actively losing brick faces needs the coating removed, which means chemical strippers and poultices rather than abrasion — $6 to $18 per square foot — followed by repointing in a permeable lime mix and probably some unit replacement.
Budget on a typical two-story front elevation: $3,000 to $9,000 for stripping, plus the repointing. It is the most expensive way to arrive at a wall you could have had for free by doing nothing.
Frequently asked questions
How do I tell if my brick has been sealed? Spray a small area with water. Untreated brick darkens and absorbs within seconds; sealed brick beads or stays light. A visible sheen in raking light, or a hard stop line where treatment ended, confirms it.
My contractor says the sealer is breathable. Is that true? It may be. Ask for the product data sheet and look for a water vapor transmission figure. Silane and siloxane repellents typically retain 80 to 90 percent of the wall’s vapor permeability. Elastomeric coatings do not, whatever the marketing says.
What about the interior side? Painting or sealing the inside face of a solid masonry wall blocks the wall’s other drying path and pushes all the moisture outward through the freeze zone. If a wall is damp, the answer is on the outside and at the gutters, not in a can of interior sealer.