Why sealing the wall will not stop the leak.

When a wall shows damp, the instinct is to seal it. It sounds right — keep the water out. But on a masonry or stucco building, a sealer applied over a wall that is already wet, or that is taking water in from somewhere else, can make the problem considerably worse. This is well documented, and the clearest statement of it comes from the National Park Service.

First, two different products

The words get used interchangeably and they should not be. A water-repellent coating is intended to stop liquid water while still letting water vapour move through. A waterproof coating seals against both. On a wall that needs to dry outward, that difference decides whether you have helped or done harm.

What actually causes the damp

Before reaching for either, it is worth knowing what the Park Service says causes wall water penetration in the first place:

Water penetration … results from poor or deferred maintenance. Leaking roofs, clogged or deteriorated gutters and downspouts, missing mortar, or cracks … are almost always the cause.

NPS Preservation Brief 1, Mack and Grimmer

That list is worth reading slowly, because every item on it is a repair, not a coating. A blocked downspout dumping against a wall does not get fixed by sealing the wall. Neither does an open mortar joint or a failed flashing.

And why sealing can make it worse

The same brief is direct about what happens when a coating goes over a wall that is taking water in from another direction:

If water does enter the wall from the ground or from the inside of a building, the coating can intensify the damage because the water will not be able to escape.

NPS Preservation Brief 1

Most historic masonry buildings, unless they are painted, have survived for decades without a water-repellent coating and, thus, probably do not need one now.

NPS Preservation Brief 1

The same trap, in mortar

There is a parallel version of this mistake in repointing. Using a hard, modern, cement-rich mortar in an old soft-brick wall feels like an upgrade. It is not:

A mortar that is stronger in compressive strength than the masonry units, will not ‘give,’ thus causing the stresses to be relieved through the masonry units — resulting in permanent damage.

NPS Preservation Brief 2, Mack and Speweik, 1998

If the mortar does not permit moisture or moisture vapor to migrate out of the wall and evaporate, the result will be damage to the masonry units.

NPS Preservation Brief 2

The rule in both cases is the same: the repair should be softer and more permeable than what it is going into, not harder and tighter. Done properly, the Park Service reckons repointing should last “at least 30 years, and preferably 50–100 years.”

Where leaks usually turn out to be

In practice, envelope leaks concentrate at edges and transitions rather than in the middle of a wall — roof-to-parapet junctions, facade-to-coping connections, window and door perimeters, and anywhere a pipe, vent or fixture penetrates. The building enclosure consultant Michael Kubal, writing in the trade journal of the International Institute of Building Enclosure Consultants, put it as ninety percent of water intrusion happening in about one percent of the exterior surface area. We would note that is his professional estimate rather than a published study — but it matches what we find on job walks.

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