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Brick Houses and Vapor Barriers

William Kibbel III, The Home Inspector

Dear Home Inspector: The previous owner stripped the plaster from the interior walls of our brick home. We will be studding out the walls for insulation, outlets and wiring. We are concerned about moisture seeping through the bricks. Should we cover apply plastic sheeting to the inside of the brick walls?

Since you're adding insulation, I'll assume that you're in a climate that has cold weather part of the year and that you will be heating the building. In these climates, the vapor barrier of faced insulation is installed toward the heated space.

Water vapor migrates from warm to cold. Daily living activities -- showers, clothes and dish washing, even breathing -- generate a tremendous amount of moisture inside a home. Damp basements and crawlspaces, typical in older homes, surface water evaporation (from fish tanks, toilets, etc.), and furnace humidifiers also contribute to the moisture in a home.

This moisture, now a vapor, is trying to get through the walls to the exterior. If you put plastic, a vapor barrier, on the brick wall, significant amounts of moisture will condense on the inside of the plastic. This can cause two problems:

  • Trapped water can saturate your insulation, and
  • it can result in enough trapped moisture to create a thriving environment for that four letter word that starts with "M" (mold).

The major moisture issues I find with old brick walls are usually a result of the application of some type of exterior coating or sealer. These coatings were originally intended to preserve, rejuvenate or restore the exterior masonry.

Peeling paint is a symptom of moisture trapped between the brick and the exterior paint coating.

Think of an old brick wall as a big hard sponge -- it absorbs moisture to some degree, but then releases it quickly. Products like stucco, portland cement, oil-based paints and miracle "sealers" tend to trap moisture. This eventually results in major failure of the individual bricks and mortar that make up the wall system.

Old brick buildings always allowed water vapor to migrate to the exterior -- until we started messing with them in the latter part of the last century. Now, thanks to improper installation of vapor barriers and application of exterior sealers, stuff is growing inside old walls. Outside, bricks and mortar are turning to powder.

In many older homes, plaster was applied directly to the interior surface of the bricks. If moisture migration to the interior were a major issue, most plaster would be perpetually damp and efflorescence would be "growing" on the plaster. Most old brick buildings have wood joists that pocket into the walls. Again, if excessive moisture were entering, I would expect to see some serious issues with the ends of the joists rotting. But I rarely see such moisture problems in old brick homes in original condition.

If you are still concerned about moisture from the outside, you could consider a weather-resistant barrier that prevents water intrusion to some degree. A vapor-permeable barrier would still allow water vapor to migrate to the exterior.

By the way, folks much smarter than me say that plain old building felt (a.k.a. tarpaper) is just as good as, if not better than the expensive "house wrap" products. Felt is an excellent weather barrier and is even better at allowing vapor to escape.

I would only consider installing a vapor barrier on the inside of a brick wall if the building were in the deep south. In climates with high heat and humidity, air conditioning is running more often than not. This means cool interior and a hot, humid, exterior walls. But even in this situation, a ventilated cavity between the brick and the plastic, as well as pathway for water to drain to the exterior need to be provided.

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