Moisture and Structural Masonry

Brief Study of Moisture

Mitigating moisture must be the primary focus for maintaining and renovating historic buildings.
I have a saying: “A historic building must have a good hat, good boots and a raincoat to keep the head, feet and body of the building dry.”
When working on historic solid structural masonry buildings, it is critical to be cognizant and understand moisture in these two areas:

Understanding how Moisture Moves

When initially thinking about how moisture travels, we usually picture water dripping downwards like rain falling or a leaking roof or gutter.

However, water does not always obey the law of gravity. Water also moves upward when in the correct environment. This is defined as capillary action. For example, put a dry cloth in a glass of water. Hang the cloth over the edge of the glass. The moisture wicks up the cloth and empties the glass of water. It takes approx. 7 hours for a cloth to empty a glass of water.
Another example of capillary action of water is a tree. There is no pump that pushes water 150’ up into the branches and leaves of a tree. It is the fibrous nature of a tree that allows capillary action to wick water upwards from deep underground to 150’ feet in height and higher.
If you pile soil around a mature tree. The tree will die. The excess soil changes the way the tree deals with the moisture.

Moisture and Historic Stone Foundations

Solid structural masonry buildings are built completely differently than modern buildings. Historic buildings were built with stone foundations that were laid directly against the excavated soil and with no footer under them. These stone foundations were laid using earth mortar as the bedding mortar. The earth mortar consists primarily of the subsoil from the excavation with very little if any lime. The stone foundation was laid in the same soil that surrounds the structure. This allows the foundation of the building to move in tune with surrounding soils.
Lime mortar was used to point the inside (exposed basement walls) to keep the earth mortar from falling into the basement. This interior pointing is usually 2” or 3” deep.

The Capillary Action of Water

Historic solid structural masonry buildings are like trees in the way they handle moisture. Masonry buildings are constantly wicking moisture out of the ground and allowing the moisture to evaporate. Just like piling soil around a tree, kills the tree. Piling soil around an existing building (higher than the way it was built) changes the way the building handles moisture. It will eventually cause irreparable moisture damage to the building.

Old stone foundations start to fail when the moisture levels in the surrounding soil and in the foundation change. If the earth mortar between the stones of the foundation dries, it will shrink, causing movement in the foundation. If the earth mortar between the stones of the foundation absorbs more moisture, the foundation will swell. This causes movement in the foundation.

The stone foundations of brick structures were built having at least 8”-12” of exposed stone. The upper brick wall portion is laid on top of the stone. This 8”-12” of above grade/exposed stone area allows ground moisture to evaporate before wicking into the brick portion of the building.

Tar or other waterproofing membranes should never be applied to the exterior below-grade levels of solid structural masonry buildings. The earth mortar in these old walls needs to stay in contact with the surrounding earth. So they stay in tune with each other. Remember there is no way to seal moisture out of solid structural walls. Especially stone foundations that were laid directly against subsoil, as there is no impervious concrete footer to keep moisture out of the foundation.

Perm Ratings

Breathability, is the permeability rating (perm rating) is how fast moisture penetrates a material. The perm rating scale goes from 0 to 100. 100 being the most permeable and 0 being the least permeable. All materials have some type of perm rating. For example glass would have a perm rating of 0 and a sponge might have a perm rating of 100.
Any coating or material that is applied to a historic masonry or plaster must have the same or a greater perm rating than the substrate it is being applied onto. For example, historic lime mortars generally have a perm rating in the 70’s. This means that any coating, mortar or plaster that is applied over old lime mortar/plaster walls must have a perm rating that equals or is greater than 70. Oherwise moisture will be trapped.
Here are some general perm ratings of materials:

Limewash - perm in the 80’s
Historic lime mortar/plasters - perm in the 70’s
Gypsum wallboard 1/2” - 50 perms
Portland cement - usually in the 20’s
Gypsum plaster @ 3/4” - 15 perms
Latex paint - usually around 7
30 lb roofing felt - 6 perms
Oil paint with primer - 1.6 perms
Polyethylene 6 mils - .06 perms
Aluminum Foil - 0.00 perms

The question should not be “is the paint or plaster breathable?" The question should be “is the paint or plaster breathable enough?"
Structolite, latex paint, oil paint, cements etc. cannot be used over historic lime structures. The perm ratings are too low.

A Brief Word about Moisture and Historic Brick

Old historic bricks are normally soft with a PSI of under 500. For comparison, a new modern brick can have a PSI of 13,000 or more. It is rare to find a new modern brick with a PSI less than 7,000.
Old bricks have an open pore structure causing them to be very porous. The high porosity of old bricks allow water to saturate them quickly.

When standing an old brick in a pan of water, it takes about 3 ½ hours for the water to wick to the top of the brick. Each old brick will hold about 1 ¾ cups of water. 100 bricks will hold over ten gallons of water. 1,000 wet bricks will hold over 100 gallons of water.

If the masonry is sealed on the exterior, this moisture will migrate into the interior of the building or to “dry” areas to escape. If the wall is sealed on both sides, the moisture will wick upwards. I have observed moisture that has wicked to a height of 55’ when both sides of a structural masonry wall were sealed.

Old bricks will “rot” or turn back into powder when they stay wet and damp for long periods of time. This is one reason to never pile dirt against historic brick walls.
Because old bricks are soft and because they hold a lot of water, they will fall apart quickly when freezing. Water expands when it freezes, and the freezing expansion of water will cause the bricks to spall and “blow apart.”

Wet/damp masonry walls also weigh more, and they lower the R-value of the wall.
Piling soil higher against historic structural masonry walls than the way it was built is not healthy for the structure. It will cause damage long term.

Identify and Eliminate Sources of Moisture

Obvious sources of moisture are things like a leaking roof or gutter. Not so obvious sources of moisture are things like a broken underground sewer line.

A very important but often overlooked moisture culprit is condensation/dew point issues. Much like the outside of a cup of cold water on a warm day will get wet, solid masonry walls will get wet during temperature and humidity swings. It is o paramount importance that historic masonry buildings be allowed to dry as quickly as possible when they get wet/damp.

Here is a list of where moisture hides in solid structural masonry walls:

Ensure that no Portland cement mortar, silicone, latex sealants, plasters, vapor barriers or latex/oil paints are on either the exterior or interior of the masonry wall. These materials trap moisture within the wall.
Condensation/dewpoint temperature and humidity changes will saturate the wall and penetrate impervious coatings.
Grade that is higher than when the structure was built, i.e. face brick covered with soil
Be sure all roofing, flashing, spouting and gutters are working correctly to carry water away from the building.
Improperly installed roof drip edges.
Check underground water supply or drainage pipes are not broken.
Look for old tree stumps that were not properly removed. Every roof when it rots can become a "pipe" to carry water into the foundation.
Ensure windowsills and other protrusions are allowing water to drip free from the masonry and not run down the side of the building.
Electric and cable wires should not slope towards the building.
Fire escapes that direct water towards the building.
Ensure all surface grading is sloping away from the foundation. Flat surfaces against the structure will not work.
Gravel, stone, sand, or pervious soil/backfill against the building will allow moisture to drain quickly into the masonry wall.
Do not sure asphalt or concrete up against the foundation. This includes basebent floors. Concrete and asphalt trap moisture under the, causing the trapped moisture to migrate into the foundation. The stone base under concrete and asphalt provides a "water holding" area.
Do not use mulch against the foundation. mulch by design is for trapping moisture in the ground.
Plant only grass against the foundation. Do not put trees, flower beds or shrubbery against the foundation.
Sub-soil around the foundation should consist of a sloped clay apron that extends about 6'-8' out from the foundation with 4" of topsoil on top of the clay sub-soil.
Never use "French Drains" gravel, or perforated pipes within 15 feet of the foundation.
Make sure AC and dehumidifier condensation water is not discharged along the foundation.
Avoid the use of any deicing salts around the foundation. Salt will migrate into the foundation.
Avoid using acid masonry cleaners. Acid digests lime and produces salt.
Avoid interior vapor barriers, spray foam insulation, modern insulation techniques. Use breathable lime insulating plasters. A dry wall provides better R-value than a damp wall.

We hope this helps you work through how to best repair your building. If you need further help, feel free to reach out via the contact form or texting. Please text or submit pictures of your project so we can give project specific advice.