Dealing with foundation moisture

Q. We are in the process of renovating the basement because of a sewage backup last October. The house is a Cape Cod built in 1943. The foundation is 4 feet, 5 inches of concrete and 3 feet more with two layers of brick. There is some efflorescence on the brick, mostly where the porch is in the front of the house and on some low parts of the foundation.

We had the entire plumbing system replaced with an overhead/underground ejector system. Also we added a trench drain around the majority of the interior foundation and installed simple board so that any water that might make its way into the foundation will run down into the trench. I am having the most trouble with deciding how to insulate the walls. Do I need to let the inside layer of brick breathe so that it doesn't get subjected to freeze/thaw in the winter?

The wall is being framed with a treated two-by-four bottom plate against the dimple board and studded out with 2-by-3s. For insulation, I am putting 2-inch foam board between the studs, and I'm not sure about using a vapor barrier. I'm hoping this will allow the brick to breathe. We will have some protection from the cold.

The gap behind the wall will also allow some movement of air from the slightly leaking HVAC system and should keep it somewhat tempered in the 1.5-inch gap. Does it sound like we are on the right track or are we setting ourselves up for failure? We just purchased this house in 2017 and weren't expecting any issues and, since then, it's been one thing after another. We would really appreciate your opinion.

A. The efflorescence on the bricks is caused by moisture absorbed by the bricks. If the lower parts of the brick walls are in contact with the soil outside, you may want to dig the soil out to below 6 inches from the top of the concrete foundation wall and apply 6-mil black plastic to the portions of the brick wall in contact with the soil. Put the soil back, add more soil to obtain a gentle slope away from the foundation and tamp it firmly. Grow grass on the soil.

The greater efflorescence on the brick wall under the porch tells me the soil under the porch slopes down toward the foundation wall. The efflorescence seen on the lower concrete parts of the wall indicates that the moisture penetration at this location is quite deep.

If this is the case, and it is reachable, the same treatment should be effected against the brick wall only and more soil added to reverse the slope under the porch. If this is not possible, there are ways to prevent water intrusion under covered porches.

Efflorescence is usually easily removed by brushing it off with a stiff brush. If needed, the brush may be dampened.

The outside work should be done before attempting to insulate the walls, and the affected brick walls allowed to dry from inside; a high velocity fan should help a lot.

Once this is done, there should no longer be a concern about letting the bricks breathe, but do not apply a vapor retarder against them so that any possible future moisture can evaporate in the 1.5-inch space you are leaving between the studding and the foundation wall. Rigid insulation is, in itself, a vapor retarder on the inside face of the new walls where it belongs.

Be aware that insulating a foundation can increase the frost pressure on these walls. It is important to know that the backfill was made of coarse material that allows any water penetration to drain safely to a footing drain. But one of the best insurances against damaging frost pressure is a gently sloping grade away from the foundation that moves surface water away from it.

What you refer to as a slight leakage in the HVAC system is not clear to me. Are you referring to a duct system leaking at the joints? And how does this leakage cause some air movement into the 1.5-inch space?

Q. I have two wood burning fireplaces in my home, and they and the chimney are in the center of the home. The roof line has a gentle slope with two shed roof lines sloping away from the chimney on two sides. The peak of the roof meets the chimney on the other two sides and I have a ridge vent on the ridge of those two peaks. I assume there is moisture entering from somewhere because the inside walls have the plaster bubbled around the chimney area inside the house on the two sides under the ridge vents sides of the chimney.

First I had the chimney flashing replaced, and the ridge vents installed in place of the old box vents. I thought it might help with reducing condensation. Then, I had the chimney re-pointed. Then I had the chimney sealed. It is a flagstone exterior to match the house. Then I had the whole roof replaced and of course the flashing replaced again. And I had the top re-cemented and sealed. And it continued to damage the interior. I asked the roofing company to return and redo the flashing and they said they did. The man was here and did the work. And it is again bubbled and damaging the interior plaster walls. Each time, I have redone the plaster and paint, of course. But now I am looking for a solution.

There is no visible running or dripping water, I have not seen it wet during heavy storms. Can you suggest any way to correct this condition? Could it be from broken tiles inside the chimney? And if so how would that be repaired?

I have to do something besides redoing the plaster and painting every year.

A. I wish you had sent me some photos illustrating the roof lines you describe; (interior photos would have also helped me see how extensive the plaster damage is.)

I am sorry to say that I am unable to visualize the roof lines and how they relate to each other from your description, and to determine how the new ridge vents fit in relation to the chimneys and the roof planes, but your statement that the plaster damage is under the two sides where the ridge vents are located tells me they are probably responsible for it.

Most ridge vents installed by builders are not externally baffled and can admit rain and snow under windy conditions. Externally baffled ridge vents have a far better record of keeping the weather out, but are not a total cure. The damage to the plaster coming from the ridge vents sounds more plausible to me than if it were coming from broken tiles inside the chimneys.

To test this hypothesis, you could have the ridge vents covered with temporary tarps through several rainfalls and see if further damage occurs; if it does, then the problem is elsewhere. In that case, if you so desire, send me photos and I'll see if I can do better.

I am sorry that you have spent so much time and money needlessly dealing with this mysterious problem.

• Henri de Marne, a former remodeling contractor turned columnist and consultant, is the author of "About the House with Henri de Marne" (Upper Access Publishing). He continues to take questions from readers for this column and his website, www.henridemarne.com. Email questions to aboutthehouse@gmavt.net.