Published on March 11th, 2013 | by Henri


Drafty basement in a historic house

Q.    We live in an 1831 brick house with a stone foundation. We recently had an energy audit, because some parts of the house are very drafty. The auditor found that a lot of the cold is coming from the basement around the foundation. (I know this is true because I can hardly stand in front of my stove in the winter because there is such a draft coming from underneath.)

He suggested spraying some sort of insulating stuff all over the stones that would create walls with a relatively smooth appearance. I don’t know this product, but I’m concerned about whether that is a good approach in a historic house. How would you handle this? Most of the basement is pretty dry, though one wall can get wet in a heavy rain.

A.    There are some serious concerns about spraying insulation on a historic stone-wall foundation.There is a great risk that insulating the foundation may cause frost to penetrate deeper and push the wall in.

Depending on the type of insulation, there is a concern about flammability, and in the case of foam insulation, off-gassing for an extended period of time, which can result in health problems.

In case of a fire in the basement, foam insulation emits poisonous gas, an obvious danger to the occupants and the fire responders. Therefore, if used, it should be covered with a fireproofing material. In some areas, building or fire codes may prohibit spraying on insulation in this manner.

A better solution to seal off air infiltration that is causing the drafts you feel is to ascertain the condition of the stone foundation, clean it if needed, and tuck-point it with an appropriate mortar. You should retain the services of a mason experienced in historic restoration, as using the wrong mortar can cause problems.

There may also be some air infiltration around the sills; these spaces can also be sealed with mortar or canned foam (which would represent much less of a fire danger, being such a small amount).

If you still feel some draft, you may want to consider stapling Tyvek to the bottom of the floor system. Do not use insulation or plastic, which may cause problems by trapping moisture within the floor system. Tyvek is vapor permeable, providing an air barrier without sealing in water.

As to the wall that gets wet, you should check the grade around the entire foundation (water can travel far). If it is possible to raise it to slope it away from the foundation, but still leave some of the foundation showing, consider adding some topsoil and maintaining a healthy stand of grass on it. Tuck-pointing this wall may not be too successful if there is a strong hydrostatic pressure on the wall, and the pressure could damage the wall.

If there is not enough of the foundation showing to raise the grade, a swale can be built from the foundation out a few feet with a positive outlet to move the water away and prevent it from percolating and leaking in the basement.

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