Here in New England most of our older houses have rubble foundations. Rubble foundations are uncut stone, often called field stone. Even the best builders of rubble foundations had spaces between the stones that essentially connect the soil directly to the basement. Sometimes there is mortar in those spaces, but it’s very moisture permeable and only slows water down a little. Our present drought not withstanding, soil tends to be wet. The amount of water that evaporates from soil varies depending on many factors. Lots of numbers get presented with not enough information about how they were arrived at. Let’s just use the relative amounts “a lot” and “too much”. For our purposes that’s all we need to know.
Just a note here. I love rubble foundations. The best ones are tributes to the skills of the men (usually) who put them together. They’re beautiful. The stones were selected out of the piles to fit together really well and provide an impressively strong and durable foundation. Just don’t ask a modern structural engineer to certify them for a certain load. You’re likely to see a lot of twitching. It’s not a nice thing to do to an engineer. Also, don’t dig a trench next to the wall below the base level of the stones. While these things are really strong in compression* they do not deal well with tension*. I know at least a couple of structural engineers who should have known that. But I’m not going to digress that far.
Back to this water problem. Even if you’re not getting liquid water through your rubble foundation (lucky you) the evaporation is not helping at all. I routinely measure relative humidity levels in the high 90’s in rubble basements. Another point, houses with rubble foundations are our older housing stock. The leakage from floor to floor to attic is usually greater in these houses. So every level has higher humidity levels and that presents a whole series of possible bad outcomes. Read the post, And One to Grow On, for background on why that’s a bad thing.
How do you fix this? You’ve got all these rocks sticking out at different levels, there’s dirt, cobwebs and probably some unidentifiable stuff that you don’t even want to think about. First, read my post, Oh, that’s gonna smell…. DO NOT, under any circumstances, wood frame a wall, install some porous batt insulation, and cover it with drywall. Read the title of that post again. Now, let’s get in the way-back machine and watch the opening scene of “The Graduate” and listen to the wise word spoken to Dustin Hoffman as Ben Braddock. “Plastics.”
That covers a lot of ground. Spray foam, sheets, and rolls of various types. Some are thick, some thin and some seem too flimsy to do anything. The next post will cover more specifics and we’ll start going over the pros and cons of different approaches.
Until then, don’t raise a stink.
*Better folks than I have used this, it seems.
*Compression: The squeezing force applied to a material. Most rocks resist compression pretty well. Sandstone, not so much.
*Tension: Pulling force. An individual rock has good strength in tension. A pile of rocks does not.