How to Insulate a Stone Foundation w. Dirt Floor
Hello,
I live in Maine and own a home built in the 1880s. We’ve been here for a few years now and have been trying to make heating the house more efficient and less like heating a lobster trap.
We have a high-efficiency furnace that runs our hot water/radiator system. Our basement is unfinished with a dirt floor and a granite/brick foundation. We have no plans to finish the basement so cosmetics don’t matter, I just want it to be a little warmer and energy-efficient.
The insulation contractor who did our attic recommended using spray foam to insulate the sill and then gluing foam board on the brick portion of the foundation (this section is above grade and about 4″ high. Would that help? Would that cause moisture issues?
We are on a pretty tight budget so I’d like to do as much of this work myself as possible.
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Replies
Hi Ryan,
If the block wall is fairly uniform, consider using reclaimed rigid foam. It will be much more affordable and greener than spray foam. GBA usually only recommends closed cell when the wall face is irregular. (See this article for more info: https://www.greenbuildingadvisor.com/article/how-to-insulate-a-basement-wall.)
It's also important to cover the dirt floor with a durable liner and then monitor humidity levels to ensure they stay in the safe zone. If you have any bulk moisture issues, those should be addressed as well before you do anything else.
Also... You might want to install a capillary break under the mud sill if that is possible (especially if you end up foaming this part of the building assembly.) I'm sure the construction experts will offer additional (and probably better) suggestions.
Hi- I am working with some clients in Westchester Ny- also a 1880's home! The house is over a basement and a kitchen addition over a unvented crawlspace. I am planning on adding a "swimming pool" liner to the crawl, rat slab and insulating the walls per code- but will be adding floor insulation in floor/clg to separate the cold basement from the first floor.
>"We have a high-efficiency furnace that runs our hot water/radiator system."
No you don't. You have a high efficiency BOILER that runs the hot water/radiator system. In the HVAC world furnace hot air is the fluid delivering the heat, not water.
Using rigid foam over the more uniform flat brick surfaces is fine, or even quarried granite if it's flat enough. Closed cell spray foam is usually necessary to air & moisture seal fieldstone or uneven sill ledges, etc.
Most hard brick doesn't wick moisture nearly as strongly as poured concrete or CMU, and granite hardly wicks at all. If there is reasonable roof overhangs and a foot+ of above grade exposure on the foundation it's unlikely to need capillary break retrofit. But it's worth taking some wood moisture readings before proceeding with the insulation.
This article describes the process:
https://www.buildingscience.com/documents/insights/bsi-041-rubble-foundations#P08
Lstiburek now believes that you don't need a capillary break on stone foundations because stone is generally not porous enough to transport water into the framing, unlike concrete.