Basement insulation for a 1930 house retrofit (wall and slab)
The northern Massachusetts house where I am now living had some very bad energy efficiency characteristics when we moved in last summer. These included:
* a poorly insulated attic with an HVAC system in the cold zone
* an uninsulated basement with 18 inch thick fieldstone walls
Last December we took care of the problem with the attic by installing closed cell spray foam insulation. I got some great advice in this forum — see https://www.greenbuildingadvisor.com/search/node/1930 . Now the attic is toasty and the house is less drafty.
However we have yet to tackle the basement . We now need to address it, because we now have VERY cold floors in the downstairs. In fact, due to the thermal mass of the basement walls we expect to have cold floors well into May, even when temps outside are in the low 70s. Certainly some insulation under the floor may be needed. One contractor also recommended a second heating system dedicated to the first floor. I don’t really want to do this, it seems like a waste to have two heating systems in a house that is 1800 sq. ft.
I would also like to get more use out of the basement for a workshop and for storage. Currently it is a damp space with 60% humidity in the winter (temperatures in the low 40s) and 100% humidity in the summer (keeping a fan going lowers the humidity, but it is almost too damp to dehumidify). If we don’t do something to warm up the basement floor and/or walls the humidity alone will eventually rot the house.
My first question is on replacing the basement slab, which is only 3″ thick and crumbling. I have a quote to dig the whole basement down a foot, and then pour a more proper floating 6 inch slab with drainage underneath. This would have the effect of lowering the basement a bit. My contractor said nothing about under slab insulation or thermal bridging to separate the slab from the cold wall. Has anyone ever tried to add an insulated slab — or a heated slab — as a retrofit in such an old house?
My second question is on insulating the fieldstone wall. I am reluctant to insulate from the inside as I don’t want to trap the water from the spring thaw in the wall. The downslope portion of the house has the fieldstone wall 60% exposed so I would potentially like to do cover that from the outside — it makes this part of the house very cold almost froze our sewer line. But digging below grade to insulate (we have a 3′ frost depth) all around the house would be expensive.
I think it makes sense for us to try things with the greatest chance of success, so I am wondering if anyone has an idea what we should try first — tackle the slab or tackle the wall? And should we insulate underslab or not, and should we insulate the fieldstone wall or not?
Thanks in advance for all your suggestions,
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