Moisture sealing on top of stone foundation
I have an 1890s balloon framed 2 story hour. It has a basement, and the foundation is mortared stone. The rim joist and floor joists sit directly on the stone foundation wall, ther is no sill plate. The rim joists are pulling away from the floor joists at the bottome (forming an upside down V). I am planning to (working in sections) jack up the floor joists about 1/2 inch and then using structural screws from the outside of the rim joist through the end of each floor joist to try to pull them back together, or at least to stabilize them so they don’t move further. While I have the joists up I would like to insert a moisture barrier between the stone and the joists. Future plan is to have the rim joist and foundation walls sprayed with closed cell foam for insulation and air sealing. I am aware of the risk to the rim and floor joists from moisture that has nowhere to dry, thus the moisture barrier.
My question has to do with choice of moisture barrier materials, and how to detail them. My first thought was to use the standard roll of foam sill seal. If I use that would I use an adhesive to seal it on the outside to the top of the foundation and find a way to hide it visibly, possibly with a piece of trim? Or would it work better to staple the outside to the rim joist and then tape it to seal the edge of the sill seal. My worry with that is that any failure of the tape would allow water to enter and be held against the rim joist.
Second choice would be to use a self adhering membrane of some sort. If I use that, I still have the question of putting the sticky side down and sealing the outside edge to the stone, or put sticky side up and sealing against the outside of the rim joist, with the same moisture issues.
Any advice is appreciated!
John
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This is a good article on dealing with foundations of that type:
https://buildingscience.com/documents/insights/bsi-041-rubble-foundations#P08
I did hear Lstiburek say that he no longer feels a capillary break is necessary with stone, as unlike concrete stone doesn't wick appreciable amounts of water. But the kind of spreading you're experiencing is often a sign that the bottom of the rim joist is rotting from the outside in, which causes it to roll outward as it gets undermined on the outside.
Thanks for the post. I had read that article, which is where I got the idea to jack the joists up and put the capillary break in. I need to jack them up anyway to secure the rim and floor joists back together, but if I read you right you don't think I should need the capillary break? If it makes a difference, the top of the stone wall is four foot above the outside grade.
The bigger question is why are your joists separating from your rim joist? Is the foundation wall bowing out?
As far as I can tell the walls are not bowing out. I haven't put a stringline to them, but visually they are straight. We have been in the house for 23 years, and the rim joist has always had the same tilt to it.