Some More Questions around Rubble Foundation Insulation – GreenBuildingAdvisor

 Hi,

I have a 1500 ft2 house built in 1900 in coastal Rhode Island (climate zone 5A).  The house has a rubble foundation in good condition with consistently mortared joints.  The basement floor is an uneven, discontinuously poured “rat slab” of approx. 1-2” thickness, poured right on the dirt.  I run a dehumidifier continuously, which keeps the RH low.  In the eight years I’ve owned the house, there has been no evidence of water intrusion.  There is no sump.

I would like to bring the basement into the conditioned space of the house, similar to what is described by Joe Lstiburek’s Septemmber 2021 article, BSI-041 “Rubble Foundations”.  Like Joe’s example, my house has a separate crawlspace area under an addition that was added in the early 2000s, that area is also rubble foundation with dirt floor.  Currently there is no connection between it and the original basement.

I have some questions:

1.      Following Joe’s template, I was planning to use drainage mat on top of the current slab and up the base of the walls, with 2” of EPS laid over it.  Then pour a new slab above that.  What thickness concrete slab?  There’s only 6″ of headroom currently, and I’d like to keep all I can. 

Also, do I need to add a sump and pump, or can I eliminate that since there is not evidence of bulk water currently?

2.       The house was lifted off the foundation in the early 2000s, and the sills replaced.  There is no evidence of rot in the current sills.  I would prefer not to jack the house up to put a membrane between the sills and rubble foundation.  I would spray foam up the walls to the wooden sill.  What’s the potential risk of eliminating the membrane?

3.      There are two carrying beams running the length of the basement, set in pockets of the rubble foundation.  How should these be treated relative to the spray foam?  I am concerned that the interior foam would effectively seal the beam ends off from the interior drying conditions and invite rot.

4.      There is a bulkhead door leading outside via stairs in a “bump out” of the foundation.  Should I treat that bump out as outside the envelop, and build a well sealed door between it and the basement?

5.      The main basement has five ground-level windows, and the crawlspace has one.  These windows are old and would either need to be replaced or eliminated.  If replaced, I assume adding bucks to mount new windows, and spray foaming up to the bucks.  If eliminated, what are the recommended options?  Note:  they are visible from the exterior, and glass block would not fit with the architecture of the house.

6.      There is plumbing, electrical, and an oil tank running along the perimeter of the basement.  All these would need to be moved inward to make room for the spray foam.  What thickness of foam is needed?  (this will influence how easy it will be to relocate those mechanicals).

Thanks!