I lived in a home for seven years that first had an addition, then a new basement added under the addition. Leaks were never a problem.

I've never done such a reno myself. But I don't see waterproofing as a problem. Just overlap the join below-grade with dampproofing spray followed by Delta-type membrane.

How tightly the old and new foundations should be tied together is another question. My inclination would be to butt them together, not interweave them. In any case, I would follow the recommendations of a P. Eng. with experience in such a project.

Worthy,

Thx for the info.. My only question is... What is a "Delta-type membrane"?

Thx

Delta membranes. It was the first; there are a number of competitors now.

For overkill, you can add a rubberized spray before the membrane.

Looks like a good idea.. thx.
:-)

We are doing this under our whole addition instead of a crawl space. It is fairly common where I live in NJ. Contractor is connecting old and new just by a cut out doorway because it would have cost a lot to deal with removing a lot of wall structurally re support. The new area will have a seperate door from outside as well and will double as a guest suite and a home office. we had a water problem in our current basement until we put in french drains, so the contractor is a priori building them into the new part. He is also putting a few inches (at lesast of gravel under the floor to prevent water problems. Don't know anything about a membrame. He does this a lot in my neighborhood and thought it was a waste of space not to do it. There is an upcharge vs. a crawl space, but cost much less for the space that if we added that much space on the first floor, because in his worde he is digging anyway, it is just digging a few more feet down.

We have done many basement foundation additions in block and the usual way to tie the 2 together is by drilling 16" rebar dowels into the existing walls and into the new blockwork then corefill the first 2 blockcores to tie the 2 walls together, then as worthy described proceed with the waterproofing . At least thats the way we do it in Ontario.
cheers
Martin