I just discovered that my sill plate and joist setup is different than others I've seen (and I was a Realtor, so I've seen a lot of sills).

As you can see from the picture, there's a gap between the exterior board and the subfloor. Covering that gap, on the exterior, is just some tar paper and vinyl siding. So as you can imagine, cold air here in Maine is just pouring in.
I have two questions - is there a potential structural issue? and what should I do to fix it? spray foam? cut 2 x 6's and attach to inside?
More information: house built in '78, floor joists are insulated, area is a crawlspace, joists rest on on full length steel I-beam.
Any help would be appreciated.
