I would highly recommend doing the repair from the topside, not from the bottom. First, getting a properly laid up laminate without voids is much more difficult from the bottom—since gravity is working against you. Second, you have to really be careful when glassing inside the boat or you can make a really amazing mess...that will take forever to clean up—which isn't the case when working from the top down. Third, it will go much faster if you work from the top down. You also do need to put a layer of fiberglass on the side you're working from—since that is what gives the cored laminate its strength, as others have stated.
The way a cored laminate works is that the two layers of fiberglass act as the top and bottom of an I-beam effectively, one in tension and one in compression with a stress web, the core material, between them. This makes it far stronger than solid laminate of the same weight would be. Not glassing the interior will result in a one-sided I-beam...which is pretty weak.
For core material, I'd go with divinylcell or Airex foam rather than balsa.
It is probably going to be a lot easier to work with. ContourKore balsa is great to work with, but have you priced it recently???
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You know what the first rule of sailing is? ...Love. You can learn all the math in the 'verse, but you take
a boat to the sea you don't love, she'll shake you off just as sure as the turning of the worlds. Love keeps
her going when she oughta fall down, tells you she's hurting 'fore she keens. Makes her a home.
—Cpt. Mal Reynolds, Serenity (edited)
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Still—DON'T READ THAT POST AGAIN.