I don't think it is needed, but it is extremely beneficial.

Interior glass actually helps add a lot of strength for when you hit objects like submerged rocks. You kayak will probably be adequately strong without interior glass, but if you glass intead of taping, you get much more strength for a VERY minimal weight gain.

The interior glass also makes sure you get enough epoxy to adequately seal the inside to prevent the dreaded black ick. I believe that is why black ick is a common topic on the CLC board, as some of their designs do not call for interior glass. I have a kayak that I put away wet. Water soaked into the wood in places that were not glassed, even though I had two seal coats on the plywood. The places with glass were fine. From now on all of my S&G kayaks get fully encapsulated with glass, even if I only use 3.25 oz satin glass, it does wonders for the logevity of the craft.

Interior glass also adds some texture to the interior, that makes it less slippery.

: Since fiberglass is strong in tension, is it structurally necessary to glass
: the inside of the hull, since this side of the "sandwich" should
: be in compression when the boat is floating? Apart from waterproofing the
: wood and using a glass layer for abrasion resistance in the cockpit area,
: I'm wondering if taping the seams (we're talking S&G here) isn't enough
: treatment for the remainder of the inside of the hull. I would think the
: deck should probably be glassed both sides, but maybe not. Am I missing
: something? I'm sure the group has already figured this out.

: Still in the planning stages and (truthfully) trying to get up the nerve to
: order plans (probably Cirrus).