In article ,
says...
The spar cap is glued to the inner skin, and there is foam between the
spar cap and the outer skin. The diagrams in this tech note pretty
much show it:

http://www.schempp-hirth.com/tmdocs/396-8-489.pdf

(Somehow, when I look at those photos, I smell epoxy resin.)

The big advantage of that method is that the sandwich skins are more
stable, and less prone to mirroring the spar after aging. One of the
disadvantages is that the spar is less stiff than it would be if it
went from outer skin to outer skin, and didn't have its depth reduced
by the sandwich foam.

I don't think this is a disadvantage, but is instead an advantage,
because the wing isn't so stiff, and the pilot enjoys a smoother ride
in turbulence. The disadvantage is it likely takes more material, so
the wing is a bit heavier and more expensive.
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Eric Greenwell
Richland, WA (USA)