  We study the renormalization group flow of the interactions in the
two-dimensional t-t' Hubbard model near half filling in a N-patch
representation of the whole Fermi surface. Starting from weak to intermediate
couplings the flows are to strong coupling with different character depending
on the choice of parameters. In a large parameter region elastic Umklapp
scatterings drive an instability which on parts of the Fermi surface exhibits
the key signatures of an insulating spin liquid (ISL), as proposed by Furukawa
et al., rather than a conventional symmetry-broken state. The ISL is
characterized by both strong d-wave pairing and antiferromagnetic correlations,
however it is insulating due to the vanishing local charge compressibility and
a spin liquid because of the spin gap arising from the pairing correlations. We
find that the ISL is a consequence of a Fermi surface close to the saddle
points at the Brillouin zone boundaries which provides an intrinsic and
mutually reinforcing coupling between pairing and Umklapp channels.
