U value means the heat transmitted through the glass,the less the better.
Normal 6-12-6 insulated glass has an U value 1.65w/m2.k, triple glazing(thickness 48mm) with 2 low-E coating is 0.65w/m2.k.
Vacuum glass U value can be 0.47 w/m2.k-double silver low-E, 0.71w/m2.k- Online low-E.