Im in the process of taping up and putting mastic over all the leaks in my air ducts under my house. If i manage to seal it up and insulate all the ductwork, will i see more savings in the winter or summer?
It really depends on where you live. In more northern areas the winter heating load tends to be much more than the summer cooling load. In more southern areas the cooling load in summer can be much higher. . Where I live in East Texas Nov, Dec, Jan are the highest loads even more than Jul or Aug in the summer.
the savings overall may be minimal, but helping your HVAC to work more efficiently is always good. Be sure you insulate properly so that moisture will not build up between the ducts and the insulation causing mold.
Without SOME hint at local weather conditions, your target internal temp, and the efficiency of your HVAC system, flip a coin and you will get as good an answer.