Good air-sealing where the ceiling meets the attic is important not only to save energy and reduce fuel bills, but also to prevent moisture problems and even help reduce radon entry (as discussed below).
Warm air has a tendency to rise, and this can increase pressure on the upper floor of a home. Thus, air leaks here can be especially problematic. In cold climates, for example, the warm, moist air leaving living space can enter the attic where is can condense in the colder attic possibly leading to water damage and mold growth. Frequent air leakage sites in the attic include staircases, the chimney chase, recessed light fixtures, plumbing and electrical penetrations, and some framing details.
There are a variety of techniques for, and approaches to, sealing the ceiling of the top floor.
Insulation issues, including how much, where you put it, and it's relation to air and vapor barriers are also concerns for energy-use, comfort, and moisture control. Improper use of insulation can lead not only to energy problems but also to condensation and mold growth.
Attic Conversions
Converting attic space to living space is popular and can be very economical. However, as you bring an attic into your living space, you should use care to ensure the attic is brought all the way into the living space to avoid comfort problems (too hot/too cold) and to prevent other conditions which could impact your health or the structure of your home. These include the air-sealing and insulation issues discussed above, as well as other considerations mentioned below.
Ventilation
Good ventilation protects both your health and your home. If it will be living space, the attic should be included your home's ventilation strategy. If your home currently has no mechanical ventilation this may be an opportunity to install a system. As a rule of thumb, you should install roughly 1 square foot of "net free" ventilation area per 300 square feet of attic floor area. (Net free vent area refers to the actual area of open holes in a manufactured vent. It is usually about one-half the total vent hole area.)
Converting your attic to living space may present a good opportunity to replace old windows with new ENERGY STAR® windows. While costs do not always justify the change from purely an energy savings perspective, there may be other benefits of new windows. More efficient windows may be less prone to condensation and related mold growth. Painted window sashes and frames in homes built before 1978 may contain lead-based paint; this is a special concern because the friction of opening and closing windows can release lead dust into the home; new windows can solve this problem.
General Recommendations
You should also review the general recommendations for all remodeling and renovation projects in the guidance "Addressing Indoor Environmental Concerns During Remodeling."