You should seal your attic prior to any insulation improvement.
It can save you many thousands of dollars over the years, and avoid a few moisture problems. Your home's energy consumption depends greatly on it.
Typical attics have more air leaks than any other part of the house;
Attic air leaks are a cause...
» Heat and air conditioning loss
» Moisture problems
Heat losses and Unwanted Heat Gains Through the Attic
Attic leaks are among the most important sources of energy loss, in wood frame homes.
In the winter, indoor hot air will leak in through any possible gaps and openings in the attic floor; in the summer outside hot air will enter the living space through the same air paths, making colder inside air to sink and to leak out through openings at the bottom of the house.
In non-energy efficient homnes, the function of the heating and cooling systems is more to replace the heat that is lost in the winter or to remove the heat that leaks into the house in the summer, than to heat or to cool the house… And the attics play a major role in these losses.
But leaky attics are also a cause of moisture problems. Indoor humidity moves into the attic through leakage paths, where it may condense and cause serious damage.
Inspecting your attic leaks
The bottom line: you should inspect your attic, looking for air leakage paths.
Use lights and a dust mask when inspecting or working in an attic; be aware of electrical wiring concelead in many possible places, and support your weight on the ceiling joists.
Look for leakage pathways, namely those that connect the attic to the living space and to the walls and the framing of the house. A trained eye can learn a lot by just observing it.
Attic Points to inspect
Images: Dow Chemical and EERE/Energy Star/DOE
Pay special attention to potential leakage points in dropped soffits, or where the ceiling is interrupted or the ceiling height varies.
Look for holes close to the chimney and ducts. Plumbing penetrations, wiring holes and light fixture boxes are also common sources of attic air leakage; and do not forget the top plate…
Consider also possible knee walls, hatches and doors associated to the attic, and the attic exterior and interior walls and joints: caulk along the top of these walls along the top plates.
See if there is a chase for pipes and plumbing running inside your walls up to the attic (the chase often runs from the basement to the attic, with a opening at each floor). Chases should be properly sealed with expanding foam.
Attic Air Sealing materials
Caulks are inexpensive for small jobs, but the high number of gaps and the size of many of them (and the many dirty and dusty surfaces) make low expansion foams a better option for attics.
Use a professional foam gun. That's more accurate and easy to operate.
In some places you will need special materials: non-flammable caulks (for air leaks associated with chimneys or electrical fixtures...), special elastic caulks (to respond to wide variations in temperatures), or expanding foams (large openings), or foam boards and air barriers.
Penetrations and holes associated with recessed lighting fixtures, plumbing connections or chimneys will require high-temperature fire-resistant products.