So I caution you not to blindly throw solutions at a problem that may not need to be fixed.

When I worked in the Pharmaceutical industry our manufacturing facility had rooms of various "pressures" These contributed to the various levels of "clean room" and ran from -.1" water pressure to +.2" These pressures were meet by adding or removing more air flow than the inlet or outlets would allow. Thereby creating this pressure differential. If you attic is negative pressure due to a large gable vent fan then small leaks around light fixtures, plumbing vents etc. will allow the cooler conditioned air to get "sucked" up into the attic...and therefore cause more energy waste.

Little was known at the time about attic air flow, energy costs, moisture and mold. In the 70's when we had our first energy issues many people began to seal up their attics and began to turn off their attic fans in order to save some money.

In the 80's we began to realize that shingles on roofs began to fail faster, and plywood used to sheath roofs were failing as well. A lot of research went into this issue and the emergence of ridge vents came into the picture. Also we began to see the use of de-humidistats on the fans to control moisture that resulted in mold.

In the 90's we began to see lots of mold in the attics. We began to see that attics that had ridge vents, and soffit vents had much less mold issues then those with fan systems. Were we on to something? Perhaps not?

In the 2000's we began to look at energy with much more emphasis on the house as a system approach. Meaning that for any one thing you change or alter, it effects something else.

We began as an industry that all of our ideas of sealing and venting regardless of what or how we did it changed something else within the home. This brings to light why some homes that were the same style and construction and had the same type of ventilation system employed worked and some did not and resulted in some sort of an ill effect such as mold, odors and the like.

Using the whole house as a system approach you will begin to understand a bit more on the hows and whys you need to ventilate and how to do it properly so not to effect something else within the home.

Picture your house as a big chimney. As with any chimney heated air flows up. when this occurs the air in the lowest part as in a basement of a house becomes slightly negative in pressure. If you were to open a window in the middle of the house or put a hole in your chimney in the middle it would effect that airflow and less would be pulled in at the bottom. With the addition of newer windows and doors along with better house sealing methods you or the industry as a whole in effect has created a better chimney in your home. More basement air is being introduced into the home. Which results in more pollutants from poorly drafting appliances. Radon is drawn in and so are odors from crawl spaces. So along comes a better mouse trap. Basement fans are installed to counter against this. So a tug of war begins with a natural occurrence of air flow and mechanical methods to counter act those occurrences. All in an effort to save money.

Now comes into play the attic. As we now understand your attic is part of a greater system which is the whole house. If we begin to change anything in the attic it too will being to change how the rest of the house reacts. By installing a fan or roof vent it begins to draw more air out of the attic, in return more must come in. But where? does it come in from the sofffit vents? Or does it get pulled out of poorly installed duct systems, or light fixtures or from holes created for wires that extend all the way to the basement? Add this to outside conditions such as wind flows, or house designs. And you end up with a recipe of good intentions causing more issues then you began with.

If you can cap your chimney your air flow from the rest of it will stop flowing up. Although air currents will still rise and as they cool will fall. The condition of that air other then temps will or should remain the same. If we take that a bit further If we seal off the attic properly. such as lights, ducts, access hatches and the like we now only need to control that system by itself. If properly done, your attic area then becomes a system all by itself.

At that point you can then determine what type of ventilation would work best. And there are about as many methods as Basking Robbins has flavors of ice cream. The current trend is using natural methods. No power fans. These items waste money using electrical energy and can draw in abnormal amounts of air containing excessive moisture resulting in mold.

With properly sized ridge vents and correctly matching and located soffit venting the natural heating and cooling will bring in just what the attic requires. Don't believe me? Go to a house that has a detached garage. No mold in attic, roof lasts about twice as long as the house it belongs to. So you have to ask yourself why? Because the natural ventilation that the garage has because of leaks and no indoor conditions such as heated and cooled air is introduced shows us that this is what we should be doing in our homes.

So by properly sealing the attic from the rest of the house, controlling indoor humidity and installing properly sized natural ventilation systems such a ridge and matching soffit ventilation your doing your very best to preserve your roof, and lower your utility bills as a result.

But I have repaired dozens of them that were improperly sealed, designed, maintained, etc

Every penetration to the attic is a potential source of moisture leaking from the living space to the attic. Holes for wiring, plumbing vents, errors in VB screws through the VB, attic access hatches, etc, etc, etc

A properly performed audit using blower door and precise air flow testing equipment can quantify this air loss for you. And is strongly suggested to be done before and then after the work is done.