Attic ventilation and insulation is critical to reduce cooling costs, to prevent shingle damage, to minimize ice damming, and to prevent moisture buildup. Installation is not complicated.
An unvented attic is like a politician or me—–full of hot air.
If the area directly under your roof does not have adequate air movement for cooling, the sun can raise temperatures to levels damaging to asphalt shingles. Heat is also transferred to the living space below, and increases cooling costs. In the winter, heat rising through the ceiling to a poorly vented attic space can melt snow on the roof. The resulting water can run downward, and refreeze on colder areas of the roof to form ice dams. The water pooling behind the ice dams can overwhelm the roofing and leak through. This will cause severe damage to insulation, ceilings, and interior decorating. Excess humidity needs to be removed from the attic space to prevent damage to the roof and insulation from mold and rot.
In one severe case, I saw where asphalt shingles badly curled in 5 years. They normally would not have been as bad after 20 years. This was a shed type roof on an addition to the south side of a two story house. The shingles were black. The wall above was painted white, reflecting heat onto the roof, and a wall opposite was also white, reflecting heat back again. The original cedar shingle roof had been removed, and insulation had been stuffed in from the top. No attic ventilation of any kind had been provided.
Admittedly, this is an extreme example, but it serves to demonstrate the importance of attic ventilation.
Complicated roofs can complicate the equation considerably, but adequate ventilation on a simple gable roof is not difficult to achieve.
A continuous ridge vent combined with vented soffits is a low maintenance passive system that works well. Ridge vents should not be combined with other types of vents in snow areas, as snow may blow into the attic. There is much less chance of serious leaks with ridge vents.
In older houses, the 3” x 10” screened vents about every 4 feet in the soffits combined with one or two square vents near the ridge, is not really adequate.
I believe that passive attic ventilation is a better option for attic ventilation than powered fans, or systems with moving parts. It is dangerous to be on the roof to affect repairs when there is snow. Working on the roof in hot weather can damage asphalt shingles. There can be noise problems. Power outages render fans useless. Electricity consumption is a cost, and an environmental concern.
A complicated roof may require more complicated solutions. Vaulted ceilings make insulating and venting difficult, as do attic living or storage space. Of course, this is another argument for simpler roof designs.
There are minimum requirements in building codes for attic ventilation, but once again I advocate going a step further, and providing somewhat more.
Care must be taken to not block the attic ventilation at the eaves when insulating. Insulation stops will be needed in many cases.
There are a great many attic ventilation devices on the market.
Most require a hole through the roof which means they may leak, if not properly installed, or if they become damaged from wind or hail. Made of metal or plastic, they are usually obvious, and not necessarily attractive. Most will work fairly well, if there is enough of them, and they are well placed.
Gable vents seen on older houses are not that effective, due to poor air distribution.
Turbine vents are popular, but I really don’t think they are much more effective than static vents. They can be noisy, and more so if they are damaged by wind or hail.
Power vents can move a lot of air, but care must be taken to provide extra inlet vents, if they are to be any more effective. Distribution of air movement may not be that even. Their true effectiveness is arguable. Solar powered models are available to reduce energy cost and eliminate wiring.
For cost and effectiveness, I believe that ridge venting with vented soffits is the better option for attic ventilation. It works the best on a gable type roof. If you choose the type that you shingle over, it is virtually invisible from the ground. Some other roof designs will require different solutions.