The attic is a strange, yet useful, space within every home. In the old days, attics could breathe freely through large gable vents, and heating of the air in the attic was not the problem it is today. With today's tighter buildings, the problem of super-heated air - air that can be as much as 40 of 50 degrees hotter than the outside air temperature - is a huge problem. This heat, if not kept in check, will eventually make its way into the conditioned living space, placing further load on your air conditioner.
To make an attic more efficient, you need to take a three-pronged approach to fixing the problem. First, the attic should never be sealed to the outside - it must breathe and exchange air with the outside. Secondly, radiant heat into the attic should be minimized. The less radiant heat you have in the attic, the less heat there is to transfer through the ceiling and into the living space. Finally, a strong thermal barrier should separate the attic from the living space, regardless of what the temperature is in the attic. I will talk about all three here in detail.
Let the Attic Breathe
Letting the attic breathe properly is paramount to anything you can do to make an attic more efficient. First of all, air circulation helps control moisture levels - extremely important in humid climates. Secondly, air circulation carries super-heated air out of the attic space, replacing it with the cooler outside air. Replacing super-heated air with cooler air means less heat that could be transferred into the living space. So how do you circulate the air?
There are several methods of accomplishing this. Regardless of which method you have or choose, you must have some method of allowing air into the attic, and some method to exhaust it out of the attic. First, the air inlets. This is most commonly accomplished on newer homes through the use of soffit vents. These are the vents you see under the eves of your home. Other methods of getting air into the attic could also be gable vents. Regardless, the air inlet should be as low as possible in the attic area, and thus my preference is soffit vents. Why? Because heat rises, and as it does, it will pull air in through the lowest point it can. Since soffit vents are typically the lowest air inlets in an attic space (unless you haven't sealed up your home envelope), that means more air is circulated through the attic as it moves around and it will take more heat with it as it is exhausted.
Now for the exhaust portion. Your first option is power vents. These are wind-driven, electric, or solar fans that either penetrate the roof decking and covering to exhaust out the top, or they can penetrate the side of the house through gable vents. Many are thermostatically controlled and only run once the attic temperature rises above a certain level. Power fans are not my preference for peaked roofs, because they will eventually fail and need replacing, and they cost money to run. Most draw the current equivalent to a 60 or 75 watt light bulb when they're operating. However, they are cheaper to install than my preference, and if you have them or want to install one or more of them (or you don't have a choice because you have a flat roof), make sure you get one that is thermostatically controlled so that it doesn't run all the time (or require you to flip a switch to operate). Wind-driven turbines are free to operate; however, they can be squeaky, and they are not effective if there is no breeze.
My personal preference are ridge vents. These are modifications made to the peak of roofs that actually allow air to exhaust out of the highest point of a roof. And they cost nothing to operate, because they work on the principles of convection and negative pressure. As heat builds up in the attic, it will rise to the highest points in the attic (the peaks). The movement of heat will also cause the movement of air. As the heat rises towards the peaks and reaches the ridge vents, it will exhaust the heat and air out the top. This will cause a pressure differential, and will draw cooler air from the outside up through the soffit vents. Wind can also cause pressure differentials, and when it does, it will also help to circulate air through the attic.
Ridge vents can be an expensive retrofit to an existing roof, but could be worth it in the long run even if you have a relatively new roof - again, because they cost nothing to operate and work wonders on keeping the attic temperature down. I would highly recommend you have them installed, though, if you have to put a new roof on your house. The shingles will be off and the roof decking will be exposed, and it will not be nearly as expensive to install them at that point.
Controlling Radiant Heat
So how does the air in an attic become super-heated and hotter than the outside air? The sun shining on your roof, of course! Darker roofing materials compound the problem because they absorb more heat than lighter-colored surfaces - heat that is transferred into your attic. But that is conductive heat, and installing a lighter roof surface is about the only cure to that problem. The other problem - and one that you can definitely control - is radiant heat.
Radiant heat is heat that is radiated from a heat source (in this case, the sun) and transmitted across space (with or without air). Radiant heat is infrared energy, and that energy travels in a straight line from the source until it is either absorbed or reflected by an object. In the case of the roof and attic, the roofing material absorbs some of the radiant energy, and the attic air space absorbs the rest.
Radiant barrier stops the majority of this transfer. It comes in several forms, but it must have a reflective quality to it. In most cases, this is accomplished by either reflective metallic sheeting or metallic particles in a spray. Radiant barrier sheeting very effective at reflecting radiation (and thus helping to keep the attic cooler), and it is available at many internet sites and home improvement stores. I helped install it several years ago in my folks' attic, and I did it in the attic at a home I used to live in a few years back. Those were my first and last times to do that. While it is the most effective at reflecting radiant heat, it is also a bear to install. The concept of installation sounds simple: unroll it, cut it to length, and staple it to the rafters. Try doing that in a confined and potentially perilous attic environment! Even though we did it in the winter time when being in the attic was actually comfortable, it was still a nightmare, and I'll never do it again. The other downside is that there are certain areas where you just can't reach to stable the barrier.
If you're in the market for a new roof, investigate having your roof decking replaced with decking that has the radiant barrier integrated on it. One of these products is called TechShield. Just a few years ago, TechShield was an upgrade option on all new spec homes being built in the Houston area. However, after seeing how effective it is and many builders wanting to comply with EnergyStar standards, they began offering TechShield as part of the standard building process. The folks' new home has TechShield, and it along with good ventilation makes the attic actually bearable in the heat of the summer. The downside to this type of radiant barrier is that it can sometimes interfere with cell signal strength.
Since I bought the folks' old house, I inhereted the staple-up radiant sheeting. It is fine for now, but certain areas have come unstapled and are falling due to variou contractors having to be in the attic to repair the water heater or HVAC units, etc. In the not too distant future I will need to put a new roof on this house. At that point, I will be looking at two options: either replacing the roof decking with TechShield or removing the sheeting we installed and having a spray-on barrier applied. The spray-on barrier is not as efficient as the sheets or the TechShield, blocking around 70% of the radiant heat entering the attic (whereas TechShield claims their product blocks over 95%. But having that much blocking is way better than nothing - the final decision will come down to economics, and I have yet to do that research.
Insulation
The final step in any attic efficiency project should be insulation. Why is this the final step? Because you don't want to be walking on and squishing down insulation as you (or contractors) work. The fluffier your insulation is, the better it performs; squish it down, and it won't come close to meeting its rated R-value.
What is an R-value? It is a measure of a material's ability to block the transfer of heat. The higher the R-value, the better it is at blocking heat transfer. Higher R-values typically imply thicker material. But more, more, more is typically not better. You reach a point of diminishing returns after about R-49 (roughly 18 inches of blown cellulose insulation) where it is not economical to apply more insulation. That R-value might be higher in northern climates, but you typically shouldn't need to go any higher than that in warmer, southern climates...and even then, some people will claim R-49 is overkill and your money is better spent not going over about R-38. Remember, when shopping for and buying insulation, do not buy inches, buy R value, because the thickness of material required to reach a specific R value varies with the type of product used (fiberglass, cellulose, etc.).
Before insulating the attic, you should have all of your attic sealing projects and any other projects up there complete, including the application of radiant barrier. If you use your attic for storage, and want to maintain that storage space, you should consider building a raised floor platform at least 18 inches (24" would be better) above the level of the ceiling joists. This will allow a sufficient layer of insulation to be blown or installed (if you decide to use rolled fiberglass batts) under the attic floor decking while still providing an adequate thermal barrier between the attic floor and the living space below.
Regarding types of insulation, you have rolled fiberglass batts, blown fiberglass, blown cellulose, and others. I haven't done enough research to give an opinion on which is better, but from what I have seen so far, it looks like cellulose insulation outperforms fiberglass in a blown-in application. You will have to decide which is best for your particular application.
One item that you should check and insulate if necessary: your HVAC ductwork. Ducting can be made out of duct board or it could be flex duct. Either way, it should be well insulated. Insulating the ductwork insures that the air exiting the vents is as close as possible to the temperature the air was when it passed over the evaporator coils (or the heat exchanger). Ultimately, delivering colder air means the living space cools faster and the air conditioner runs for a shorter period of time. Many times ducting will have an R value printed on it, but sometimes it won't. If it doesn't and looks or feels relatively thin, you should consider adding more insulation or replacing the ductwork with ducting that is insulated.
Keep in mind, though, that no matter how much insulation you have blown in, eventually its effeciency will degrade. Over time, not even lightweight insulation can escape the effects of gravity, and the insulation will settle. This in turn reduces the R value and effectiveness of the insulation. Also, if your attic is entered regularly, it is likely that some of it has been compacted. Periodically, you need to check the insulation and supplement the layer if necessary, especially if you've been in the house for some years.
Putting It All Together
To make an efficient attic, it must breathe, it must reflect radiant heat, and it must have a barrier to the conditioned living space. The three-pronged approach is extremely important to an efficient attic. Will one or two items done separately help? Absolutely. But for maximum efficiency, all three should be completed. Don't forget, though, that you should do as much work in the attic as possible before insulation is installed. This includes any ductwork insulation or repairs and sealing the home envelope so as to minimize the amount of conditioned air that escapes into the attic.