There has bee a lot of talk about this very topic here on Hometalk and in the industry as a whole.
A few things to remember about this product. Yes it will lower some of the radiant heat entering into the living area of the house, but there are some serious drawbacks to this product that they often do not tell you about.
With any reflective surface the heat is reflected back out of the space. Depending upon where you place this material will determine what if any issues you may have as a result. The heat must be able to be removed once its reflected away from the living side of the home. So if your attic is lacking in proper ventilation that is one thing you will need to address first.
The 2nd thing you need to be aware of is, dust. As dust begins to collect on the shiny side of this material its ability to reflect heat goes down. So you need to ask yourself, is my attic dusty? Will it become more dusty if I add more ventilation to remove the heat gain that results with this product? And how if at all am I going to be able to clean it if it does.
The 3rd thing you need to understand is that no two reflective radiant barriers are the same. Meaning does or does it not have vapor transferable qualities? What I am getting at is moisture. If you take a tin foil sheet and place it over a warm container of food and put it in fridge, what happens when you remove the foil later for that late night snack? You find moisture trapped on the back side of the foil right? This can also happen in your attic as well. Some foil barriers have what they refer to as micro holes. These little perforations in the barrier allow moisture to transmit through the surface of the barrier. Or so they claim. Try taking a toothpick and punch holes in your warm container of food in the fridge. What do you think is going to happen? Correct your still going to have moisture trapped on the back side of the foil. So what does that tell us. These little micro holes as they say will do little to nothing to allow moisture to travel out of the attic where it will do no harm. And lets say just for the heck of it that they do work. How long do you think these holes will remain open once dust gets on the surface of the foil barrier.
I am not trying to talk you out of this product. In theory it works really well. But you must do a lot of research on this, both on the product side and the installation location side.
Ideally in my opinion you would be much better served if you air sealed your attic and basement/crawl space if you have one. Then foam your underside of your attic to bring the attic into the living space of the house. this assumes your AC system is located in the attic of the house. If not, foam the floor area instead and increase your ventilation rate in the attic.
You may spend a lot more money doing this, but your energy savings will go way up as compared to just removing the heating load with the shiny film system.
Hard to add much to Woodbridge's essay, but here is some more info:
Opinions are all over the board on radiant barriers but Woodbridge is as comprehensive an answer as you can get. There are similar issues and concerns with spray foam & urethane type products. Get as many opinions as possible in your local market.
Well I have been thinking about this topic for quite awhile now. And I had submitted my feelings and questions to several folks who know a whole lot more then me. I just got a response back from on of my cohorts about this very topic which makes me even more concerned about using these foil systems in an attic. As an inspector and home improvement contractor I cannot tell you how many times I see exposed or improperly wired attics.
This link provided gets to some really big engineering stuff, but the few pages in the beginning will provide some very useful information to all that are considering this type of improvement in their home. All of which makes a lot of sense. For some reason it brings you to the middle of the topic so scroll to the top to start reading.