There is some debate in the radiant barrier business whether to use a perforated or a solid radiant barrier product in an attic application.
In an attic application, you should use a perforated product.
In other situations, it can depend on your specific application, but in an attic that is vented you ALWAYS want to use a perforated product.
Why? Because you want dry walls and ceilings and the perforations allow for moisture to flow freely and escape/dry out. Over 70% (that's 7 out of 10) of home issues are due to mold, mildew, rot, decay, etc. and moisture is the common theme.
Dry products don't grow mold, rot or decay; therefore, moisture in wall and ceiling assemblies is not a good thing. This is why in an attic you would choose a perforated product (that allows moisture vapor to pass through freely) over a solid product (that traps moisture behind it).
AtticFoil's perforated product is the main product we sell. We also sell a SOLID non-perforated VAPOR BARRIER version of our standard perforated radiant barrier foil. The perm rating is simply a way to measure the diffusion of water vapor (moisture) through a material. The higher the perm rating, the more vapor/moisture can escape. The lower the perm rating the less amount of vapor/moisture can escape. In a vented attic you want a perforated product - one that allows moisture to pass through freely and easily, so that it can evaporate and dry out. Even at a perm rating of 14, AtticFoil®'s perforations are still small enough that in some cases it's practically an air barrier. This just means the perforations (pin holes) are small enough that they don't decrease the foil's effectiveness, but large enough to allow moisture to easily pass though and dry out.
When you are dealing with a low perm rating, your main concern is trapping moisture; any material with a perm rating of 1.0 or less is considered a vapor retarder (meaning it doesn't allow vapor to pass though). This is of concern especially when installing over the insulation, as you could end up trapping moisture under the radiant barrier foil if you do not have a perforated product; so we urge you to look at the fine print and make sure you know what you are purchasing!
Bottom line: in any application that has the potential for moisture accrual (like a vented attic space), you want to use a perforated product, NOT a solid/vapor barrier.