Under the “Myths and Misconceptions,” you say it is fine to install reflective insulation over existing mass material. I assume this means the rolled fiberglass I have in the attic now. I was reading somewhere else that applying it on top of the fiberglass can trap moisture and it is not recommended. I do understand, and you as well as another source, say dust can render it ineffective. My house is 34 years old and I want to add insulation to the existing insulation layer. The attic is tight – you definitely can’t stand up. Is a radiant or reflective the best type for me? It is a 1400 sq. ft. house including the garage. A salesman was at the house today and showed me a sample of his product. It was extremely thin. Home Depot has rolls that look like foil bubble wrap. Just how many different reflective barrier products are there?
Installing a radiant barrier on top of the mass in the attic floor should only be done in cases that won’t allow installation on the roof rafters. If that is the only choice, then it is very important that you use a radiant barrier that is perforated so that moisture that collects in the mass can escape to the attic air. If there are AC ducts in the attic, the radiant barrier should cover them.
As to the dust issue, over time dust can accumulate and this will reduce the effectiveness of the radiant barrier. Yours is an older home, so you should be able to determine just how dusty the attic is. It could take years for enough dust to accumulate to seriously affect the radiant barrier performance – or it could happen much sooner. It all depends upon the environment and your visual inspection should be your best judge. Be sure to order a radiant barrier with both surfaces being reflective. That way, if after several years there is evidence of a significant amount of dust you can turn the radiant barrier over and have a clean reflective surface again.
The point of a radiant barrier is to reduce energy costs mainly in the hot months and provide better comfort in the living space. You need to weigh those benefits against the cost and possible trouble of dealing with dust. Many people have had radiant barrier installed on the attic floor for a number of years with no significant decrease in their perceived performance.