The idea is that this thin material can act as a type of insulation in tight spaces where you cannot put thick fiberglass insulation. There is no metal content; it is all plastic.
They are difficult to carry home, and if you have a car it's that much harder. Once you get them home, you have to suit up from head to toe to protect yourself from all of that itchy fiberglass.
Is there a better way? Does Reflectix offer up a reasonable substitute?
Not exactly: Reflextix is not a fiberglass alternative. It is a different kind of insulation.
The Fine Print and What It Means
Each of Reflectix's statements about R-value on the label are valid, but they must be accompanied by a reading of the finer print off to the side. The label says:
This is a Reflective Insulation. Note: The stated R-value is only valid for the application specified when installed per the installation instructions on the label or at our website...
For example, the R 6.1 masonry value is only good if you provide two 3/4" air spaces, one between Reflectix and the masonry wall and another space between the drywall and the Reflectix.
Reflectix's stated R-14 R values for exterior walls are good only if the Reflectix is paired up with R-13 fiberglass batt insulation.
Air Space Equals R-Value
In fact, if you don't provide an air space, this product provides almost no insulating value. The rubber meets the road on Reflectix's website, where they say
No Air Space = No Reflective Insulation Benefit
(An R-1.1 is provided from the product itself for the Reflective/Double Bubble material.)
If you've got air space, why laboriously suspend a shiny bubble wrap in the center of it when you can just fill it with far more effective fiberglass batt insulation?
That is a question you will need to answer yourself.
Where It Helps
In some cases, Reflectix squarely falls into the category of better-than-nothing. You might have a cold workshop with heat escaping through the roof. Stapling a layer of Reflectix along the rafters will cut down on some heat loss, but not as well as would fiberglass batt. Yet if you can't stand the idea of wrestling with fiberglass batt over your head, Reflectix is an alternative. Also, its reflective surface would give your shop more ambient light.
Also, it can help with any areas where cold air is blowing into your house. While caulking is the recommended procedure for this kind of thing, there may be situations where a large expanse of Reflectix will stop airflow.
Buy or Not?
In the end, Reflectix is exactly what it says it is. All of the information about Reflectix is laid out on the label, and their site is highly informative.
You would never use it for wall cavity insulation--why spend your time tacking this stuff to the center of your wall studs? But you might wrap pipes with it or use it for less-critical areas, like shops.
It's popular for uses outside of the home, too. People use it in trailers or for camping or to insulate the insides of coolers.
The key to using Reflectix properly is that you should take a little time to understand how reflective insulation works in the first place. The Reflectix site provides a great range of information about this, and about how it differs from other types of insulation. Michael Tokarski of Reflectix likens his product to a Thermos jug, in that it reflects heat back rather than a mass insulation such as fiberglass.