Reflectix Foil Insulation Overview

R-values, best uses, and where to use it

Reflectix foil insulation

The Spruce / Jason Donnelly

When you're shopping for insulation, those giant bales of fiberglass, rigid foam, mineral wool, or denim can look daunting on the shelves of the home center. So, when you see a thin compact product like Reflectix, it might seem like the easy answer to your insulation issues, especially if you don't have a truck to carry those large bales back home.

Not only is Reflectix clean, neat, and tidy, but the product literature also states that Reflectix offers thermal-blocking R-values of up to R-21. With fiberglass insulation R-values ranging from R-15 to R-19 for two-by-four wall systems, Reflectix appears to be a better insulating material than fiberglass or other thick, dense materials. Or is it?

What Reflectix Insulation Is

Reflectix is a brand of double-reflective insulation. Found in the insulation aisle, Reflectix comes in tightly spooled rolls of shiny, reflective, foil-look insulation that resembles bubble wrap. There is no metal content; it is all plastic.

Reflectix is a little thicker than a quarter-inch (5/16-inch). It's very lightweight (about 1-1/4 pounds per square foot). It's flexible enough to bend back on itself.

Reflectix is not a competitor to fiberglass. It's a different kind of insulation for different applications. You'll still want to install fiberglass, foam, denim, or mineral wood in your home's large wall and ceiling cavities. But Reflectix can supplement those types of dense insulation with reflective insulation to provide a more total, insulated home.

Reflectix can insulate tight spaces where you cannot put thick insulation or where other types of insulation are not appropriate.

Reflectix R-values

Since Reflectix is reflective insulation, the stated R-values on the Reflectix label are valid only when the insulation is applied in a certain way.

To begin with the highest R-value stated in Reflectix literature (R-21), that's two layers of Reflectix insulation under a radiant heating floor on wood joists. More specifically:

Two air spaces a minimum of 4.75”- one between the product and the bottom of the floor decking and the second between the two layers of product

The R-6.1 masonry value stated on the Reflectix label is applicable if you provide two 3/4-inch airspaces. One airspace must be 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 apply if the Reflectix is paired up with R-13 fiberglass batt insulation.

If you do not provide air space, Reflectix has less insulating value. Reflectix states this in its sales literature:

No Air Space = No Reflective Insulation Benefit. (An R-1.1 is provided from the product itself for the Reflective/Double Bubble material.)

Where to Use Reflectix

Reflectix lets you insulate spaces that might ordinarily go uninsulated because it would be prohibitive to insulate them with fiberglass.

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 and stop air infiltration. If you cannot or are unable to insulate the ceiling with fiberglass or foam, Reflectix is an alternative. As a nice byproduct, its reflective surface would give your shop more ambient light.

Reflectix can help with any areas where cold air is blowing into your house. While caulking is the recommended procedure for this, there may be situations where a large expanse of Reflectix will stop airflow.

Because Reflectix is so flexible, it can be wrapped around pipes and heating vents. If your local code allows, Reflectix can be wrapped around the water heater as a thermal blanket.

Some homeowners face the inside of their garage doors with Reflectix to help insulate these spaces for use as workshops.

Reflectix can act as housewrap, too. Behind brick, Reflectix has an R-value of R-3.4. Behind aluminum or vinyl siding, its R-value is R-2.5.

So, even though Reflectix functions best with airspaces, there are some applications where it is impossible to add airspaces, such as around pipes, vents, and water heaters.

Reflectix can be used as basement ceiling insulation when precisely stapled to the centers of the floor joists above. The product must be applied tightly, with room for airflow.

One benefit of using Reflectix over fiberglass insulation is that Reflectix resists mold and mildew well. If it does develop mold and mildew, it can be cleaned off. When fiberglass insulation becomes moldy, it must be discarded.

When Should You Buy Reflectix?

Reflectix can be used as wall cavity insulation—but tacking it to the centers of the studs in each cavity is a laborious process. Even if you managed to do this, the R-value of this system would hover around R-3.9. It makes no sense to use Reflectix where other products that are easier to install provide better insulating properties.

A 50-foot roll of Reflectix that covers 200 square feet costs around $120. Five rolls of R-13 fiberglass insulation also covering 200 square feet cost $150. The fiberglass would provide three times more insulation than Reflectix for just $20 more. Plus, installing the fiberglass would be much easier than installing the Reflectix.

The key to using Reflectix properly is to understand how reflective insulation works in the first place, and then sticking with the types of applications where it excels. The Reflectix site provides a great range of information about this and about how it differs from other types of insulation. The product is more like a Thermos jug, in that it reflects heat back, rather than mass insulation such as fiberglass.

Article Sources
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  1. Insulation. U.S. Department of Energy