If you look at a duct wrap insulation data sheet, you may notice that there are two different R-Values listed: Installed R-Value and Out-of-Package R-Value. These two R-Values reflect the difference in thermal performance that occurs when you compress the insulation during installation (installed R-value) from the thermal performance of the insulation directly out of the package (out-of-package R-value).
The installed R-Values on JM’s duct wrap data sheets reflect the thermal performance of material that has been compressed by 25%, the maximum allowable compression during installation. Anything beyond that hinders the materials’ thermal performance by over-compressing the insulation. The more compressed the insulation gets, the lower its R-value becomes and the less effective it will be as a thermal insulator. The out-of-package R-value correlates to the material’s labeled thickness.
Compression of the duct wrap happens naturally during installation as the insulation is wrapped around the duct. A good installer will ensure that the entire duct has insulation flush against the metal without any air gaps between the insulation and the surface of the metal duct. Additionally, they will gauge the compression of the insulation to ensure that it’s not wrapped too tightly and being over-compressed.
Since it can often be difficult to gauge what 25% compression looks like during an actual installation, data sheets will often have measurements called “stretch-out” tables, like the one shown below. These allow you to determine how much insulation you need to cut to wrap a specific duct size while avoiding unnecessary compression.
Duct Wrap Stretch-out Table (Stretch-outs include 2" (51mm)overlap. P = Perimeter of duct)
| Labeled Thickness (in) | Installed Compressed Thickness | Round | Square | Rectangular |
| 1 | 0.75 | P+7.0" | P+6.0" | P+5.0" |
| 1.5 | 1.125 | P+9.5" | P+8.0" | P+7.0" |
| 2 | 1.50 | P+12.0" | P+10.0" | P+8.0" |
| 2.2 | 1.65 | P+13.0" | P+11.0" | P+8.5" |
| 3 | 2.25 | P+17.0" | P+14.5" | P+11.5" |
| 4.4 | 3.30 | P+24.5" | P+21.0" | P+19.0" |
In this stretch-out table, P stands for the measurement of the perimeter of the duct. Add the suggested amount of insulation to the total measurement of the perimeter of the duct, and you will have sufficient insulation to wrap the entirety of the duct with 25% compression or less. Note that different duct configurations (round, square, and rectangular) require different amounts of insulation to achieve 25% compression, and the less compression you have, the better the thermal performance will be.
In addition to compressing the insulation, the installed R-value can also be impacted by the insulation’s density. At JM, we offer three “types” of insulation densities: Type 75, Type 100 and Type 150. Type 75 has a density of three-quarter pounds per cubic foot, Type 100 has a density of 1 pound per cubic foot, and Type 150 has a density of 1.5 pounds per cubic foot. While many people feel that the denser insulations (Type 100 and Type 150) will offer much more robust thermal performance, the truth is, while there is a slight increase in R-value, the bigger benefit is the handleability of the material. Density is most influential during the installation; the greater density offers a more robust material that can help prevent installers from over-compressing the insulation at the corners.
As a reminder, it is important to use outward clinching staples when installing duct wrap insulation. This helps ensure that the staple doesn’t shoot through the insulation and create a hole or a void that can hinder thermal performance, damage the vapor barrier, or create a potential condensation hazard. Outward clinching staples grip the facing to secure the material in place. Installers should tape or seal the staple afterward to maintain the integrity of the vapor barrier.
To learn more about Johns Manville’s duct wrap offerings, including our brand new Microlite Black PSK Duct wrap visit www.jm.com/ductwrap.