The IsoJoist, commonly referred to as a joist or stud isolator or u-boat, is a two inch wide piece of EPDM rubber u-channel designed to soften the connection between a newly framed 2x floor and an existing sub-floor. The most common applications are for creating isolated platforms, home theater risers, recording studios, vocal booths, and sometimes entire rooms. We do not recommend the use of the IsoJoist in any wall or ceiling application. To decouple walls or ceilings we recommend the use of resilient clips or specialty framing.
For higher deflection, damping, and less resonance issues, we recommend isolation floors be built using a combination of either the thicker styles of GenieMat RST or GenieMat FF Roll-Out, several mass layers above or below the GenieMat, and Green Glue Compound between the layers of mass.
With 16" spacing of both the joists and IsoJoist, the average floor will use 1 IsoJoist per square foot. An IsoJoist is required within 6” of each end of the joist for support which may increase the number of IsoJoists required, depending on the dimensions of the floor. A scale drawing of your floor plan is the best way to determine the exact number of IsoJoists required.
The IsoJoist pressure fits into 2x framing without the aid of additional fasteners or adhesives. The joists should be spaced 16" on center with 16" on center spacing between each IsoJoist. An IsoJoist is also required within 6" of each end of the joist for support. Once the IsoJoists and framing are set, insulate the new joist cavity with basic fiberglass insulation sized to fit (R-13 for 2x4, R-19 for 2x6) and attach a minimum of two 1/2" layers of plywood or similar mass to the new floor. Applying Green Glue Compound between these two layers of plywood will help immensely with damping the new floor.
Walls cannot be built on top of a floating floor created using IsoJoists.
All u-channel joist isolators like our IsoJoist and other name brand versions, have limitations in performance because of how the material is manufactured, and the specs required for manufacturing. The ideal Shore A durometer for rubber isolators is in the 35 to 45 range. The u-channel requires a higher Shore A durometer (firmer rubber). This means that u-channel rubber is not really the most ideal material for floor isolation because the dynamic stiffness of the rubber is too high. Isolation in the low to mid frequency range will be minimal with most isolation gains in the upper-mid to high frequency range.
Resonance issues are created in floating floors with u-channel. One resonance issue is from the air cavity created over an existing sub-floor, commonly referred to as a retrofit installation or triple leaf. This problem is unavoidable with this type of floating floor, but the effects can be minimized by insulating the new joist cavity with basic fiberglass sized to fit. The other resonance issue can be avoided by maintaining 16” on center spacing of the joists and each IsoJoist as well as installing at least two 1/2” layers of plywood or similar mass over the isolated assembly. Without proper spacing, absorption, and significant mass over the IsoJoist floor, you will create what is essentially a drum for a floor.
Because of the issues listed above, we recommend the use of thick rubber underlayment in place of the IsoJoist or other u-channel products. The Shore A durometer of GenieMat RST is 40, an ideal softness rating for rubber isolation, and the thickness of the RST-12 is equal to the base of the IsoJoist. Also, an air cavity is not created when using rubber underlayment, so the resonance issues described above are avoided.
IsoJoist Product Specs PDF 230KB
||2-1/8" wide x 2" long x 1-1/4" tall
||1-1/2" wide with base height of 1/2"
||750 pounds per piece
|Shore A Durometer
||-65°F to 250+°F