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Acoustic Underlayment: Which Type Is Best?
What Is Acoustic Underlayment?
Acoustic underlayment is used anywhere a higher level of isolation is required in both STC ratings and IIC ratings. The materials used in acoustic underlayment are usually recycled rubber, cork, a combination of rubber and cork, MLV, foam, and a combination of MLV and foam. Other underlayment includes adding Green Glue between the existing sub-floor and a new layer of sub-floor or a "mass-based" rigid underlayment usually made of concrete type materials. The higher performing underlayment (rubber or a combination of rubber and cork) are usually required in multi-family housing and other commercial applications to achieve a field test IIC rating of 45+ and a lab test IIC rating of 50+. Most condominium structures and large commercial buildings use thick concrete floors and steel framing so these rating requirements are achievable with an underlayment of minimal thickness (typically around 2mm) depending on the flooring used on top of the underlayment. For example, tile floors will require a thicker underlayment as they will generate considerably more noise than a vinyl floor or even a hardwood laminate.
Different Ratings For Floors
STC ratings largely cover the higher frequencies like speech, radio, and sounds from the TV (not including sub-woofers). The IIC rating is a way to calculate the level of isolation in relation to impact noise or structure-borne noise which would include footfall, a chair dragging along the floor, or other common noises typically caused by every day living. The IIC rating is important because a floor can have a sufficient STC rating, say in the low 50's, but won't do much at all for isolating impact noise. Or in the case of carpet pad, a product can have a high IIC rating, but provide no barrier for airborne noise. With that said, IIC ratings can be very misleading in most cases because it is a rating for the entire system and not just for the material added to the current construction. Most IIC tests include several inches of concrete, a layer of Gyp-Crete or similar, and usually an acoustically rated ceiling on the underside including either steel stud framing or resilient channel.
So the solution for this dilemma, to cut through the bogus claims of 70+ IIC is to find out the Delta IIC rating. The Delta IIC rating, or usually written as ΔIIC, is a rating for what the material truly isolates on its own without the contributions of concrete, sound rated ceilings, and other common products. Like every sound rating, the Delta IIC can be misleading to some extent as well because performance of a product on a concrete sub-floor will vary from the performance of a product on a wood floor. But it is a rating that cuts through most the fluff claims you will find online.
Sorting Through Bogus Sound Ratings
Unless you can verify the materials used in a test then you can either trust the person trying to make the sale on the product or educate yourself on what type of materials and methods can actually isolate sound. For walls and ceilings, the best way to isolate sound is to decouple the room you are in from the existing structure as much as possible. The same is true for floors, but the cost of doing this in floors, at least properly, can be ridiculously expensive. The next best option is to dampen the connection between the flooring and the sub-floor. To do this you really only have two options. You can either use a resilient underlayment or add a layer of sub-floor to the existing assembly with a damping glue between the existing sub-floor and the new layer of sub-floor.
Using Rubber Underlayment
Solid rubber is without a doubt the most resilient underlayment available. In fact, 2mm of rubber underlayment is equal to 6mm of the common underlayment cork. Rubber underlayment usually comes in 4' wide rolls in either 25', 30', 50', or 75' lengths and 2mm, 3mm, 5mm, 6mm, 8mm, 10mm, and 12mm. The advantage rubber underlayment has over Green Glue Compound is that it can be used on wood floors, concrete floors, wherever, and installation only involves rolling out the underlayment and taping the seams together. You can glue it down, but you definitely don't have to and really you shouldn't glue it down if you are using a floating floor on top of the underlayment.
Acoustical underlayment manufactured from sustainable recycled rubber composite offers optimal sound reduction while remaining impervious to the elements for life. Unlike natural cork, recycled rubber remains permanently resilient and will not embrittle with air exposure. Recycled rubber underlayment can be used with most all wood flooring installations, including glued down, nailed down and stapled down. It is also specified for use beneath ceramic tile, cork floors, carpets and a variety of resilient sheet and tile floors. As is the case with acoustic cork, whenever you are installing flooring by a method other than free-floating, the underlayment must be attached to the sub-floor as opposed to simply laying it down.
Using Green Glue Compound To Dampen
Adding a second layer of sub-floor with a damping glue like Green Glue Compound will definitely provide some great results. The only problem is that installation can be very complicated and limited to certain floor constructions. One of the limitations is that you can only properly add a layer of sub-floor and Green Glue Compound to the existing floor if the existing floor is wood rather than concrete. This is because the new layer of sub-floor will need to be screwed into the existing layer so that the Green Glue Compound can properly compress between the two layers of sub-floor.
Another limitation of using Green Glue Compound and an extra layer of sub-floor is simply the labor of installing another layer of sub-floor. It really isn't much of a DIY project, at least not for a novice, and can be pretty close to impossible to accomplish in a retrofit situation if you don't have the space to setup a table saw to cut the wood to fit your existing floor. The added thickness of a layer of sub-floor can also cause issues with clearance under doors. So adding a layer of sub-floor and using Green Glue Compound is a great option and can be very affordable in comparison to rubber underlayment, but only if all the labor is done yourself or free by someone else.
Using Cork Underlayment
True acoustic cork underlayment will provide a decent level of sound reduction as long as the cork used is 6mm or thicker. Acoustic cork underlayment is spec'd for use under all types of flooring including glued, nailed, and stapled down installations. With some flooring types (typically hardwoods and hardwood laminates) you may need to install a moisture barrier under the cork. Most manufacturers will accept a 6mil plastic sheet as a moisture barrier. Be sure to consult the manufacturer before installing your flooring over cork without a moisture barrier.
One advantage to cork is that it is a very affordable build-up material that can be used to create level transitions from tile, hardwood, and carpeted surfaces. As with rubber underlayment, whenever you are installing flooring by a method other than free-floating, the underlayment must be glued to the sub-floor instead of utilizing a free floating installation of the underlayment.
Using Other Underlayment
A basic alternative to traditional underlayment is mass loaded vinyl. Our 1 LB TotalMass Barrier will provide a noticeable improvement to any floor with a pretty minimal overall cost. Our 2 LB TotalMass Barrier will perform better with airborne noise than our QuietGround 250 and very similar to the QG 250 in terms of the IIC rating. Beyond that, any composite MLV underlayment is typically an over-hyped alternative to rubber underlayment. Open and closed cell foam, recycled insulation fibers, thin crumb rubber, whatever a manufacturer wants to attach to the bottom of the MLV, the main performing component will always be the MLV. The added cost of the additional attached product is just for higher profit margin and/or an attempt to have a different product line than their competitors.
The only effective way to isolate impact noise and reduce sound transmission between floors is to use an underlayment that can properly dampen. Rubber underlayment will always perform better than the rest and also has several other advantages in regards to longevity and providing a quality moisture barrier. With that said, no underlayment exists that can achieve a rating well into the 60's and 70's for either STC or IIC without the aid of another product (i.e. 8" concrete, resilient channel on the ceiling below, or a Gyp-Crete type product over the underlayment).