How To Soundproof A Floor

Flooring

Similar to wall soundproofing the sound traveling through floors is dependent on structure-borne and airborne noise. These two transmission mechanisms should be addressed in tandem to ensure that the maximum amount of noise transfer is attenuated. Unlike walls, flooring is often-times a major contributor for structure-borne noise, due to the constant footfall or impact imparted on the floor from moving objects (unless of course, someone is hammering into the wall.) This is particularly problematic in high-rise condos and apartments, and sometimes even in 2-3 story homes.

First, evaluate the flooring system to determine the primary paths of sound transfer. Once identified, we always address the “low hanging fruit” first – those elements that are fixed in the most cost effective manner. Finally, determine the amount of sound-blocking you need in the floor, and develop a budget that will allow you to meet those goals. Solutions range from decoupling joists from subfloors, adding soundproofing layers below the top floor, and even using localized isolation pads for loud speakers and dancing areas.

There are 5 primary layers in any floor, as well as two primary contact points, show in the diagram below,:

floor-layers

  1. Floor Supports – The rafters or joists running within a floor to support it from caving in or sagging when a load is applied. These are often 2×4, 2×6, or even 2×8 boards of construction lumber, installed so that their greater dimension is vertical to more effectively combat bending moments.
  2. Substrates – also known as a subfloor, this is the layer installed directly on to the underlying floor supports. It is not the aesthetic layer that you can see when walking into a room, but provides support and additional insulation. The most common substrates are plywood or concrete flooring – with plywood used often in residential or small-commercial applications, and concrete used for large commercial and industrial applications, and grade-level flooring. Other substrates include particle board or chipboard (underlayment grade), vinyl ceramic tile (VCT), cement backer board, or even gypsum and terrazzo (stone & glass composite)
  3. Acoustic Layer – the acoustic layer is sometimes included in flooring, especially when there are sound-blocking or absorption requirements. These may range from cork board and carpet cushions to specialized acoustical sound-blocking materials (including our Floor Blocker). While cork board may be effective at reducing some vibration, acoustic floor underlayments provide sound-blocking for structure-borne and airborne noises.
  4. Top Flooring – the top flooring layer is the aesthetic and functional layer that you see when entering a room. This can range from wood (hardwood, engineered wood, parquet mosaic, or bamboo) to concrete, carpet, or tile.

The performance of any flooring assembly depends on the interaction and makeup of each of these layers, and the contact points between them.

There is a contact point between each layer. With 4 layers, that leaves three primary contact points (we have consolidated the substrate-acoustic underlayment-top flooring below), listed as follows.

  1. Floor Support-Substrate Contact: the substrate is typically screwed directly into the supports, ensuring a structural contact directly between the two layers. This contact allows structure-borne noises to flow directly through the assembly, and may be minimized by using a non-hardening caulk or strips of acoustical underlayment between them.
  2. Substrate-Top Flooring Contact: There is also an opportunity to decouple the connection between the substrate and top flooring layers, via use of non-hardening caulk or thin-set adhesive membranes (when the top floor can be adhered, such as in the case of hardwood flooring or tiles).

Floors have a number of unique performance characteristics from other construction elements, implicit in their function to support weight above them. These include:

  1. Thermal: act as a temperature gradient between the room and underlying space
  2. Moisture: this is critical on the 1st floor or at grade-level. Non-permeable, moisture-resistant materials should be used to ensure moisture does not seep into the living space. The primary difference between the Floor Blocker and other wall and ceiling sound-blocking materials is its moisture-resistant properties.
  3. Acoustics: as mentioned below, flooring not only provides a substantial STC from tenants above and below, but is also responsible for IIC (structure-borne). Unlike walls and ceilings, carpeted flooring can also significantly contribute to absorption in an open space.
  4. Structural Support: As we’ve already discussed, the flooring also needs to support weight from above, evenly and uniformly, without excessive deflection. To reduce creaking below, it’s critical to ensure the floor is level (i.e. less than 3/16” vertical change every 10’). Acoustic underlayment or non-hardening caulk may be used to “shim” the floor to approach this level criteria. Furthermore, plywood and other underlayment should be “scored” or “kerfed” during install, to release tension and prevent warping.

Generally speaking, when looking to improve the sound-blocking capability of a floor, we are looking to improve its ability to block structure-borne noises (shown in its IIC rating) and air-borne noises (shown in its STC rating). Floor are unique in that IIC is more important than most other structural elements. This is due to frequent impact noises they endure, from footfall and support moving objects (unlike most walls and ceilings, that do not receive regular impact). To improve the STC of the floor, we increase the mass of the floor by adding additional, functional layers, and the IIC by decoupling adjacent layers and adding impact-resistant materials.

Approach each layer independently, adding thickness to plywood, gypsum board and concrete wherever possible. Add decoupling between joists and subfloor, and then between the subfloor and top floor. Finally, add a reliable acoustic underlayment that will increase both the IIC and STC of the floor. Acoustic underlayment should be installed under framing (if necessary) on the floor above, and should completely cover the floor so that paths of least resistance do not develop.

A typical IIC for high-rise construction is 50, meaning that the floor reduces structure-borne noise by 50 dB on average. By applying the Floor Blokker, decoupling joists and sub-floors, and using other techniques discussed above, you can achieve and exceed IIC values of 60 or more.

Acoustics Expert

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