How to Soundproof Duplex Walls and Floors

A friend of mine recently moved into a duplex and was surprised at the amount of noise that came from their neighbor’s apartment. They asked me how to soundproof duplex walls and floors so I came up with a few solutions:

  1. Decouple the walls
  2. Use resilient clips and hat channels
  3. Add mass to the walls
  4. Build a floating floor
  5. Use acoustic underlayment
  6. Lay carpet
  7. Use sound damping materials

How to Soundproof Duplex Walls and Floors

I’ll go over these in more detail below, but first it’s worth going over the type of noise pollution you’ll typically deal with when living in a duplex.

What Kind of Noise Pollution Will You Find in a Duplex?

As you may already know, a duplex is a residential home divided into 2 apartments. While this offers convenience in terms of living space, it also means having to deal with a neighbor’s noise.

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Although the specific noise pollution will vary depending on the neighbor, it can generally be broken down into 2 categories:

  1. Impact noise
  2. Airborne noise

Sounds are pressure waves that cause vibrations, either through air or through solid objects.

The pressure of the sound wave causes particles to vibrate, which the ear registers as sound.

Depending on the noise source and objects between you and it, you’ll either hear the sound as impact noise or airborne noise. They can be defined as follows:

Impact noise is when an object makes direct contact with a surface, producing vibrations that pass through the surface and come out the other side as sound. Common examples include:

  • Footsteps
  • Furniture being moved
  • Something hitting a wall (such as hammering in a nail)

Impact noises require their own soundproofing solutions, as I’ll cover below.

Airborne noise, however, is sounds that transmit through the air. While you might hear them through a wall, they’re not made by direct contact with the surface. Examples include:

  • TV or music
  • People talking
  • Dogs barking

As airborne noises aren’t made through direct contact with a surface, they have less energy when they make contact with it (such as a partition wall between apartments). This is why they often sound muffled when they reach you.

Airborne noises require slightly different soundproofing solutions, although there is some crossover between the 2.

You might also experience flanking noise in your duplex, which is simply indirect sound transfer. For example, noise traveling from your neighbor’s apartment into yours through the HVAC vent would be flanking noise.

When it comes to soundproofing a duplex, you’ll want to try and block both airborne and impact noises. Luckily this is possible with some fairly straightforward options.

Preparation for Soundproofing Your Duplex

Preparation for Soundproofing Your Duplex

Before you start your soundproofing project, it’s worth thinking about what you’re working against.

For example, are you soundproofing against your neighbor, against street noise, or both?

It’s worth considering this because it’ll impact where you apply your soundproofing materials, and how much you use.

If you’re only working against street noise, you only really need to soundproof the street-facing part of your apartment. The same is true for soundproofing against your neighbors.

That said, if you want the best results, it might be worth soundproofing all the walls, the floor, and even the ceiling if you’re in a downstairs apartment.

How to Soundproof Duplex Walls 

Soundproofing a duplex isn’t a complicated project, but you should be prepared to get your hands dirty with a bit of DIY. You’ll have much greater success if you’re willing to do a bit of gentle remodeling.

Below are my top tips for soundproofing both walls and floors, divided into the relevant sections.

1. Decouple the walls

If your duplex has partition walls made from studs and drywall, there’s plenty you can do to reduce sound transmission. My first suggestion would be to decouple the walls.

This is quite invasive and one of the most resource-heavy options, but it’ll do a great job.

Decoupling is also known as mechanical isolation and involves separating the 2 sides of drywall so vibrations can’t pass all the way through the structure.

In short, you’ll build a new set of wall studs for each side of the drywall. You can either do:

Double stud walls involve a set of studs for each side of drywall, often in line with each other. There’s a small cavity between the 2 for better isolation.

Staggered stud walls still involve 2 sets of studs but in a staggered pattern. While they’re isolated, there’s less room between the 2.

Be sure to fill the wall cavity with mineral wool insulation (Amazon link), as this is great for absorbing and damping sounds.

Also, use Green Glue for sealing and sticking. It’s acoustic caulk and converts sound waves into heat. It makes a surprising difference to the reliability of your structure.

You can watch this video on how to decouple a wall for more visual information.

Sound Proof Shed IV - Decoupled Walls
Watch this video on YouTube.

While this is easily the most effective way to soundproof a wall, it’s also the most invasive and resource-heavy.

What’s more, it takes up a lot of room because you’re basically building 2 separate walls with space in between.

2. Use resilient clips and hat channels

Possibly a better option for duplex walls is to use resilient clips and hat channels (Amazon links).

These offer almost the same results because they isolate one side of drywall from the studs. The resilient clips have rubber feet that absorb some of the vibrations too.

Rather than completely isolating both sides, they provide the isolated side with some flexibility so it can move with the vibrations rather than transit them.

I’d recommend this option for a duplex because it’ll take up much less space and will still offer some good results.

Follow these steps:

  1. Remove the old drywall to expose the wall studs.
  2. If the wall cavity is empty, now is a good time to add some insulation. Again, use mineral wool.
  3. Fix the resilient clips to the studs using screws.
  4. Fit the hat channels into the resilient clips.
  5. Mount the new drywall onto the hat channels using screws.
  6. Seal any gaps with Green Glue (Amazon link).

This is a pretty straightforward process but it delivers good results. Also, it’s effective against airborne and impact noise.

The benefit of using mineral wool insulation in the air cavity is that it’s an effective sound damper, particularly against airborne noise.

While impact noises will mainly travel through the structure itself (the wall studs), airborne noise will travel through the drywall into the cavity.

As this cavity is usually empty, it’s the perfect space for sound waves to bounce around and amplify, making them sound louder than they are.

Mineral wool is effective against this because it has an open fibrous structure. It traps sound waves, which expend their energy moving the fibers. This energy is then converted to heat and is considered to be “absorbed”.

If you’ve got a bit of time, I’d highly recommend this option for your duplex walls. It’ll make a massive difference and isn’t as invasive as full decoupling.

3. Add mass to the walls

Mass is one of the easiest and most effective ways to soundproof a space. Simply put, more mass means a structure is more difficult to vibrate. This makes it more difficult for sound waves to transmit through the structure.

Drywall is fine for construction but is thin and fairly lightweight. Adding more mass to the wall with therefore reduce noise transmission, particularly for impact noises.

Although you can use anything heavy, I’d recommend either:

Both of these products are designed for soundproofing and are dense mass, meaning they won’t take up much space.

Mass loaded vinyl comes in various weights, but I’d recommend using the 2lb per square foot version for best results.

The main advantage of sound deadening mats is that they have a self-adhesive backing, although both options will be great for this project.

Simply apply the product to the walls using Green Glue or another acoustic caulk. While you can use screws or nails, they can compromise the material’s effectiveness.

If you’re trying one of the remodeling options above, I’d recommend fixing the soundproofing material to the back of the drywall.

It makes no difference which side it’s on, but putting it on the back means you don’t have to look at it, as it’s not particularly attractive.

However, if you’re not, then simply fix another layer of drywall over the top so you have an easier surface to decorate.

How to Soundproof Duplex Floors

Soundproofing the floor in your duplex follows many of the same processes listed above, but with one notable difference.

Whereas you might have previously been soundproofing against impact noise transmission, it’s worth considering here about the impact noises you’ll make in your apartment.

Cutting them off at the source using sound damping materials will make a big difference. Follow the tips below.

1. Build a floating floor

A floating floor is the same as a decoupled wall. In short, you’re isolating the floor from the rest of the structure.

If you’re able to pull up the floor joists, the best option is to fit some floor floaters (Amazon link).

These are little rubber U-shapes that absorb vibrations before they pass into the building’s structure.

Once they’re installed, you can go ahead and relay the floor, but make sure you insulate the floor cavity first.

It’s also worth laying some sound deadening materials either under the floorboards or the main floor surface, but I’ll cover that in more detail below.

2. Use acoustic underlayment

acoustic underlayment

Regardless of whether your floor is carpet or wood, it’s a good idea to lay some acoustic underlayment beneath it.

You can either buy specific acoustic underlayment or use cork sheets.

Acoustic underlayment follows the same principles as mass loaded vinyl: it’s dense and mass rich, meaning it reduces vibration and adds a layer of sound damping.

But cork is just as good at damping sound, and is much less expensive. You can realistically use either for soundproofing the floor.

3. Lay carpet

When it comes to a floor covering for a soundproofing project, I’d highly recommend using carpet.

It’s a fairly effective sound damping material and will reduce the level of noise made by footsteps at the source.

In turn, this’ll mean that any soundproofing you put underneath will be more effective at its job because there’s less noise to block or absorb.

The thicker the carpet the better, as you’ll want a fairly soft floor surface.

If you already have carpet and it’s not that thick, bulk it up with some rugs or other floor coverings. This also works on laminate and wooden floors but obviously won’t be as effective.

4. Use sound damping materials

If you can get away with ripping the floor up and insulating it, your next best option will be to lay some sound damping materials on top. This is particularly effective on hard floor surfaces.

The best thing I’d recommend is EVA foam gym tiles (Amazon link) or similar.

EVA foam is a good shock absorber, meaning it’s also good at reducing noise transmission. While this isn’t the material’s main purpose, it’s a handy side effect.

These are easy to source, but you could also use cork tiles or sheets instead. Hiding them under a rug or mat will make things look nicer too.

Bear in mind that this won’t deliver as good results as a floating floor or proper insulation, but it’ll make a difference if you’re limited in the level of construction work you can do.

Some Final Thoughts

I hope this guide has given you some useful information on how to soundproof duplex walls and floors.

The more mass and insulation you throw at the structure, the better your results will be. But that said; nothing beats proper decoupling. If you can justify doing this, then go ahead.