Posted: 21st September 2022
When converting an existing building into residential flats, it is essential that you consider the sound insulation performance of the separating floors at an early stage. Getting this wrong can result in failed Building Regulations sound insulation tests, significantly increasing costs and putting back final completion. This design guide discusses the best ways to make sure your timber floors pass the tests first time. We’ve also highlighted common mistakes and pitfalls.
Sound insulation criteria for flats being converted from an existing building – technically termed “being formed by material change of use” – is detailed in The Building Regulations Approved Document E.
Table 0.1a requires the following performance is achieved.
|Airborne Sound Insulation
DnT,w + C’tr
|Impact Sound Insulation
|Floors and Stairs
It is important to remember that Building Regulations specifies the airborne sound insulation performance as a DnT,w + C’tr. DnT,w + C’tr is the performance that is achieved by a test on site. Most product manufacturers publish data as an Rw or Rw + C’tr. These are the performance tested in a laboratory. The Rw + C’tr for a timber floor is usually 7dB to 10dB higher than the equivalent DnT,w + C’tr, although this previous article discusses how that isn’t always the case and the correlation between Rw and DnT,w is variable.
What this means, is that manufacturer’s published data is very unlikely to be what you will achieve on your site.
Without doubt, the most effective way to improve the sound insulation of timber floors is to install an acoustic ceiling below. Because it uses generic building products (plasterboard and insulation), rather than specialist products, it is also usually the cheapest method.
The only time we’d recommend an alternative method to improve the sound insulation of a timber floor is if you don’t have the ceiling height to accommodate the acoustic ceiling.
There are various other methods to improve the sound insulation, some better than others. We’ve given a run through of the most common treatment types.
As mentioned above, an independent ceiling gives the best improvement to the sound insulation performance of an existing timber floor. This construction will pass Building Regulations sound tests, unless you have flanking noise problems. We discuss flanking noise below. There is very little that can go wrong, and even poor workmanship isn’t usually a problem.
If the ceiling is lath and plaster then this can remain so long as it complies with Building Regulations Approved Document B (Fire Safety). Otherwise, upgrade the existing ceiling to at least two layers of plasterboard (minimum 20kg/m2) with joints staggered. Then form the new ceiling from two layers of plasterboard (minimum 20kg/m2), using either independent timber joists or a metal frame system with acoustic hangers. The ceiling should create a void of 125mm or more. Lay 100mm mineral wool in the void. This mineral wool only needs to be 10kg/m3 density. This is a typical loft roll type – there is no need for expensive acoustic mineral wool. The existing floor is retained, and you do not need to do any work above the floor.
This system is specified as Floor Treatment 1 in Approved Document E. It will comfortably pass both the airborne and impact sound tests.
If desired, it is easy to improve the performance beyond the minimum standard of Building Regulations. Airborne sound insulation is improved by increasing the depth of the cavity or choosing higher density plasterboard (e.g. SoundBloc rather than standard WallBoard). Including a resilient layer above the floor will improve the impact sound transmission (noise from footsteps).
If ceiling heights or other restrictions mean that you can’t install an independent ceiling below, then you will need to look at alternative options.
Resilient timber battens with a floating floor above typically provide the highest performance of the other options available.
The exact specification of the floating floor varies for different manufacturers. Typically, these comprise plank plasterboard or a cementitious board with a chipboard deck above. Most resilient timber batten systems also require the ceiling to be replaced and a new plasterboard ceiling installed on resilient bars. Insulation is fitted between the joists.
Batten systems have the added benefit of allowing services to be run within the void below the floating floor. On the downside, the finished floor height is increased by around 80mm to 110mm. Work is also required to both sides of the separating floor.
The most common results when searching Google for acoustic flooring are acoustic overlay boards. There is a massive range of options from different manufacturers. Options range from 9mm thick MDF to 40+mm composite boards. Each overlay board is bonded to a resilient layer – typically foam or felt/fibre.
Most overlay board systems require existing ceilings to be removed and replaced with two layers of plasterboard on resilient bars. Insulation is fitted between the joists. Resilient flanking strips are installed around the perimeter of the floor. Usually, the resilient bars and mineral wool must be purchased from the overlay board manufacturer. This forms a complete system to ensure compliance with their published installation instructions.
Some of the thicker overlay board systems can achieve Building Regulations performance without having to replace the ceiling below, if installed exactly as the manufacturer’s instructions.
Although these systems are popular, they are dependent on correct workmanship. A contractor screwing down through the resilient layer or just one screw through the plasterboard in the wrong place and bridging the resilient bar can cause the sound test to fail. Generally, they are also more expensive than the independent ceiling option.
For this reason, unless no work can be carried out to below the ceiling then we recommend there are usually better alternative options.
The most common systems to improve the sound insulation of timber floors fall into the above categories. There are other options available from various manufacturers, including hybrid options. These have a plasterboard ceiling on acoustic hangers but also acoustic flooring above.
Building Regulations Approved Document E specifies a second system – Floor Treatment 2. This involves creating a floating mass floor on a 25mm thick dense Rockwool resilient layer over the existing floor. Mineral wool is laid between the joists. The existing ceiling will usually require an additional layer of plasterboard directly screwed below. The performance of this system usually only just complies with Building Regulations. It has many of the same concerns about workmanship as the overlay boards and is much more time consuming. For these reasons it’s rarely used.
Alongside contractors screwing through overlay boards or bridging resilient bars, the most common reason for a timber floor to fail the sound insulation tests is flanking noise via common cavity blockwork walls.
Acoustic energy gets into the wall, bypasses the floor, and radiates in the adjoining flat. If you have this scenario then it doesn’t matter what you’ve done to treat the floor, your tests will fail unless you control the flanking noise transmission.
Flanking sound occurs when the mass of the wall is not sufficient. Approved Document E advises that walls which are continuous between flats should have a mass per unit area greater than 375kg/m2. This equates to a 9” brick wall or dense concrete blocks laid flat. The inner leaf of a cavity wall has a mass significantly lower than this minimum level. Timber frame buildings don’t have this issue, as the internal leaf of the wall is broken at the line of the floor.
Modern buildings with blockwork cavity walls and timber joist floors are at significant risk of sound tests failing due to flanking noise.
If you have cavity blockwork walls and timber joist floors you will need to install independent wall linings to the flank walls. A suitable wall lining comprises two layers of plasterboard on independent timber or metal studs. Mineral wool insulation is hung in the void.
Any services which penetrate timber floors should be wrapped with 25mm mineral wool and enclosed for their full height with two layers of plasterboard.
It is possible to install downlighters and recessed lighting when using the independent acoustic ceiling method. These should be kept to no more than one light per 2m2 of ceiling area, at centres not less than 0.75m and into openings not more than 100mm diameter. If you are using alternative floor treatments, then it’s usually best to check the manufacturer’s advice on whether downlighters can be used.
Some elements – such as existing concrete floors or masonry walls – may already achieve the minimum sound insulation performance standards without having to do any further work. In these situations, we sometimes recommend that it’s worth getting a preliminary test carried out. This can save the costs of doing any further work and give you that peace of mind at the start of the job that you will pass your sound tests on completion.
However, the sound insulation of traditional timber floors is usually very poor. Without further works these floors wouldn’t achieve the criteria in Building Regulations. Because of this we normally recommend it’s not worth carrying out preliminary sound insulation tests on timber floors.
Using this design guide is a good starting point for the most appropriate way to achieve the sound insulation requirements of Building Regulations for timber joist floors. However, getting the design of separating floors wrong could cost you £000’s if your sound insulation tests fail.
We have extensive experience in assessing the sound insulation performance of buildings and undertaking sound insulation tests.
ACA Acoustics will review your drawings and construction details so that you can be sure your sound tests will pass first time.
To discuss your project in more detail with one of our experienced acoustic consultants, and to find out how we can assist in making sure your timber floors achieve the required standard of sound insulation, please call us at your local office number, or use the contact form.