Posted: 6th January 2023
There is a current trend for converting offices and other commercial buildings into residential flats. Many of these buildings include concrete separating floors. The mass of a concrete floor means that they will usually provide a good standard of airborne sound insulation. However, the impact sound insulation of concrete floors is often poor. Without appropriate treatments these floors would fail Building Regulations sound insulation tests.
It is essential that you consider the sound insulation performance of the separating floors at an early stage. This design guide discusses the best ways to make sure your concrete floors pass your tests first time.
Sound insulation criteria for flats being formed by material change of use – the technical term for a building being converted – are detailed in The Building Regulations Approved Document E.
The required performance is shown in Table 0.1a.
|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. This is the performance that is achieved by a test on site and shouldn’t be confused with the Rw or Rw + C’tr which are laboratory tested values.
As mentioned above, normally the focus will be to improve the impact sound insulation of concrete floors. You would achieve this by installing a resilient layer above the floor.
Typically, concrete floors around 150mm thick or more should provide sufficient airborne sound insulation to pass Building Regulations. If your concrete slab is thinner than this, or you have infill sections or other details which may reduce the airborne sound insulation of your floor then you may need to consider adding mass layers or an acoustic ceiling on resilient hangers.
In our previous article on the best way to improve the sound insulation for timber joist floors, we discuss how, in our view, the most effective – and cheapest – methods use generic building products (plasterboard and mineral wool insulation). However, to increase the sound insulation of concrete floors it is the other way around. You can create your own floating floor using gypsum or cement particle boards (CPB) on dense mineral wool insulation, but it is usually easier, cheaper, and better performing to use proprietary acoustic products.
There are various methods to improve the sound insulation for concrete floors, some better than others. We’ve given a run through of the most common treatment types.
One of the most common methods to improve the acoustic performance of a concrete floor is to pour a screed over a specialist resilient layer. Approved Document E describes this as Floor Type 2. Examples of this type of construction are Robust Details systems E-FC-4, E-FC-5, and E-FC-6.
It is important that the screed does not touch either the base concrete slab, or any flanking walls. This can bridge the resilient layer and cause sound tests to fail.
The resilient layer is usually overlapped and taped, and resilient flanking strips are installed around the perimeter. It is important to follow the manufacturer’s installation instructions. Installed correctly, this type of system will perform well and should comfortably achieve Building Regulations minimum criteria.
Acoustic underlay systems are designed to be installed below a screed, whereas resilient overlay floor coverings are designed to be bonded directly to the top of the concrete slab or a bonded screed. This is as Floor Type 1 in Approved Document E. Examples are Robust Details systems E-FC-9 and E-FC-10.
Assuming the concrete slab is suitable, then these should ensure compliance with Approved Document E.
Using overlay boards in place of a wet screed can have other, non-acoustic, benefits. Some overlay boards are constructed as a dry screed panel, while others use plywood or MDF layers. Each of the systems have a resilient layer bonded to the underside of the board. Again, this can be of various materials such as foam or felt and of different thicknesses.
Whilst different configurations will influence the achieved sound insulation performance, as far as you – the developer – is concerned, any of the available options should be sufficient to improve the sound insulation of concrete floors sufficiently to comply with The Building Regulations.
Resilient timber battens or battens and cradle systems can provide the highest performance of the options available. Depending on the depth of the battens, any insulation used in the void, and the build-up of the floating layer, it is possible to achieve a very high standard for sound insulation of concrete floors.
Resilient battens are fitted over the existing floor and a floating floor installed above. The exact specification of the floating floor varies for different manufacturers, but is typically a chipboard deck. If a higher performance is desired, this can be achieved with plank plasterboard or a cementitious board along with the chipboard.
These 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 60mm to 90mm.
Unlike timber joist floors, concrete floors do not normally suffer from excessive flanking sound transmission. However, if you have separating walls between flats built up from a common concrete slab then there is the potential for horizontal flanking within the concrete floor.
A screed on resilient layer or resilient batten system, which stops at the separating wall, would control horizontal flanking. Otherwise, if proposing to use overlay boards or bonded resilient layers, then the mass of the concrete floor is key to control horizontal flanking. For flats formed by conversion of existing buildings, then usually the concrete slab should have a minimum mass of 365kg/m2, although the exact detail should be checked by your acoustic consultant.
Any services which penetrate concrete separating 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 in the ceiling below a concrete floor. Keep these 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 methods, then it is usually best to check the manufacturer’s advice on whether downlighters can be used.
Using the above design guide is a good starting point to determine the most appropriate way to improve the sound insulation of concrete floors to comply with the requirements of Building Regulations. However, getting the design of concrete 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. 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 the sound insulation of concrete floors on your development achieves the required standard, please call us at your local office number, or use the contact form.
Image credit – Robust Details Handbook