Timber joist floors in residential buildings
With refurbishment projects of historic/listed status and the strive for more sustainable buildings, developments with suspended timber floors are becoming commonplace. Residential timber floors are often formed from joists with floorboards above and ceilings below. More commonly encountered in refurbishment projects, they can also be used in new build projects.
Typically, refurbishment projects have a lower environmental impact than new builds. However, to be fit for purpose and sustainable, good acoustic performance is needed.
The relatively low mass of a timber floor results in a lower sound insulation performance when compared to concrete, which has become standard in modern multi-dwelling buildings.
There are two types of sound insulation that need to be considered for floors:
- Airborne sound insulation – to control airborne sound transfer from one space to another
(eg, the transfer of speech, music or building services plant noise)
- Impact sound insulation – to control impact sound transfer from one floor to another
(eg, footsteps or the movement of furniture).
Floors separating residential dwellings are generally required to comply with the sound insulation criteria defined in Approved Document E (ADE) of the Building Regulations. This sets minimum requirements but enhanced standards are increasingly common. ADE recognises the need to conserve special characteristics of historic buildings. It accepts that it may not be practical to achieve the minimum sound insulation performance standards and in these cases, states that the aim should be to improve the sound insulation to the extent that is practically possible.
While timber floor constructions can achieve ADE requirements, there is an increased risk of being able to hear people walking around upstairs. This has the potential to result in complaints and reduces the perceived acoustic quality of the development. Timber floors have an inherently weak low frequency sound insulation performance. A comparison of the impact sound insulation of a timber and concrete floor is shown below. Both achieve the same overall impact sound insulation performance of L’nT,w 52 dB, but the comparatively poor low frequency sound insulation performance of the timber floor is apparent.
The floor finishes need to be carefully considered to minimise impact sound transfer. Use of carpeted floors or resilient layers below hard floor finishes are usually adopted for this purpose.
To meet the requirements of ADE, floating floors are usually installed, typically in the form of a cradle and batten type systems, or by use of a resilient layer between the deck and flooring (or dense floor grade boarding with a resilient layer backing).
The floating floor adds mass that is acoustically decoupled from the structure. This increases the airborne sound insulation performance and provides resilience to reduce impact sound transmission. The cradle and batten floors provide a better enhancement than the resilient layer approach but with the penalty of increased depth. Use of resilient floor channels above the joists, with heavy boarding hung between can be adopted to minimise raising the floor height.
Use of heavier duty floated screeds or screed-board flooring can be considered as an enhancement (where weight restrictions permit) to maximise the sound insulation performance, particularly at low-frequencies.
Low frequency performance improvements can also be achieved through works to stiffen the floor structure which could take the form of additional noggins between floor joists. In traditional buildings deafening such as ash was often used to add mass and provide stiffness to the floor, and making this good and reinstating it where it is missing can provide good improvements to the floor sound insulation.
Further enhancement can be achieved by using staggered or twin floor joists.
Historic buildings may have ornamental ceiling features such as panelling and cornicing which needs to be retained from a conservation perspective. It is also often a design aspiration to maintain the look of an exposed beam ceiling. In such cases, acoustic enhancement above floor level is the only option. This approach limits the sound insulation performance in practice and requires larger floor zones which is usually not ideal.
Where ceilings are permitted, these are typically plasterboard mounted using a resilient system to decouple them from the underside of the joists. Resilient bars (or resilient clips/hangers) are often used but care needs to be taken to ensure no bridging occurs.
Ceilings would ideally be imperforate, without recessed fixtures or fittings. Where required these would need to adopt some form of backing ether using plasterboard or some other form of proprietary system. To avoid these weaknesses, secondary sacrificial ceilings are often used to accommodate recessed fixtures, fittings and building services.
Flanking sound transmission
Control of horizontal and vertical sound flanking requires careful consideration. This is where sound enters the structure and travels from one side of a partition to another, ie, bypassing the main separating element.
Lateral sound transfer via the relatively lightweight floorboards, timber joists or floor voids has the potential to undermine the sound insulation provided by a wall. The acoustic enhancements outlined above (ie, floated floors and plasterboard ceilings) help to control this in combination with timber blocking to close-off the floor void. Ideally, light-weight elements are discontinuous between dwellings to maximise control of flanking transmission, particularly where enhanced sound insulation standards are adopted. Perimeter resilient flanking strips are used between floating floors and adjacent elements to avoid sound bridging. The principles of this are shown below.
In traditional buildings, vertical flanking of sound can occur via continuous vertical walls, particularly if these are thin or lightweight walls. Impact sound can also flank down through continuous vertical walls and installing new enhanced ceilings does not address this, so the installation of floating floors which are isolated from the surrounding walls is necessary to control this.
- Enhanced performance requirements in mixed-use buildings, between dwellings or noisy non-residential spaces: Sound insulation performance criteria better than the ‘minimum’ ADE standards are often targeted between such adjacencies. Enhanced sound insulation standards may be achievable in practice where spatial or weight restrictions apply, which can be a limiting factor for historic buildings.
- Unfavourable adjacencies: Noise transfer and subsequent risk of disturbance is minimized when similar uses are stacked above each other. Spaces with hard flooring such as kitchens, living rooms and bathrooms above bedrooms would ideally be avoided. Where this is not possible, enhanced standards may be appropriate. Washing machines also have the potential to generate high levels of noise and vibration and so avoiding positioning these above living spaces, in particular bedrooms is often advised.
- Creaking floorboards: Timber floors need to be appropriately fixed/braced etc to avoid creaking floorboards which otherwise would have the potential to cause noise disturbance.
By James Atha
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