Sound new targets
Timber in Construction, July-August 2007

The code for sustainable homes will set tough new targets for designers in the timber industry. TIC hears from Dr. Sean Smith at Napier University, about how designers can meet these goals.

New challenges and performance factors await the construction industry under the new Code for Sustainable Homes (CSH). As of April 2007 new build public funded housing projects in England are required to meet at least CSH level 3, with level 6 being the highest and ‘zero carbon’. One of the aims of the Code is to encourage house builders and developers to build new homes to standards significantly beyond the regulatory minima.

Each section of the Code is classed as an Environmental Impact Category and has a weighting (or Points). The points are derived from credits for each sub-section. Within the array of factors to be considered sound insulation is included under the non-mandatory category Section 7 “Health and well-being”. Detached houses are automatically awarded 4 credits for sound insulation. Credits are awarded for attached houses and apartments based on either pre-completion testing or registration with Robust Details for Part E.

Credits, Points or Weightings?
Each category in the Code has a weighting which is assumed to be relative to their environmental impact and this weighting is expressed in “points”. For Code level 3 the new build dwellings must achieve at least 57 points. The highest level of available points is understandably for Energy. However, the second highest level of “points availability” is under Health and Well-being, where sound insulation is located. Health and Well-being has a potential 12 credits available, which is re-weighted to be worth 14 points. Of the 12 credits potential sound insulation is worth 4 credits. Given that specifiers, technical directors and architects will be looking to gain points from wherever possible this is particularly good news for timber frame and attached houses, as discussed below.

Sound Insulation and the Code
Table 1 shows the credits available for airborne sound insulation (separating walls and floors) and impact sound transmission (floors). Under pre-completion testing for sound insulation at least 10% of the dwellings must be tested and there are further 10% subcategories for changes in flanking conditions, such as inner leafs. Attached houses and apartments based on the same site must each have 10% of their dwelling stock tested. The plots to be tested should be identified by the building control officer or building inspector.

For example if four tests are carried out for eight attached houses and the results are 54 dB (4 credits), 52 dB (3 credits), 49 dB (1 credit) and 47 dB (0 credits) this will translate into the respective credits for the plots tested. However, for all the remaining site plots not tested these will be awarded the credits for sound insulation based on the lowest test, in this case 0. Therefore it is important that the design specification of the separating wall should repeatedly be achieving the target credits, when built correctly.

For attached houses and apartments registered with Robust Details and built in accordance with the RD specification pre-completion testing is not required.  Robust Details will be publishing shortly on their web site the credits available for each RD.

Attached houses and +8dB (4 credits)
Timber frame separating walls generally have good levels of sound insulation. However, current designs do not always achieve +8dB repeatedly. A new design solution developed by Cullen Building Products will allow developers to aim for the maximum 4 credits available for attached timber frame houses. This has been possible through a research partnership between Cullen Building Products, Napier University's Building Performance Centre and TimberSolve.

The new design requires no changes to the separating wall leafs, for example with minimum 89mm depth studs. Instead the acoustic performance is dictated by Cullen’s new Acoustic Wall Strap (AWS-75). The cavity spacing between the frames for open frame is normally 50-65mm and for sheathed cavity walls is minimum 50mm. By changing the cavity width to 75mm and using the innovative AWS-75 the sound insulation is increased for low, mid and high frequencies.

Blockwork separating wall cavities are minimum 75mm and may struggle to repeatedly hit 3 credits for the Code. Part of the reason for this is due to mortar collecting on wall ties and collecting at the base of the cavity, bridging blockwork leafs. Timber frame wall designs using the Cullen AWS-75 will be able to be incorporated into existing house plan designs.

In addition, the new acoustic wall strap has significant structural advantages. These include the ability to double the load transfer of conventional party wall straps which can be incorporated into the racking resistance of adjacent timber frame units in terraced or multi-family units. As a result there will be less need to resort to additional sheathing layers or dense nailing to overcome racking issues.

The Cullen acoustic wall strap was also designed to address a range of features including sustainability, installation, site practice and robustness. Table 2 lists some of these features.

The Future
The Code presents a number of challenges across construction and performance issues. However, such challenges also encourage new innovative approaches which must be multi-functional in their objectives. Napier University has formed specialist interdisciplinary teams to work with industry and address various Code challenges. The teams are brought together from the Building Performance Centre, Scottish Energy Centre, Centre for Timber Engineering and Centre for Infrastructure.

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