Part E of the Building Regulations changed on 1 July 2003 and now requires higher acoustic standards in both new build and conversions (material change of use). In any conversion, the economics are usually better if as much of the original structure as possible is incorporated in the new layout. Architects have to design for all the routes by which noise can travel through the structure, new and old. They will have no control over what has been done in the original construction and may not be aware of some of the indirect routes that noise may take - so-called flanking transmission. Nonetheless, the building has to be tested to show compliance. If it fails, it has to be put right and retested.
By comparison, new builds can be simpler. Designing from scratch means that many of the awkward junctions can be avoided. Furthermore, there may be a Robust Detail that can be used to avoid the need for pre-completion testing altogether, although a small proportion of dwellings using Robust Details will be tested as part of the Robust Details quality-checking procedure.
To provide guidance on conversion work, the ODPM has commissioned a series of case studies over two years, of which this is the first. Each conversion will be different and therefore will require a different solution. It is hoped that giving examples will make builders and architects aware of the sorts of constructions that give satisfactory results, and of the typical problems they are likely to encounter. The work also involves a quantity surveyor to estimate the costs of the various solutions. Thermal requirements and fire-safety issues form part of the standard requirements for conversions: all parts of the Building Regulations must be satisfied as normal.
One of the cases in the study, described here, concerns the conversion of a Victorian cottage hospital into dwellings. Although the case study is specific to the building under examination, the lessons learned can be translated to similar projects. This case comprises the conversion of a 100-year-old, single-storey cavitywall building into three two-storey, individual dwellings by opening up the roof space and installing an additional floor with dormer windows in the pitched roof.
Structural assessment The hospital comprised:
l cavity brick external walls, with windows in the front facade. Some door and window openings had been made in the rear wall. A pitched tiled roof with gables at each end was set at right angles to the main roof.
The main roof was supported via central posts between roof trusses and the apex;l solid internal brick walls;l ventilated, timber-joist ground floors;l plaster on lath ceilings.
The new Part E has brought a change in emphasis towards good sound insulation at low frequencies, so structures that had satisfied the previous regulations do not necessarily comply now. Broadly speaking, existing masonry structures are more likely to be adequate than lightweight ones. The original 240mm solid brick walls in this case gave adequate sound insulation as separating walls.
As part of the conversion, the retained brick separating walls had been extended upwards to form the first-floor separating walls and close off the remaining roof void. These new walls comprised two leaves of 100mm dense solid-concrete blockwork mortared together to form a solid wall, nominally 210mm thick, and plastered to match the existing wall. It would have been better acoustic practice to lay the blocks flat to make a 215mm thick wall - and a single leaf would have been adequate above the first-floor ceiling level - but this survey was set up to examine what was built rather than what should have been built.
There were gaps in the brick walls where corridors or doorways had been in the earlier building. Where these became separating walls, the gaps had been filled in with two leaves of 100mm dense solid-concrete blockwork, which were plastered.
The potential degradation of sound insulation penetrations is always an acoustic issue if they are not treated adequately, but in this instance no services had been laid through separating walls.
The new stairs had been built away from the separating walls, reducing the possibility of structure-borne noise being carried through to the adjacent property, which is a particular problem with solid separating walls. (This aspect of noise control is not covered specifically by Approved Document E, but should be considered for good sound insulation. ) Separating walls and roof The junction of the separating wall and the roof is an important factor in limiting flanking noise transmission between dwellings. Here, the thermal insulation had been incorporated in the sloping roof up as far as the ceiling level, comprising two urethane boards, one fixed between the rafters and the other under the rafters.
This was faced with 12.5mm plasterboard (10kg/m 2). Above ceiling level, 250mm mineral wool quilt (10kg/m 3) had been laid on top of the ceiling.
The new first-floor ceiling had been fixed to new timber ceiling joists spanning between new steel purlins, which were supported at each end on either a new separating wall or an external wall. Sloping walls were of 12.5mm wallboard, fixed to the underside of the rafters, a section that is not on a separating wall line.
The masonry that separates walls had been extended full height into the roof void and cut to follow the line of the pitched roof, the gaps filled with intumescent packing.
The separating walls penetrated the ceiling, finishing between the rafters. Therefore, the plasterboard lining, which formed the sloping walls of the rooms, was not continuous across the separating wall, essentially to avoid a flanking path across it.
Openings had been left in the blockwork of the separating walls to carry the steel purlins, which were bedded on mortar. As steel is a good carrier of sound, a gap was left between the purlins and packed with mineral fibre (35kg/m 3), then the flanges were packed with 'ordinary' mortar mix to the depth of the separating wall. The purlins had then been faced with wallboard between the ceiling and sloping walls.
External walls The external walls comprised cavity brick with a plaster finish, with the cavity closed on the separating wall lines with mineral-fibre fire-stopping. The walls were heavy enough not to need other upgrading to reduce flanking noise.
One of the separating walls lined up with a rear window. In order that the neighbours' gardens had privacy, the window openings on this facade had been filled in with blockwork.
It would have been better acoustic practice to have built the separating wall through the inner leaf, or to have block-bonded them together. Separating walls must not be built against window openings because the sound insulation will be seriously degraded by flanking and inadequate sealing.
Internal walls Internal floors and walls within the dwelling do not have to be tested, but must meet a laboratory rating.
This can usually be obtained from the suppliers of proprietary wall and flooring systems and plasterboard manufacturers. Alternatively, a robust construction described in Approved Document E can be used. If internal walls and floors are kept in conversion projects they are not subject to the sound-insulation requirement of Part E at all. Many of the internal walls in these new units were retained brick and half-brick walls that actually achieved the standard of 40dB Rw.
Former doorways were closed with one or two leaves of dense solid-concrete blockwork to match the existing wall, then plastered.
Internal floor The new first floor had been constructed lower than the existing ceiling. This aligned it below the head of the ground-floor windows on the front facade, so the floor level had been stepped up above the window head, forming a bulkhead.
On the first floor, the small step in the floor level was hidden by boxing in the space above the step and below the level of the window-sill. This formed a continuous shelf across the room and also extended horizontally into the dormer window opening. It was essential that the void formed was totally closed off and sealed at each separating wall line.
The new internal floor complied with internal floor type C, given in Approved Document E 2003 and comprised:
l 25mm flooring grade chipboard deck (17kg/m 2);
l 175 x 50mm timber joists at 400mm centres;
l a ceiling of 12.5mm plasterboard (10kg/m 2) and skim, with 100mm mineral wool between the joists.
Costs The elements of construction affected by the requirements of Part E have been costed to reflect the build cost.
Separating walls, internal walls, internal floors and ceilings costs do not include doors, surface finishes, floor coverings, fittings and fixtures, but do include the structural element plus the acoustic treatment. The cost is the total cost of installing these items, including a proportion (15 per cent) of site facilities overheads and profit, but excluding VAT.
The estimated costs for alterations to the Victorian building, compiled by QS Franklin & Andrews, comprise:
l separating wall (extension of existing wall) - £115/m 2l internal floor - £62/m 2; l new ceiling (over first floor) - £40/m 2.Pre-completion testing Building Regulations Approved Documents require pre-completion testing to be done on at least one in every 10 units completed, including the first ones, to show that the specified levels of airborne and impact sound insulation have been achieved. If a different construction is used for some of the separating walls (or floors) within a development, tests must be done on each type of construction. Tests on the aforementioned solid brick and blockwork walls were 47dB and 48dB (DnT, w + Ctr), which met the performance standard in the Approved Document.
The design and the high standard of workmanship on this project indicated that great care had been taken to minimise flanking noise transmission, and maximise the potential sound insulation of the separating walls. This might not always be the case in other developments. Furthermore, on each development each construction must pass every time it is tested. It would be unwise to assume that the separating walls built here could have a lower spec and still consistently achieve the sound insulation required.
Malcolm Every is managing director of the acoustic consultancy Sound Research Laboratories. For further information tel: 01787 247595, email:
srl@soundresearch. co. uk or visit www. soundresearch. co. uk to see more detailed results.
This work is part of an ongoing study funded by the Office of the Deputy Prime Minister. The author would be pleased to hear from architects involved in conversion projects suitable for inclusion in the overall study.