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 | | NEVER SAY NEVER | | | Icopal | | 18/03/2008 | | | For the past 50 years of standards, building regulations and digests it has always been stated that masonry cavity separating walls “should never be built off continuous slabs”.
Well finally, we can remove the words “should never” and replace them with “can”. The reason for this step change in building design and practice is an innovative solution called Bridgestop, which is undergoing final testing to qualify for Robust Detail accreditation
Bridgestop provides performance levels for 75mm masonry cavity walls, which would easily achieve three credits under the new Code for Sustainable Homes (CfSH), and with 100mm cavities, four credits.
The system’s properties have high acoustic damping to absorb more sound energy and reduce flanking, performing approximately 4-5dB better than solid walls and 2-3dB better than standard cavity walls such as RD E-WM-4.
The history and rationale behind not allowing cavity masonry walls off slabs is for a number of reasons. Direct structural bridging between the wall leafs via the continuous slab, such as a raft foundation or upper floor level slab, leads to significant reductions in sound insulation performance.
Figure 1 shows the transmission path between the wall leafs which bypasses the effective isolation provided by the cavity. In addition, as the blockwork coursing continues vertically so more and more mortar collects at the base of the wall which also bridges the cavity wall leafs.
Dr Sean Smith, depute director of the Building Performance Centre at Napier University in Edinburgh, said; “Collectively, such bridging formed by the slab and the mortar reduces the sound insulation significantly and may result in failure to meet target performance levels for Part E, Robust Details and the CfSH.”
However, the Bridgestop system developed by the Monarfloor Acoustic Systems division of Icopal is cleverly designed to permit the use of the Bridgestop high-performance DPC membrane, as shown in Figure 1.
Bridgestop DPC is laid over the surface of floor slab along the centre line of the cavity wall. After the first course of blockwork, the Bridgestop quilt is installed using Bridgestop ties to hold it in place. A second layer of quilt is required for robustness and held in place by ties on the second course of blockwork.
Dr Smith added: “With a thickness of only 3mm, Bridgestop DPC can sit within the existing design details without major changes to the floor-to-ceiling heights. The Bridgestop quilt lines the cavity on one side to prevent mortar bridging between the wall leafs.
“The quilt is also made from low-resonance acoustic chip foam that is more sustainable. The inherent natural damping of the Bridgestop membrane, where vibration energy changes to heat, reduces vibration and sound waves travelling into the slab and adjoining rooms.”
He said: “Previous to the arrival of the Bridgestop solution, continuous slabs with cavity walls would require dense blockwork solid walls. Solid walls can be restrictive in design options, buildability, robustness and particularly for stepped layouts between attached dwellings for floor and wall junctions and exposed gables.”
Compared with solid walls, Bridgestop uses only one coat of render instead of two and only standard plasterboard (8kg/m²) instead of 12.5kg/m² for solid Part E solution walls, which saves 4.5kg/m² of gypsum-based board per m².
“With 70% of new build now on brownfield sites where contamination may require raft foundations or gas barriers, the Bridgestop system is likely to prove invaluable to housebuilders, developers and structural engineers in the refurbishment sector as well,” said Dr Smith.
“Gas membranes and radon barriers can also cause mortar to collect and lower sound insulation performance for the target CfSH, Robust Details and Part E levels. Being a radon and gas membrane barrier as well, the Bridgestop system can also be used without detriment to sound insulation or membrane criteria.”
Bridgestop is now being used wherever a masonry party cavity wall has to be built off the same support mechanism that is at or near the floor level. In addition, larger buildings with continuous slabs, such as schools, hospitals and office blocks, would also be able to use the system.
The attraction of the security features of masonry cavity walls, thermal mass and the introduction of Bridgestop suggests there may be increasing use of blockwork cavity walls for so many situations which have previously been utilising stud and plasterboard.
“This solution has resulted in a greater range of options for housebuilders for Part E in their separating wall designs and provided the blockwork and concrete industry with wider scope in the increasing brownfield site market.
“At present many housebuilders are having to revert to 215mm dense block solid walls but the introduction of a high acoustic performing solution for a range of dense and lightweight blockwork type cavity walls has been in the wish list of many housebuilders and developers,” he said.
The initial response from within the industry has also been very positive. Paul Goring, principal acoustic consultant with the NHBC, said: “Test results have been very good,” and Nigel Cox, senior design manager at George Wimpey Central added: “Bridgestop offers a fail-save method of complying with Part E in modern construction.” | |
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