Why using curtain walls in residential design could be a mistake
For those who don’t know, a curtain wall is a commonly used system that acts as a non-structural, cladding system for large, multi-storey buildings.
The frames of the systems are generally made from lightweight building materials such as extruded aluminium and are often filled with glass, creating a pleasing aesthetic.
Curtain Wall Design
The framing of a curtain wall system is constructed using vertical elements called mullions and horizontal elements called transoms. The framework is then used to house a glass panel or louvre for ventilation.
There are three types of commonly used curtain wall systems; Stick systems, Ladder systems and Unitized systems.
Where the vertical elements (sticks or mullions) are installed on site first. Once the mullions are fully fitted and secure, the horizontal elements (transoms) are fitted and then finally the glazing elements.
Using the same principles as the stick system, the mullions are installed on site first. The main difference being that the mullions can be split apart and screwed or snapped together. The main advantage of this is that it reduces the time spent on site.
A unitised curtain wall system is created in a factory as fully assembled frames that can often be interlocked on site. These are quick to fit on site and contain factory fitted seals meaning that they can reduce errors associated with workmanship.
Issues with acoustics and Sound Insulation Testing for Building Regs
When you are designing rooms for residential purposes, sound insulation between the rooms must meet the minimum requirements of The Building Regulations Approved Document E for airborne and impact sound insulation (other types of space adhere to their own standards). Sound insulation is usually measured on site by playing loud pink or white noise through a loudspeaker and measuring the difference in sound pressure between two adjacent rooms.
For airborne sound transmission between rooms there are two things you should consider; Direct Transmission, and flanking transmission.
Direct transmission considers the build-up of the separating partition wall or floor between two adjacent residences. The sound insulation of the partition is dictated by the mass, stiffness and isolation of the system. You can find common build-ups for these partitions in Approved Document E itself.
Flanking transmission is where most issues are found during sound insulation testing. This is where sound avoids the direct path and follows the path of least resistance (an area that has less sound insulation than the wall or floor). Common problems for flanking noise are found at junction details of partitions, which is where curtain wall systems can become an issue.
Imagine that you have two adjacent apartments that are separated by a partition wall/floor that has a very high level of sound insulation and that the apartment building has been constructed using a full curtain walling system. If a loudspeaker is turned on in once room the direct transmission is blocked by the partition, but the junction detail between the partition and mullion/transom would be significantly weaker than the wall itself.
There are a number of possible weak points in the detail:
- The glazing itself,
- sealing junction
- continuous / interconnecting mullions that are not split between rooms (vertical transmission).
What can you do to fix it?
No two schemes are the same and as such it should be considered good practice to consider sound insulation at an early stage of design. It can be very difficult to correct a curtain wall detail if it has already been installed incorrectly. It is essential that the following mitigation measures (and more) are considered during the design stage of your project.
- Create a break in the mullion/transom: Do not allow continuous mullions or transoms to interconnect between adjacent spaces. There should always be a break between separating walls and floors ensuring that there are no rigid connections. Custom inserts may be available from suppliers.
- Use two mullions/transoms: Introducing another mullion or transom can significantly increase the sound insulation provided by the partition detail if designed correctly.
- Use high quality fire stop: Battens that have a high level of sound insulation should not be solely relied upon, but can help to increase the sound insulation performance provided by the detail (providing they are fitted to create an airtight seal).
- Use a suspended ceiling: The use of a suspended ceiling is very important when looking at sound insulation with curtain wall details. The edge detail can be boxed out with plasterboard at the second mullion mentioned earlier. The detail should be looked at and designed by a qualified acoustic professional.
Curtain wall systems are commonly used in modern architecture for offices, hotels and apartments. If sound insulation is not considered at an early stage in the design of the building, there will likely be problems at a later date. This is very important if you are designing a residential building as it could result in failing sound insulation testing.
This is not to say that curtain wall systems should not be used in residential developments, only that considering acoustics at an early stage should result in less problems later on.