Installing an effective radon barrier in a traditional, masonry-based building is relatively straightforward. Where radon gas barriers in timber-frame buildings are required, builders face a trickier task knowing exactly where to locate the barrier. The more obvious location potentially conflicts with the need to ventilate the sole plate area. How do you install an effective barrier without impacting on timber-frame build standards?
Radon gas barriers in timber-frame buildings – what’s the main issue?
Difficulties arise where the proposed timber frame superstructure and sole plate are sited above damp proof course (DPC) level and a suspended ground floor construction. Trying to locate the optimum position of the radon barrier in this case becomes problematic because it conflicts with the positions of periscope ventilation, cavity trays and prepends and the need to ventilate the sole plate area.
The following possible solutions all have their drawbacks:
- Sole plates sitting on radon barriers which then continue across the cavity at the same level – leading to possible moisture back-tracking across to the timber sole plate
- Cavity trays over the radon barrier / periscope vents - which then block ventilation to the sole plate
- Dropping the periscope vent to ground level to site the radon barrier lower – raising concerns of ground water ingress
- Rigid full fill insulation being used to support the radon barrier across the cavity – but there are only a limited number of products that have been tested and approved to full fill a cavity!
So what is the best approach to install an effective radon gas barrier in timber-frame buildings?
A radon barrier working alongside ventilation and drainage
The following detail has been drawn up which shows the radon barrier dressed up the inside face of the outer leaf. This prevents gas getting into the wall cavity while allowing for the installation of the periscope vent above it and maintaining a clear cavity below the sole plate.
This is a complex arrangement, however! To ventilate the sole plate area, consider:
- Positioning a row of perpend cavity wall vents either side of the periscope vents in the course below the external horizontal DPC
- Including a continuous cavity tray above the DPC level needs to be included
- Adding an extra row of ventilating perpends above the DPC to ventilate the upper cavity area
Alternative approaches to consider
Perhaps it should come as little surprise that contractors are reluctant to use this detail, claiming that the radon membrane could be damaged during the beam and block installation and the protective blinding brings extra time and cost to the works.
An alternative solution, which may be more palatable from a contractor’s perspective, is to raise the timber frame superstructure and sole plate 150mm above the DPC level.
Some radon barrier manufacturers are providing details where rigid cavity insulation board has a third-party product approval for use as a full fill cavity insulation (in between periscope vents) and is suitable for positioning in the cavity below ground level, where it is subject to saturation by ground water.
Suffice to say, there are no easy solutions. Developers and designers should carefully consider their options when deciding where to position the radon barrier in timber frame construction.
- Chapter Seven of the LABC Warranty Technical Manual outlines effective gas membrane installations in a timber frame superstructure
- Radon: Guidance on protective measures for new buildings (BR 211- 2015 edition) is available from the BRE bookshop
Every care was taken to ensure the information in this article was correct at the time of publication. For the most up to date LABC Warranty technical guidance please refer to your Risk Management Surveyor and the latest version of the LABC Warranty technical manual.