Best Practice Considerations Where Timber Frame and Floors Meet
Is the way in which timber frame construction and SIPS construction integrate at ground level flawed? Currently reliance is placed on a number of separate elements working together as one, but in practice these elements are unconnected. Should the construction approach be changed so at the same time one can address the concerns raised when appraisal of Part C of the Building Regulations was conducted?
At ground level the site operative is required to establish three protective DPC’s: The oversite membrane, the DPC in the outer skin, and the DPC in the inner skin. The combination of all three is intended to provide complete and unpunctuated protection of the building footprint. But, with three unconnected elements this everyday construction arrangement has weaknesses. A different approach can eliminate those weaknesses; damp ingress, hygroscopic absorption and the entry of contaminated land gases into the building envelope.
Tackling first the subject of contaminated land gas, the World Health Organisation (WHO) announced some time ago that radon gas (which is present to some extent in most areas of the UK) is more dangerous than first realised. Furthermore, with more airtight build standards, opportunities for some natural radon dissipation/exit paths from structures are reduced. Given one of the most direct entry routes of the gas in a typical home is via the floor/wall junction, the question arises of whether increased gas concentrations cant accumulate within a house with more disciplined airtightness standards above floor level?
By merging and linking the oversite membrane with the inner skin DPC, discontinuance can be eliminated and the gas entry route can be blocked.
The alternative approach combines the inner and outer skin DPC requirements and does so in a way that offers several important accompanying benefits.
A one-placement solution providing a continuous and unpunctuated protection solution. It is called the Type TFC – The Timber Frame Cavitray.
The Type TFC protects the outer skin and then rises, transverses the cavity and at a higher-level projects inwardly. The inward projection extends through and beyond the inner skin, providing substantial inboard presence to integrate horizontally with the oversite membrane. All the separate unconnected weak areas associated with separate DPC’s are eliminated.
The arrangement also arrests rising contaminated gases from progressing upwardly in the cavity. Instead they are safely discharged out of the wall via Cavibricks located below TFC level around the base of the building. The cavity spanning portion means water penetrating the building envelope at higher level is arrested on top of the cavity barrier where Caviweeps located in various perp joints provide evacuation routes. Beneficially this arrangement eliminates volume pooling within the base of the cavity, which can be advantageous if the land under the building offers poor drainage or is of rock or has a high water table.
Within the cavity the Type TFC profile has a vertical projection that locates against the inner timber frame skin. Apart from aiding optimum placement, this laps and covers the timber plate that frequently remains partly exposed to the cavity along the bedding course. Hygroscopic exposure of the timber to cavity moisture is thus minimised.
By combining the DPC requirements for both masonry skins with a cavity barrier that extends upwardly and inwardly, the result is a functionality efficient and consistent build detail that offers greater protection against contaminated land gases and DPC misplacement.
The Type TFC is manufactured from gas-grade resistant Petheleyne in various profiles. The inboard projection can also be cranked to suit different floor/slab arrangements. Profiles are semi-rigid and supported with a range of preformed angles and steps to suit. Adjoining sections and angles link using gas-grade anchoring strip.
The Type TFC is manufactured by Cavity Trays Ltd of Yeovil, the only UK cavity tray manufacturer awarded European Technical Approval.