The provision of insulating mediums under or on top of floor slabs and to all slab edges reduces heat loss and improves the acoustic behaviour of the structure. But the question arises of whether edge insulation presence contributes towards conventional oversite membrane termination providing a weakened arrangement via which air or contaminated land gas might infiltrate?
Where an oversite membrane turns up and terminates between floor slab and wall face, there is no continuity of the oversite protection into the wall skin. The membrane and the wall dpc are two unconnected separate entities. The alternative of the dpc extending into the inner skin offers an improved arrangement, but one that in reality is difficult to achieve on site. Ordinary dpc will not readily match the structural tight right-angled conformity necessary to achieve an airtight junction. Nor is it able to stretch/cover where any inward projecting right-angle in the exterior wall is encountered – there is insufficient physical material to do so without patching. Edge insulation when present can increase the extent of discontinuance and thus increase the access opportunities. Resilience against entry can vary pending the composition of the insulation and how it is integrated. The differing pressure under a structure and the air pressure within a room directly influence infiltration behaviour. Normally the pressure in a building is less than that of soil air/gas. Consequently the soil air/gas can enter a structure if fissure opportunities exist. The floor/wall junction is a recognised entry path. If there is no physical linking of wall dpc and slab membrane, entry into the building envelope will not be challenged.
To add to the dilemma, the Health Protection Agency adopted a new stance regarding radon, a gas that until recently has not been considered one of the most onerous of dangers to inhabitants of properties.
The HPA introduced a Target Level in new construction (of houses) of 100 Becquerels. This is half the current Action Level and its introduction follows the announcement by the World Health Organisation that radon is more dangerous than first thought and ‘there is no safe level’.
So if modern construction promotes more airtight structures, will radon seeping into a property around its ground floor/wall junctions be less able to escape once within the structure?
An alternative approach growing in popularity is to incorporate edge insulation that integrates with the inner skin, so the resilience of the oversite membrane is physically extended vertically (against slab and inner face) and then inwardly through the inner skin.
Combination Edge Insulators (CEI) are preformed multi-functional dpc’s. The shape of a Combination Edge Insulator is cranked – providing a continuous dpc presence through the inner leaf, down the wall face/slab edge and finally horizontally under the floor slab. Gas rising under no longer has an entry route into ground floor rooms between slab edge and wall face because the arrangement is not discontinuous. The CEI acts as the inner skin dpc, and establishes an extension from that level that protects the structural arrangement. To the upward rising section of the CEI is bonded an acoustic and thermal barrier. Thus a CEI addresses many requirements in one placement.
A recognised contaminated land gas entry route into the building is obstructed and the dpc integrity of the interfacing is maximised.. Where the land is identified as requiring more severe measures, an adaptation of the Type CEI is available that additionally extends downwardly through the cavity and terminates on the outer face of the outer skin. This version therefore additionally protects against rising gases in the cavity.
A proficient floor/wall arrangement can be likened to bringing together glass within a window frame. The arrangement will not resist air and water if the glass fails to extend sufficiently into and integrate with the adjacent medium. The Type CEI Combination Edge Insulator in one placement provides inner skin dpc + air/gas arrestment link + thermal and acoustic edge cushioning.
Further reading: Technical Manual number 22, Cavity Trays Ltd.
For complete footprint protection against radon, see section 5 of Technical Manual.
Cavity Trays Limited Administration Centre Yeovil Somerset BA22 8HU
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