Cavity Barrier Problem

News from Cavity Trays

Cavity Barrier Problem

Cavity Barrier Problem 

Timer Frame construction is one form of construction that calls for the exterior wall cavity to be compartmented. This is a straightforward procedure, and involves the use of sleeved non-combustible insulation located to fully fill the cavity width without interruption at heights that coincide with the floor levels around the building. In the event of a fire, the insulation sleeves prevent fire progressing via the cavity from one floor to the next. 

Vertical barriers are also necessary to provide the same function. Thus every dwelling or living unit is effectively ‘framed’ within its exterior cavity wall, so if fire originates in one dwelling unit it cannot reach its neighbour via the cavity. 

Such devices can address the requirement to provide cavity fire integrity and acoustic measures, but their very presence throws up a new problem. With the cavity fully bridged, what happens to rain that penetrates the exterior skin and gravitates to the level of the cavity barrier? The insulation sleeves that started-off rectangular are intended to end up slightly compressed. This compression coupled with pushing the rectangular barriers down to locate them in the cavity results in the top of the rectangle becoming ‘hollowed’. Penetrating water can pool and reservoir on top of the barrier, sandwiched between outer skin and inner timber skin. 

How does one prevent this happening?  One method is to incorporate a dpc over the barrier, so shaped to direct water away from the inner skin. The dpc will require support at a higher level to hold it in place. There are also sleeved barriers where a surplus of sleeving on one or more sides can be used to act as a dpc. But is there a complication when forming long runs incorporating either of these methods? 

To provide continuity, individual lengths need to lap each other. Lapping methods commonly involve the end of one barrier rising to lap with the other barrier that lowers. Easily done – until you consider the dpc requirement. How do you maintain dpc integrity when the end of a barrier lowers but does not have a stopend? Whether the dpc is separately introduced or is part of the sleeve, ensuring uninterrupted integrity can be exceedingly difficult to actually achieve. Many distorted barrier installations, with questionable laps and questionable continuity can be seen every day on sites.  

What has become apparent is that cavity barriers and stops are being incorporated with a total absence of dpc protection – but there appear to be no problems. 

Similarly, cavity barriers and stops are being incorporated with leaking dpc arrangements, inconsistent dpc arrangements and discontinuous dpc arrangements. But again there appear to be no problems.But appearances can be deceptive. 

When water reservoirs (on top of or in and around a cavity barrier), penetration into the timber frame is prevented only by the timber frame membrane. The membrane physically stops the water decaying the timber inner skin. Currently there appear to be no damp ingress problems and there will be none - until such time the timber frame membrane deteriorates. 

Membrane manufacturers appear to qualify a very limited membrane life when it is directly exposed to elements.  And even the most robust of membranes is reliant on joins – will these last if water pools against them throughout the buildings projected life of sixty years minimum? 

How many housing associations and flat owners have timber frame constructed dwellings where the possibility of eventual decay of the timber frame might be dependent on a membrane not designed for such purposes? 

The alternative is to use a cavity barrier that is shaped.  A sloping top ensures water is always thrown forward – away from the inner skin. No top surface pooling can take place. And a similar sloping bottom permits adjoining lengths to be lapped without the need to raise or lower each length. Thus the fire integrity, the acoustic integrity and the dpc integrity remains uninterrupted. No other barrier provides all these advantages. 

Called the Cavi 60 Type SAF (pronounced ‘safe’) Cavi 60 denotes 60 minutes fire integrity rating and SAF stands for Sloping + Acoustic + Fire.  An optional hard-cap dpc is available. 

Type SAF sloping cavity barriers promote a consistent and functionally compliant build detail and eliminate the risk of wet transference along the barrier top. 

Cavity Trays Limited Administration Centre Yeovil Somerset BA22 8HU