Hi,

I have this slightly unusual buildup... Brick cavity wall construction house (EWI imminent), with timber frame loft conversion (large dormer to rear and hip to gable conversion).

In order for the new gable wall (hip to gable conversion) to line up with the wall below, I have 100mm PIR (non foil faced) outside of the osb+timber frame stud wall (PIR is bonded to the PIR with PU foam glue), and then there's vertical 50x38mm batten fixed over this at 600mm centres (closer near the corners) to support a 9mm render board (thin coat render to be applied to this in due course) with 38mm cavity.

The batten is fixed with m6 stainless steel rods at 1.8m centres (to hold the batten+renderboard to the building in tension i.e. under wind load). The batten is also bonded (PU foam glue) to the PIR board.

My calcs show a safety factor of 5x for the rod in tensile (i.e. to provide support for the wind load), and a factor of 26x for the glue in shear (i.e. to support the weight of the render board and render, and stop it sliding down the face of the building - the wind load requirements are approx 4 times greater than the vertical loading - incidentally, the glue is also sufficient to support the wind load requirements, but only with a safety factor of 1.8x - too low for an adhered system, hence the addition of the m6), so I'm happy that it's not going anywhere.

The build-up is now complete as far as the batten, and my building control officer seems happy (or at least he didn't object when he came to site today and looked at it for a few minutes).

A possible problem may arise with BC I feel from a rain penetration point-of-view (we're in the England + Wales building regulations' "severe" exposure zone).

Despite the fact that the PIR-bonded-to-OSB3 build-up should be pretty driven-rain impermeable, as belt+braces (and also partly to help ease it past building control), I'm also planning to stick in a breather membrane. On a conventional timber frame this would normally be under the batten, however in order for the batten to be glued to the PIR, I'm going to have to take it over the battens instead. I'm going to hot-melt-glue and/or stainless staple it to the sides of the batten in order to keep the usual ~38mm air gap between the breather membrane and the render board.

So this means that the batten-to-wall fixings don't penetrate the breather membrane like they usually do, but that the renderboard-to-batten screws do instead. I don't think this'll be a problem but just in case, I'm going to put 50mm butyl tape between the batten and the breather membrane. This butyl layer will end up under compression, and should water-seal any screw penetrations (much like Tescon Naidec, but I'm just using a generic butyl tape).

This should have the effect of keeping the batten dry in the event of any serious failures in the render.

Can anyone foresee any problems with this construction (except it probably being difficult to get past BC)?


Thanks,

Tim.