Back boxes into 100mm thick wood fibre insulation.

Question

I'm looking for a method of securing back boxes(any type) into walls that are internally insulated with 100mm thick wood fibre insulation boards(adhered to 9" brick walls) that has a 12mm thick lime render directly applied to it. The cables will be clipped direct to the brick walls behind the insulation. It is a domestic installation. Has anyone done this before?

mapj1 · Accepted Answer

Sadly solo is a brand of plaster for rendering application in the more traditional 'wet' style. In this case with an embedded mesh pressed into the first pass before it has completely gone off, and then hidden by the second pass (sometimes called the "top skim" you may recognise). The mesh reduces the risk of the plaster coming off if the board behind flexes a bit. The insulation board is not entirely rigid. 
 
 From the makers website 
 " 
 All back boxes should be plastic or stainless steel (as lime tends to react with galvanised boxes and make them rust) and again, they can be glued in place with the base coat. Make sure the recesses for the back boxes and conduits are well coated with the plaster to ensure airtightness. 
 The back boxes for electric sockets should also be mechanically fixed to the wall behind to prevent them being pulled out when plugs are pulled out of sockets. Once all in place, flush up the surface with the base coat plaster to ensure there are no pits in the surface. " 
 So I think, a few long screws and plugs though the plaster, through the warm-shell and into the masonry are unavoidable. 
 It might be possible to use thermal break fixings behind the back box in some locations. 
 https://www.ejot.com/medias/sys_master/wdvseta/wdvseta/hf7/h91/9304850038814/EJOT-eta-04-0023-ejotherm-str-u-str-u-2g-sdk-u-EN.pdf 
 where the screw head is several tens of mm down the plastic tube into which a more conventional anchor may be used. 
 There are special fixings for foamed insulation like polyurethane ( https://www.fischer.co.uk/en-gb/products/fixation-on-etics-insulation/fixing-in-insulation-material/insulation-fixing-fid) but I suspect they wont work in the warm-shell- which if it is the stuff I'm thinking of has more in common with a sort of fibrous breakfast cereal than a structural material.. 
 mechanical properties 
 https://source.thenbs.com/product/warmshell-internal-wall-insulation/2CSs9qivHmJj5Xs1KBbMFw/cSK6GyHiAp8PfoujknA3gv 
 suggest the thinner panels are denser and manage 200kPa per 10% deformation, (translating - a strong index finger poke of say 5kg on an area of 1cm2 will dent it to the tune of 15-20% of the full thickness ) and the thicker panels are less dense and weaker still,,, 
 
 If not then the skinniest normal screws/ framing fixings you can muster, and go right through. 
 However in any case no need to dig more depth out of the warm shell than absolutely necessary. If it meets UK insulation building regs it will be a lot thicker than the average back box. 
 https://www.lime-green.co.uk/Documents/Warmshell%20Internal%20typcial%20U%20values.pdMikef 
 Mike

Back boxes into 100mm thick wood fibre insulation.

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