Service Cavity and Screws in Airtight Boards

[Barney12]

We've got a couple of places where we're having built in wardrobes. In one area the house design is such that the depth of the alcove which will contain the wardrobe is only 570mm which after doors is likely to reduce down to 540-550mm of internal depth. This is slightly smaller than the recommended 600mm depth for a wardrobe (Having said that we have a couple of free standing wardrobes that only have an internal depth of 550mm and a coat-hanger is 400mm wide, so perhaps not a huge issue?).

 

Anyway it got my mind thinking about the 38mm service cavity in the back wall of this alcove. At the moment I don't envisage any services at this point so this is feeling like wasted space which I could recover for the wardrobe. However, does it also act as a ventilation gap (MBC Timber Frame) or provide other benefits (such as aiding airtightness of the frame)? 

 

The last line also leads to a further question. "If you place a screw into an airtight board do you compromise its airtightness?". I accept it might to an extent but surely its by a tiny level? Or even nil if a squirt of silicone was applied to the screw thread?

 

Typical wall build up for info:

[Nickfromwales]

You can remove the 38mm batten without problems, bar one. 

The MBC boys usually gas nail these on so you'll be left with open punctures of the airrtight board. Simply apply some Siga / other branded tape over EVERY nail hole and your good to go. 

They serve no purpose other that to provide a service ( cable / pipe / other ) void, nowt for ventilation / other . 

Penetrating the airtight panel is fine as long as the screw goes in and stays in. Try not to put longer screws in than absolutely necessary so as to mitigate any minor interstitial condensation risks. 

A squirt of CT1 ( not silicone ) wouldn't hurt TBH but if this is supervised ( so these disciplines actually get observed ) then you'll be fine. 

 

[Jeremy Harris]

Where I screwed backing boards to the airtight lining board, to space out back boxes correctly (our battens were 50mmx 50mm, so 45mm boxes had to be brought forward to ensure there was no gap) I just put a dab of PU foaming adhesive (the fast cure Evo brand stuff, that comes in gun cartridges) around the screws.  This seemed to work OK, so we did the same for all the back box screws into the spacer boards, just in case they penetrated right through.

[Barney12]

Thanks for the info. Hopefully I'll get to label the wall before the guys get to the battening. 

[Nickfromwales]

1 hour ago, Barney12 said:

Thanks for the info. Hopefully I'll get to label the wall before the guys get to the battening. 

Even better if they haven't battened yet. No nail holes . 

[CC45]

Sorry to semi hijack this thread but like Barney12 we are getting close to starting battening (at least upstairs) and Ive started looking at our options with regards to hanging things.  The current plan is not to have wall units in the kitchen but we do need to hang some stuff on the walls - the current concern is radiators upstairs (ufh downstairs), bathroom 'stuff' (toilets etc).  I was going to put 12.5mm osb on the walls behind the plasterboard in bathrooms and extra noggins inbetween the service void battens where the rads are to go BUT then I found Fermacell on a web site - it makes some impressive claims about its ability to support weight (some good vids on their web site) and thus I am now thinking of using this in the bathrooms, kitchen (just in case her in doors changes her mind) and for hanging rads on - so replacing plasterboard in these area with Fermacell.  I'm happy to accept the extra cost because of the reduced time and hassle. But I've got some questions that I suspect some of you chaps can answer...

 

As long as I use appropriate fixings is this a sensible plan?  Why not use normal plasterboard along with 'Gripit' or similar fixings?

 

I assume the 12.5mm Fermacell is the same depth as 12.5mm plasterboard?  I know they say its the same but after my experiences with PIR insulation (which is not that accurate) I need some reassurance!

 

If I mix and match Fermacell & plasterboard on the same wall - can it all be skimmed without any issues?

 

Any other observations?

 

Cheers 

CC

[PeterW]

The difference with Fermacell is you don't skim it - the edge joints are glued and then the holes are filled and its floated over with FST - fine surface treatment. 

 

fantastic stuff - hang anything on it with a single screw and it's very dense. Downsides are the dust destroys power tools and it's expensive .  

 

If you want to mix and match plasterboard then use Gyproc Habito which is their dense board. 

[CC45]

Cheers - first hand experience is always useful.

 

Thanks