Service void depth

[crispy_wafer]

I've got a small bit of roof that spans a bedroom and small bathroom, the bedroom will have a standard light fitting, bathroom I think at this stage will have downlights of some description.  I'm planning to fit a vapour barrier on the underside, then batten out for services.  What sort of depth to make the void?  Floor to bottom of wall plate is about 2350.  For the barrier, poly sheet ok, or is there a better way, I think I'm likely to put holes in the poly sheet... 

 

[tonyshouse]

Best avoid downlighters, draughts, air tightness, vapour control, wasps , fire 

[Nickfromwales]

Above 2250 is outside of ny zone, so you could fit flat panel LED slimline fittings in both rooms. They fit, literally, in the depth of the plasterboard + skim coat(s) but they do have a transformer to connect up which need a recess. 

You can cut into the VCL and make a circular recess, with for eg a plastic paint kettle or pot noodle sized plastic receiver an tape to that for integrity. A cable grommet atop would allow access for cables etc and happily house a transformer etc. The issue will be the insulation will need the same cutting and foil taping first, ideally, for belt n braces before moving on to the penetration and receptacle is created for the actual light fitting of choice. 

 

1 minute ago, tonyshouse said:

Best avoid downlighters, draughts, air tightness, vapour control, wasps , fire 

Fire is moot as this area does not have any habitable spaces above, but FR light fittings are readily available in all shapes and sizes, or you can install a integral FR ( intumescent ) hood.  Draughts, agreed, but can be mitigated with some effort. Not had any vermin / insects in mine yet. Vapour control and AT are obvious areas which also need addressing regardless, as the cable needs getting from A>B. If the OP wants to create a service cavity just to avoid some detailing at the locations of the spotlights, then a 35mm service void would suffice for flat panels + transformers, but if any type of spotlight is used, the service cavity becomes way to deep if wanting to avoid penetrations into VCL and insulation.

[tonyshouse]

Fire can and has been caused by down-lighters, less likely with led but the ballasts can catch light 

Edited January 10, 2022 by tonyshouse
Posted too soon

[crispy_wafer]

when I said downlights, I meant LED downlights.  Insulation above will be mineral wool between and over.  So maybe no need for a service cavity then, just some detailing around and above any penetrations for cabling, light fittings and MVHR.  I need to do some looking up on light fittings I think.

[Dudda]

Where's the extract from the WC or is it just a window? Are you having MVHR? Do you've anything else that has to run in the service void other than a few cables for lights and a potential smoke alarm, intruder alarm?

[Nickfromwales]

1 hour ago, tonyshouse said:

Fire can and has been caused by down-lighters, less likely with led but the ballasts can catch light 

Same as anything that has 230v going into it in fairness  

[crispy_wafer]

36 minutes ago, Dudda said:

Where's the extract from the WC or is it just a window? Are you having MVHR? Do you've anything else that has to run in the service void other than a few cables for lights and a potential smoke alarm, intruder alarm?

 

Good questions, MVHR for extract, the ducts will attach to the vent from above and be insulated.  To be honest, maybe I was overthinking it, and had gone down the route of thinking I needed a void, when in reality I probably don't.  I can easily mange the penetrations for what is essentially just over a 5m*5m area with a bedroom and bathroom.  I just need to have a look at some pictures of modern bathrooms with nice slim LED light fittings (panels/strips), Design and looks is not my bag at all.

[JohnMo]

We used 48mm battens and used slim led down lights of various diameters.  All sit below airtight vapour control layer.  So no issues with air tightness, drafts and no cutting the vcl.

 

We used an aluminium foil acvl from Passive House systems, not the cheapest, but nice to work with, it drapes down the walls and is stuck to the wall with a special mastic they sell for the product.  Joints are taped with aluminium tape.