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As ever the internet is awash with a variety of answers on this but we all know the real truth can be found here.
I need to attach some 25mm (x50mm if it matters) batons over some insulation blanket into 100mm rafters ready for plasterboard.
The batons will compress the blanket at the point with the studs so I am allowing ~ 25mm for this (maybe a bit optimistic).
Soooo
25mm baton
~25mm for insulation
Aiming for 50% penetration (fnar) into joist
Arrives me at 100mm screws - probably M5.
Sound about right or should I get a note circulated to all the local Robin families pre warning them?
We wouldn't do it that way, you don't really want to be compressing fibreglass between timbers if you can help it, it loses it's thermal efficiency and potentially it is difficult to get a flat ceiling where it compresses to different thicknesses over different joist.
Normally stick a few nails/screws in the side of the joists to let you 'hang' the fibreglass between the joists temporarily, then staple a vapour barrier (thin plastic sheet) over the joists then plasterboard.
If worried about breaks in the thermal layer due to the joists use an insulated plasterboard.
We wouldn’t do it that way, you don’t really want to be compressing fibreglass between timbers if you can help it,
This was my first thought too, but not being in anyway qualified to comment, (not that it stops a lot of people on here!) I didn't want to be the first to stick my head above the parapet.
Is this a loft space? If I've understood what you've said correctly, currently your insulation stands proud of the rafter surface by 25mm? So your battens (and plasterboard) would squish that back into the rafter space?
i read it as he is using something like Supafoil or multifoil.
Which i understand the way he is doing it is an accepted method of fixing
Ah, that would make more sense.
probably M5.
That is a metric thread designation.
Are your joists steel and are you threading the fixing holes?
Do you mean 5mm?
In my haste I clearly omitted some details ...
Yes it is a multi product. Essentially creating this:
So the fixing through the baton needs to go through the foil that will run over the joist.
Joists are wood.
Thinking 5mm diameter screws
I would use a few drywall screws. Then when you come to overboard it use drywall screws long enough for c30 mm penetration into joist. They'll drive in much easier than 5mm screws and will be plenty strong enough cos you'll have loads of em.
Is plasterboard with phenolic foam insulation not more effective insulator (Kingspan or Celotex are the common brands)?
Not answering the original question but bear in mind that current building regs equate to about 120 mm thick of phenolic foam or 280 of rock wool. For revamps or extensions it is 75 mm of phenolic foam.
Superquilt is barely tickling the edges. Like a cotton shirt in a snowstorm. If you've got the ceilings down can you insulate it better rather than later finding you wish you had and having to go back again?
You may have already considered this but if not it may be worth considering.
You can put some phenolic foam between rafters, tape seal and plasterboard. Usually need at least a 50 mm gap to the roof to allow it to breathe but may be able to eliminate if a breathable roofing felt is used.