Floor Vapour Control Layer

[nishnish]

Just had a 1st floor bedroom insulated to building regs standards. Followed the information the building control officer wanted and Celotex approved method.
25x50 treated battens against solid brick outer wall with 50mm celotex over that and then drywall. It's not the method recommended on the celotex pdf for ga4000 (they show celotex then battens, then PB), but celotex confirmed that they approve this method too.
I had blocks of wood in walls for radiator fixings, but we've decided since to put a smaller radiator in
So I was cutting a new hole to fix wood direct to outside wall, under window, for new rad bracket.
I noticed that on the back of the celotex there was condensation. My fingers got wet from back of celotex, as I was messing about in there, but the wall appeared dry.
The floorboards are all loose atm. 6mm plywood, underlay and carpet will be going on in the next few days.
However, this condensation concerns me.
The celotex on the external walls is taped for VCL plus all drywall has been Gyproc drywall sealed with two coats as an extra VCL. Thus ceiling and walls have VCL, but not the floor.
Thus vapour can get through floor and up behind walls.
Is this a concern?
At no point does anyone ever mention providing a floor VCL.

 

[^woody^]

There is always this risk of condensation when you form a void like this on a lined wall. Ideally you should have insulated between the battens too

As long as there are no direct air routes via the floor, sides or ceiling, then it may be that the existing air will stabilise or any condensation will be minimal

If there is a route for air via the floor - even if just a small continuous gap, then the risk of condensation increases.

From similar situations, I've seen puddles and damp on floors along the skirting which looks just like water is coming in from outside, when it was just streaming down the insulation instead.

 

[nishnish]

There is always this risk of condensation when you form a void like this on a lined wall. Ideally you should have insulated between the battens too

Did consider that. Celotex said the foil backing on the 50mm boards will provide the same insulation as 15mm more PIR if aganist an air gap. So it seemed a relatively expensive way to add just 10mm more PIR.
This was back when I was looking at the pretty pictures of the battens installed against the perfect wall in the insulation guides.
My Victorian walls are far from that. Getting an air pocket trapped between battens is not really practical. I suppose I could have silicon sealed them all, but that would cost a small fortune in sealant.

If there is a route for air via the floor - even if just a small continuous gap, then the risk of condensation increases.

Yes, but this will always be the case? Unless a floor VCL is used, which no one seems to mention?
It always struck me that BC want this certain U value, a VCL, etc. but they ignore the underfloor area.

From similar situations, I've seen puddles and damp on floors along the skirting which looks just like water is coming in from outside, when it was just streaming down the insulation instead.

Thanks for the advice. Despite it's worrying nature