Help please with this roof build-up - can anyone help?

[Tom]

Hi all - I started a thread a short while ago about whether I need a VCL layer in my roof buildup but it disappeared under the waves unfortunately, and I really need some help with this if possible. Can I retry?

I have been quoted £4,500 for fitting the VCL to the roof structure as part of the whole boarding/skimming quote which seems ludicrous. We have been told by BC that we need a VCL but I have the feeling this is a decision made without due consideration - and one that is going to cost me in hard earned bunce. (Like when I was told I wouuld need 2x 11kWh ASHPs by one supplier but my calcs using the heat loss spreadsheet indicated I would need a single 8kWh... but I digress)

 

So, the roof make up we have from inside is:

 

plaster board

200mm rockwool

150mm PIR (actually composite panel, so clad in metal sheet)

 

I have taped the seams between the composite panels and foamed round the edges so am happy that this layer is airtight. BC want me to put a VCL between the platerboard and rockwool, so:

 

plasterboard

VCL

200mm rockwool

150mm PIR

 

I'm worried that, at best, this superfluous or at worst will just trap moisture when humid air seeps through the inevitable screw holes etc.

 

I have tried to find an interstitial condensation calculatoir on-line but had no luck, other than this German one: https://www.ubakus.de/u-wert-rechner/?  but am struggling to make sense of it.

Can anyone shed any light on this?

 

Thank you so much!

Tom

[nod]

VCL isn’t a requirement BC often prefer it 

As a plastering contractor I price lots of jobs each year Some specifing VCL

Most not 

I didn’t use one on our previous build and won’t on this one 

I would prefer a foil backed board But the majority of house builders don’t even do that When I quote for VCL it’s normally around £9 m2 supply and fit 

 

[Tom]

Thanks nod. It's just unclear why BC say they want it - and it would be good to be able to convince them otherwise.

When you provide the VCL what do you supply?

[nod]

They should really have said that they would prefer it 

It’s normally Architects that add it to the spec About 50-50 

If it’s in your building materials spec You are stuck with it If it’s not thank BC for the ADVISE and tell them your good 

 

[Tom]

Ah, that's the rub unfortunately, the roof build-up has been altered since the architects plans were drawn up, so we're playing it all by ear tbh. 

Am still trying to get hold of BC...

[saveasteading]

16 hours ago, Tom said:

when humid air seeps through the inevitable screw holes etc.

Screws don't leave holes unless you take them out again, and they will be into timber so it isn't a worry. What you might have is holes and gaps at light fittings, but this should be avoidable.

 

16 hours ago, nod said:

£9 m2 supply and fit 

That seems rather a lot if the ceiling is at bare joist stage.

What do you reckon? 2 workers x 2 days? £800. add material, add margin 20%.

Or Nod's idea of the foil backed board will be fine for bco too.

[Tom]

Thanks both for your input. Spoke to a firm who would do a condensation risk analysis for me (taking in to account the steel rafters, z purlins etc) and it seems the calculation would take the best part of a day and cost over £1k - and their hunch is that I would need a VCL anyway. So, have bitten the bullet and instructed the boarders to put a VCL in. Gone for a membrane over foil-backed boards due to the detailing needed at board/steel junctions. What's another £4k eh?