VCL vs VAPOUR PLASTERBOARD

[kay01]

Hi there,

 

My cowboy builder did not install VCL in my cold roof, now when i asked him to install VCL he said he will replace my ordinary plaster boards with vapour plaster board.

Please suggest what is the better choice VCL with normal PB or vapour plaster boards.

[nod]

I’ve been installing both for decades and would always use a foil back board (Duplex) under any loft area 

Some building companies do Other's don’t bother I’m not why it’s not a building requirement to have some sort of vapor barrier But it isn’t 

The strange thing is A vapor barrier is always in the bill of quantities for social housing 

[jayc89]

Is this Duplex boards on the first floor ceilings? 

Would a membrane across the bottom of the roof joists, then PB, work? Could also act as an airtight layer?

[Chanmenie]

21 minutes ago, jayc89 said:

Is this Duplex boards on the first floor ceilings? 

Would a membrane across the bottom of the roof joists, then PB, work? Could also act as an airtight layer?

That should work, use double sided butyl tape on the joists which will seal the screws that should make it very air tight 

 

 

Edited September 29, 2022 by Chanmenie

[Mr Punter]

A couple of coats of latex paint on the normal will go a long way.  The main issues with moisture in a loft is via warm air air passing through the loft hatch, light fittings, cable pipe and vent holes and then condensing on the cooler structure where if not well ventilated can lead to rot.

[Radian]

11 hours ago, nod said:

Other's don’t bother I’m not why it’s not a building requirement to have some sort of vapor barrier But it isn’t 

The strange thing is A vapor barrier is always in the bill of quantities for social housing 

 

If you go up into the loft of just about any regular house built before the 21st century (and plenty now), there will be no vapor barrier. Just a layer of horrible pink glass wool directly over the plasterboard ceiling. The ventilation takes care of the vapor that makes its way through from the house below. So long as the ventilation isn't compromised in any way...

 

I had chucked a 2.4m x 0.6m x 0.1m slab of PIR up into our loft last year meaning to cut it up further and fit to the back of the loft hatch. I didn't get around to it until now and when I lifted it up off the glass wool, there was a distinctly musty smell to it all. The vapor was obviously trapped in the glass wool and condensing on the underside of the PIR despite it's 'fabulous' U-value. This is why I despaired at the non-breathable space blankets that Aldi were selling earlier this year for people to put down in their attics.

[ADLIan]

Older versions of BS 5250 on condensation always showed a VCL at ceiling level with loft insulation. In the latest version (2021) the VCL is omitted in diags but referenced in the text as an AVCL. I questioned BS on this sudden change in building physics and as Mr Punter says the main concern is mass air movement though openings/holes in the ceiling. For the cost and for a belt and braces approach I would use a VCL or duplex plasterboard.

[jayc89]

48 minutes ago, Chanmenie said:

That should work, use double sided butyl tape on the joists which will seal the screws that should make it very air tight 

 

 

 

Makes sense. I was thinking of potentially battening out after the AVCL and before PB to provide a service void for lights etc as to not penetrate the AVCL. Of course all the cables already in the loft would need pulling through first...