VCL with insulated plasterboard

[PeterW]

As it says... where does the VCL go when you use insulated plasterboard..?

 

We have around 130sqm of ceilings to do that are sloping and need an additional insulation layer inside the roof - issue is, where do I put the VCL..? I was going with a foil PIR and taping that to provide the VCL but the pricing for the plasterer to do an insulated board makes me think it may be worthwhile getting him to do it all.

 

As I've got part fibre and part PIR, do I put a VCL behind the insulated plasterboard, or does it not need one..??

[Jeremy Harris]

It's OK behind the insulated PB, as the main thing is to position the VCL to stop vapour from inside the house reaching parts of the structure where it could drop below dew point, and so allow interstitial condensation from water vapour movement from inside the house.

[PeterW]

Thanks Jeremy - it would be a lot easier to put the fibre in and seal it up and then decide on the plan for the extra insulation ..!

[Archer]

Belt and braces would be to locate a VCL behind the insulated plasterboard, but manufacturers seem to accept that the integrated VCL provides enough protection without further sealing. 2 coats of Gyproc Drywall Sealer provides vapour resistance to the plasterboard when it is edge sealed with sealant or foam.  

[Jeremy Harris]

There's no definitive answer, and leaving out the internal VCL and relying on decent workmanship to seal the PB is damned risky, unless you know for sure the relative vapour permeability of the whole structure and the quality of the workmanship.  99% of water vapour transmission is usually from outside, provided there is a decent and effective internal VCL, and the golden rule is to maintain a gradient of vapour permeability from inside to out, with the least vapour permeable layer inside and the most vapour permeable layer outside (or adjacent to a ventilated external cavity).

 

As houses are built to better insulation standards, then vapour control becomes far more significant, especially if there are structural layers within the walls and roof that may be subject to damage from interstitial condensation.  Some of the big manufacturers have got this detail seriously wrong in the past, and even a year or so ago one of the major SIPs companies was still going up the learning curve with regard to the safety of their floor to wall detail, something that they changed in the light of further analysis (and following a few strong hints from several here, when we were on the other place).

[ToughButterCup]

On 20/08/2017 at 21:50, JSHarris said:

There's no definitive answer, and leaving out the internal VCL and relying on decent workmanship to seal the PB is damned risky, unless you know for sure the relative vapour permeability of the whole structure and the quality of the workmanship.

[...]

 

OK, you've  got me worried now.

Our architect has specified 

' ... Kingspan K18 insulated plaster board under-draws rafters and creates VCL...'

 

Given your reservations above @JSHarris, might @Archer's suggestion

 

On 20/08/2017 at 20:39, Archer said:

[...]

2 costs of Gyproc Drywall Sealer provides vapour resistance to the plasterboard when it is edge sealed with sealant or foam.  

 

be a sensible thing to do?

[Nickfromwales]

49 minutes ago, recoveringacademic said:

 

OK, you've  got me worried now.

Our architect has specified 

' ... Kingspan K18 insulated plaster board under-draws rafters and creates VCL...'

 

Given your reservations above @JSHarris, might @Archer's suggestion

 

 

be a sensible thing to do?

Ian. If you have all foil faced insulation then do as I did on my last project and use 2 overlapping strips of 4" foil tape the timbers to create the VCL. Be sure to use generous patch pieces to repair the board where dinks and dings have unavoidably occurred, and job done. 

You have a good VCL already on the PIR so stick with that. Will you be overboarding with another thin layer of PIR once the joists are infilled ?

[ToughButterCup]

1 hour ago, Nickfromwales said:

[...]

Will you be overboarding with another thin layer of PIR once the joists are infilled ?

 

Yes. I have foil-faced insulation between the rafters (200mm), and '... underdraw[ing] ...' (and you call overboarding)  the rafters

All 75mm of it.  275mm in all.

 

At the moment, the rafters have insulation between them. Just to be clear, you think I should use foil tape ( 2 lots of 4" ) per rafter to seal the gap ( which already has foam between edge of the PIR and the rafter)

[Nickfromwales]

17 minutes ago, recoveringacademic said:

 

Yes. I have foil-faced insulation between the rafters (200mm), and '... underdraw[ing] ...' (and you call overboarding)  the rafters

All 75mm of it.  275mm in all.

 

At the moment, the rafters have insulation between them. Just to be clear, you think I should use foil tape ( 2 lots of 4" ) per rafter to seal the gap ( which already has foam between edge of the PIR and the rafter)

If your under-drawing then you'll be better off again. No need to tape the inset boards unless there is a risk of vapour getting between because of detail ( possible reduced integrity of ) at the wall / ceiling junction. 

Two choices, either do as my first, or ignore the taping-to-joist of the inset boards and just use the continuous internal layer as that'll be factory edge to factory edge, thus less tape consumed as you can do that confidently with one run of tape. . 

[ToughButterCup]

2 hours ago, Nickfromwales said:

[...]

thus less tape consumed as you can do that confidently with one run of tape. . 

 

Confidence and building  work and me. Ha! 

 

The only thing I'm confident about is that there is always an answer to every problem.  It's finding the answer that hits the bank balance softest .

[MikeGrahamT21]

Have you considered something like a Marmox Multiboard/Wedi Building Board? These are used in the same way as plasterboard, but have integral XPS which forms a VCL (as long as the joints are sealed with marmox multibond), and can be plastered direct, the same as a plasterboard. Come in a variety of thicknesses too, dependant on how much insulation you want.

[ToughButterCup]

17 minutes ago, MikeGrahamT21 said:

Have you considered something like a Marmox Multiboard/Wedi Building Board?[...]

 

@MikeGrahamT21 : thanks.

 

No: because I had no idea  Multiboard or Wedi board existed.

Posts like yours are why I keep coming back to BH.

 

Thanks very much indeed

[MikeGrahamT21]

Not a problem. I used Marmox many years ago as a tile backerboard, and I had no idea they could be used as a drylining board, until i stumbled across instructions on their website. Its a revelation! They are super lightweight, brilliant for a 1 man installation such as mine, and even better you can create curves with very little cost + effort. Sd value is 3.2m, not quite as high as some VCLs, but this will be more than enough to stop vapour moving in to the wrong places ? I too am using Marmox for the inside of an extension with external wall insulation applied, as the internal VCL and drylining in one.