Do I need a VCL in this wall build up ?

[bmj1]

Wall build up

100mm Brick

50mm cavity

100mm rockwool slabs

100mm concrete block

Insulated plasterboard (not yet built)

 

Does the insulated plasterboard require an integrated VCL ? (assume dot + dabbed).

[Iceverge]

No, a parge coat behind the plaster would be needed for airtightness though. 

 

[bmj1]

7 hours ago, Iceverge said:

No, a parge coat behind the plaster would be needed for airtightness though. 

 

Ok thank you. So no risk of interstitial condensation?

[Iceverge]

Assuming the insulated plasterboard isn't too thick it should be fine. 

 

Airtighess is your thing here. Drafts are how almost all moisture gets carried into a structure causing interstitial condensation, not diffusion. The airtighess layer can be anywhere, think of it like blocking a pipe, it matters not whether at the beginning of the end so long as its done properly. In in this case a parge coat inside the blocks would work. It'll need to be taped/sealed to the windows and floors and all penetrations to be effective though.  

 

If you can't guarantee this you're better off limiting the thickness of the insulated PB to about 20mm of PIR to ensure the dewpoint of the insulation doesn't move too far inboard in the wall.

 

In other words if you have drafts blowing through the wall and a hefty chunk of internal insulation there's a risk that damp air will condense on the inner block wall as it'll be too cold. 

 

Have you committed to insulated plasterboard? A MF stud internally might be an easier detail. 

 

 

 

 

 

 

 

 

[bmj1]

5 minutes ago, Iceverge said:

Assuming the insulated plasterboard isn't too thick it should be fine. 

 

Airtighess is your thing here. Drafts are how almost all moisture gets carried into a structure causing interstitial condensation, not diffusion. The airtighess layer can be anywhere, think of it like blocking a pipe, it matters not whether at the beginning of the end so long as its done properly. In in this case a parge coat inside the blocks would work. It'll need to be taped/sealed to the windows and floors and all penetrations to be effective though.  

 

If you can't guarantee this you're better off limiting the thickness of the insulated PB to about 20mm of PIR to ensure the dewpoint of the insulation doesn't move too far inboard in the wall.

 

In other words if you have drafts blowing through the wall and a hefty chunk of internal insulation there's a risk that damp air will condense on the inner block wall as it'll be too cold. 

 

Have you committed to insulated plasterboard? A MF stud internally might be an easier detail. 

 

Not committed, looking at 17mm PIR bonded to 9.5mm plasterboard, as the insulated plasterboard.

 

I'm a little nervous about the dew point also. And I do think we have to assume there will be some gaps between the rockwool locally in a few areas...

 

Does that make sense ?

 

 

 

Edited July 3, 2023 by bmj1

[Iceverge]

That thickness of PIR will be fine. 

 

A quick play with Ubakus.com makes me think about 50mm of PIR is good. 

 

What is the purpose of the 10mm high emissivity gap? It sounds like a recipe for thermal looping in the real world unless you can completely seal the gap. ( impossible in reality)

 

 

[bmj1]

53 minutes ago, Iceverge said:

That thickness of PIR will be fine. 

 

A quick play with Ubakus.com makes me think about 50mm of PIR is good. 

 

What is the purpose of the 10mm high emissivity gap? It sounds like a recipe for thermal looping in the real world unless you can completely seal the gap. ( impossible in reality)

 

 

 

Thanks for the feedback.. It's dot and dab, so I've assumed some kind of gap behind the blockwork. Does that make sense ?

 

I don't want to go so high with the thickness, to conserve floorspace !

 

So to confirm, no need to worry about a VCL, even if the insulated plasterboard has no foil component ?

[haythorn_1]

19 hours ago, Iceverge said:

No, a parge coat behind the plaster would be needed for airtightness though. 

 

What is the benefit of doing a parge coat vs VCL? Is it just cheaper and quicker?

[Iceverge]

A vapour barrier stops (tries to stop) the movement of vapour through a structure.  Hard to achieve 100% tightness and can cause trouble by trapping moisture in the wall if done badly. This is especially a risk where you have two impermeable layers. EG a sheet of polythene and a layer of PIR. You can get variable VCLs (Vapour control layer) like Intello Plus but they're dear. 

 

An airtightness layer stops the movement of air though a structure. As air carries most of the vapour the task is often given over to the VCL. However it can be a sheet of OSB, woodfiber, clay plaster etc which are all more vapour open. 

 

@bmj1

 

No need for vapour barrier (Vs VCL)  Get a really good airtightness layer and let the rest of the structure as breathable (vapour open) as possible.