Insulated Plasterboard on glass Fibre in wall

Sefo · 09-10-2019 11:16pm #1

Hi,
I am removing the plasterboard on the inside of external walls in a 1980’s semi-d bungalow so a plasterer can install insulated 82mm pir plasterboard and then skim plaster.
I found glass fibre insulation between the battens with a plastic sheet covering it when I removed the plasterboard (picture attached).
Should I leave this in place and cover with the insulated plasterboard or should I replace it with something else fo better insulation?

dermob · 10-10-2019 12:05am

I would probably remove the plastic and insulation, leave battens on and fix insulated plasterboard to them if good enough. It’s good to have a cavity behind insulated plasterboard to manage condensation and mould.

The correct thing to do would be to get a calculation done on the wall For the condensation point and address accordingly.

Depends on your overall approach

BryanF · 10-10-2019 7:31am

Has this house got a cavity between 2 leafs of block? If so consider pumping it. As above before buying the PIR consider the dew point

BryanF · 10-10-2019 7:37am

dermob wrote: »

. It’s good to have a cavity behind insulated plasterboard to manage condensation and mould.

is it? Can you elaborate?

Having a cavity as described, especially where there isn’t a vapour barrier with an insulated slab, allows hot air to condense ion a cold surface, defeating the point of insulation and increases the risk of mould/condensation. Better to pup cavity and reduce the PIR to solve dew point, if possible.

Sefo · 10-10-2019 9:21am

BryanF wrote: »

Has this house got a cavity between 2 leafs of block? If so consider pumping it. As above before buying the PIR consider the dew point

Unfortunately i can't as it is a 8" cavity block wall

Sefo · 10-10-2019 9:44am

BryanF wrote: »

is it? Can you elaborate?

Having a cavity as described, especially where there isn’t a vapour barrier with an insulated slab, allows hot air to condense ion a cold surface, defeating the point of insulation and increases the risk of mould/condensation. Better to pup cavity and reduce the PIR to solve dew point, if possible.

The wall is 8" cavity block so i cannot pump unfortunately.

The insulated plasterboard i am thinking of getting is Xtratherm Thin-R XT\TL which has a 'vapour control layer', not sure if this is the same as vapour barrier?
"The composite foil facing on both sides of XT/TL-MF incorporates an integral vapour control layer, which helps to reduce the risk of condensation."

BryanF · 10-10-2019 11:53am

Sefo wrote: »

The wall is 8" cavity block so i cannot pump unfortunately.

The insulated plasterboard i am thinking of getting is Xtratherm Thin-R XT\TL which has a 'vapour control layer', not sure if this is the same as vapour barrier?
"The composite foil facing on both sides of XT/TL-MF incorporates an integral vapour control layer, which helps to reduce the risk of condensation."

Take that blurb with a pinch reality. How will the floor, internal wall, and ceiling junctions be sealed with this system

Calahonda52 · 12-10-2019 2:00pm

the foil is 100% non breathable so the devil is in the detail as to how the joints/edges with be made airtight.
What about sockets/pipes/ etc
What will you do at window reveals?

You don't say how deep the GF is, looks in good nick so would be wasteful to remove it.
The risk as mentioned already is where the dew point will end up with an additional 70mm of PIR.

What gave rise to this project.
Was EWI considered?

Lumen · 13-10-2019 7:54am

As I understand it, a cavity behind internal insulation is only recommended if the wall is required to breathe, e.g. solid brick or lime mortared stone wall, and in that case the cavity must be ventilated to the outside.

So in this case, not.

I don't see the point in preserving the existing insulation. It's taking up space and if you add more insulation the studs are going to cool down, and if there's any water penetration through the (non cavity) wall they might rot.

Better to strip back to the wall and see what you're dealing with. It's wasteful but a better job.

edit: EWI is a far better idea but this is likely to be more expensive.

Sefo · 15-10-2019 9:20am

Calahonda52 wrote: »

the foil is 100% non breathable so the devil is in the detail as to how the joints/edges with be made airtight.
What about sockets/pipes/ etc
What will you do at window reveals?

You don't say how deep the GF is, looks in good nick so would be wasteful to remove it.
The risk as mentioned already is where the dew point will end up with an additional 70mm of PIR.

What gave rise to this project.
Was EWI considered?

The window reveals will use a thinner 26 or 38mm insulated plasterboard.
The glass fibre insulation at the battens is about 50mm.
The house is D2 ber so trying to keep the heat in so spend less on heating and single-handedly save the planet

External insulation was outside my budget.

Sefo · 15-10-2019 9:26am

Lumen wrote: »

As I understand it, a cavity behind internal insulation is only recommended if the wall is required to breathe, e.g. solid brick or lime mortared stone wall, and in that case the cavity must be ventilated to the outside.

So in this case, not.

I don't see the point in preserving the existing insulation. It's taking up space and if you add more insulation the studs are going to cool down, and if there's any water penetration through the (non cavity) wall they might rot.

Better to strip back to the wall and see what you're dealing with. It's wasteful but a better job.

edit: EWI is a far better idea but this is likely to be more expensive.

I think i would agree with you, the impression i get is the further away the insulation is from the external wall, the dew point will also come in to the point where it could be between the internal side of the external wall and the insulation board which could cause condensation and mould.

Insulated Plasterboard on glass Fibre in wall

Comments