Insulated plasterboard over Lathe and Plaster?

[whitecollar]

I have a traditional tenement flat in Edinburgh built around 1880.

I'm on the top floor and the back half of the flat is under a flat roof.

The construction of the roof is:

[outside]
[roofing felt]
[sarking boards]
[30cm air gap]
[lathe and plaster]
[inside]

I want an easy way improve the insulation in the rooms under the flat roof.

The ceilings are very high, I can easily lose 10 - 15cm from the room height. So, I want to put insulated backed plasterboard straight over the lathe and plaster ceiling and screw straight through into the ceiling joists, and then replaster.

Is what I'm suggesting sensible? Are there any pitfalls to do with damp / ventilation I need to consider?

I think as the 30cm air gap will still be there and I'm insulating "inside" the existing room, directly against the lathe and plaster, everything will be OK. Am I correct in that assumption?

Thanks!

 

[KenGMac]

whitecollar, good evening.

Things to consider??

You will need to "introduce" a Vapour barrier" between the upper side of the [proposed] insulation and the surface of the Lath and Plaster, this to prevent condensation from forming either on the face of the Lath and Plaster or??? within the 30 cm. air gap below the sarking.

Would it be possible to fit insulation [whatever thickness --- more the better] and also fit some sort of ventilation into the 30 cm void below the sarking??

Ken.

 

[Plumb Crazy]

Knauf xps laminate plus would be ideal. £51 per sheet from Wickes. Can't see any problem with damp as the extruded foam backing would act as a vapour barrier. I just used this product on a cold wall in my house. Dabbed it on and skimmed it. Fab product.

 

[^woody^]

You will need to "introduce" a Vapour barrier" between the upper side of the [proposed] insulation

Why?

The condensation risk will actually reduce with th eaddition of insulation, and the foam will be a barrier in itself.

And the situation within the air void will be no worse, so why would ventilation be required to be introduced?

 

[^woody^]

Insulated plasterboard is a compromise and a bit crap and relatively expensive for what it is, performance wise.

You are better off fitting some foil backed insulation such as celotex, as thick as you can, and then plasterboard.

 

[paulrockliffe]

Can you expand on that? I'm working on a project at the moment that is supposed to be lined with insualted PB in the next few months, so if there's a better way I'd like to understand it.

What's the benefit compared with the cost of having to fix insulation, mark stud locations, then fix plasterboard?

 

[^woody^]

Insulated plasterboard is typically half to two-thirds as efficient as PIR boards for any given thickness, and tends to be about twice as expensive per m2.

That's why it is a compromise, and if other options are available, they will tend to be better - cheaper and more efficient.

 

[paulrockliffe]

Thanks, I assumed the PIR was just a normal board with a plasterboard glued on. I had clocked that it was very expensive though!

I wasn't looking forward to carrying the boards up into my loft anyway, so if I use separate plasterboard that will be a lot easier!

 

[jonshutt2]

Did you ever get this work done? Did it work? I'm starting to think about doing the same thing on my 1850 Edinburgh house. Eg, sandstone, a few cm gap, then plaster and lathe. I did finally get Everwarm installers to come and measure up to quote, and they said they just board on top of the plaster and lathe nowadays but didn't say what board they use, but they've taken months and still not given any quote - so i'm back to the DIY option!

 

[millerman]

Did you ever get this work done? Did it work? I'm starting to think about doing the same thing on my 1850 Edinburgh house. Eg, sandstone, a few cm gap, then plaster and lathe. I did finally get Everwarm installers to come and measure up to quote, and they said they just board on top of the plaster and lathe nowadays but didn't say what board they use, but they've taken months and still not given any quote - so i'm back to the DIY option!

Would it be false economy to install insulated PB over walls that have damp/mildew as a remedy to overcome the unsightly black fungus/