Internal wall insulation

[Fezoka]

We have an old Victorian house with an extremely cold kitchen. I’m planning to insulate the suspended floor and also the walls internally. I have 100mm Rockwool slabs available, I was wondering if I can use these on the walls (obviously they will take up more space) or better to buy Celotex boards and use those ( extra expense but less space loss). Do I need to leave an air gap behind the Celotex if I decide to use that? Can I apply the Rockwool directly onto the brick wall? Thanks

 

[robinbanks]

What wall covering will you then have? What requirement for mounting kitchen cupboards on it etc?

 

[Fezoka]

planning to cover it with plasterboard. No units or anything else fixed to these walls

 

[phatboy]

50mm of PIR insulation is quite a bit better of a thermal insulator than 100mm Rockwool slab (Assume RWA45?), so I'd but the PIR to be honest.....

 

[Fezoka]

Thanks guys. Do I need to leave an air gap behind the boards? Struggling to find 50mm timber which I can use for the studs

 

[robinbanks]

Do I need to leave an air gap behind the boards

No, unless you care about the fractional improvement the silver surface offers

If you're installing services such as pipes and wires you might choose to put 19mm battens on the room side then screw the plasterboard to them. Services can run in the void between insulation and plasterboard

If you're not installing services and you're looking for relatively quick install onto a sound wall surface you might want to look at using PIR backed plasterboards and foaming plasterboard adhesive

50mm of PIR insulation is quite a bit better of a thermal insulator than 100mm Rockwool slab

I think you mean the opposite. PIR generally has a lambda of 0.022W/mk, the RWA45 has a lambda of 0.035

If the rockwool was 0.044 then 100mm of it would be equivalent to 50mm of PIR but the rockwool's lambda is lower than 0.044

100mm of rockwool is better than 50mm of PIR

 

[JohnD]

You don't need studs.

 

[mrrusty]

Do I need to leave an air gap behind the Celotex if I decide to use that?

I'm going to disagree with other posters IF you have solid walls. In a timber frame house with a brick outer skin, and an inner insulated timber frame, there is ALWAYS a ventilated air gap between the two with a breathable membrane on the cold side of the insulation. If you read the installation instructions for insulated plasterboard on solid walls they suggest using studs to keep the PB off the wall. https://www.siniat.co.uk/-/dam/0582...ermal-dl-rebrand-v2-digital.pdf?v=-1787323216 page 10.

The problem is once you insulate the brick will be much colder. Unless you have perfect sealing (difficult) the warm damp air in the kitchen will condense on the inner face of the brick inside the wall. This will make the bricks damp. This WILL happen so you have to think about where that moisture will go. Somehow it has to be able to evaporate. If the external brick skin is rendered you have another issue because it can't evaporate outwards.

Personally if I decided to do this, I would build like a timber frame house with a ventilated cavity between insulation and wall.
More reading https://www.heritage-house.org/damp...ed-consequences-of-solid-wall-insulation.html

You say you have a suspended floor. This might pose another problem. If you plan to make the brick wall colder behind insulation, you are planning to have some condensation that you then need to plan to control. That condensation is also going be around the joist ends in the brick wall. We all know what happens to timber once you get high humidity....

My 2p. Other opinions will vary.