Internal solid wall insulation

[Evie]

I'm so confused with reading conflicting advice, hoping someone can help.

 

Our house is a 1920s semi with solid walls. It is pebbledash on the outside, this was done around 35 years ago and it's fine, so must have been done properly to allow the walls to breath, we have no damp issues as such.  We are adding a back extension and at the same time want to update and insulate the existing inside wall.  I believe the wall still needs to breath so I think I can't go for insulation that seals the wall.  I was thinking of adding a frame, boarding this and leaving a small cavity, the floor space would allow this.  I will be getting a professional to do this. But we've had professionals before who don't fully appreciate issues with old buildings.

 

What would be the best way to insulate this outside wall from the inside.

Many thanks

[Redbeard]

Wood-fibre or cork on a lime 'parge-coat' (air-tightness layer) or a cork-enhanced plaster such as Diathonite.

 

I do not think you can be certain that the external render and dash is breathable so I would suggest that you try to get the house modelled in a dynamic condensation risk assessment model such as WUFI which should give you a better idea of the 'safety' of your proposals. I am a great fan of wood-fibre and have used it a great deal. I have not used Diathonite but see some of Green Building Store's Retrofit case studies for details.

[jayc89]

Diathonite is mixed with NHL5, so if breathability is what you're looking for, that ain't it. Given you already have pebbledash externally, is external wall insulation an option? It's far "safer". 

[Evie]

5 hours ago, jayc89 said:

Diathonite is mixed with NHL5, so if breathability is what you're looking for, that ain't it. Given you already have pebbledash externally, is external wall insulation an option? It's far "safer". 

Not considering external wall due to expense as the whole side and front would have to be done. The pebbledash is in good condition, also not keen on moving all waste pipes to accommodate external insulation.  We are just wanting to add extra insulation to one inside wall in a room we are extending, so will do it along with renovations.

[Evie]

8 hours ago, Redbeard said:

 

I do not think you can be certain that the external render and dash is breathable 

No, I can't. And would have to admit my ignorance in these matters.  I'm just assuming that having lived here for over 30 years and the pebbledash done a few years before that, we have not experienced any damp issues.  Which I assume would have become apparent by now.  However I could be completely wrong.

[Roger440]

18 minutes ago, Evie said:

No, I can't. And would have to admit my ignorance in these matters.  I'm just assuming that having lived here for over 30 years and the pebbledash done a few years before that, we have not experienced any damp issues.  Which I assume would have become apparent by now.  However I could be completely wrong.

 

Unless you specifically asked for something breathable, its almost certain its conventional render/pebbledash.

 

But if you dont have any damp, thats good news. Its not a given you would have.

 

The leaving a small cavity approach can work, but it will need ventilating, and needs to be done such that the cold fresh air doesnt leak into the house elsewhere.

[Redbeard]

Ok, I see what you mean that it's not damp, but that is not necessarily what the breathability thing is about. If you were using a modern non-breathable  insulant which effectively closes off the breathability on the inside, you would want a breathability path to the outside so that if any water vapour *does* get through (via a faulty vapour control layer, for example), it can find a way out to the outside.

 

Get it modelled in WUFI. I am not advertising them but a wood-fibre supply firm with a name rather like how you'd describe your position if you were sitting against a mud bank will do a WUFI assessment for you for free. Basically if you are assured that the insulant (wood-fibre or cork) is OK to let the wall breathe to inside then it may not be so critical if the exterior is less breathable. Better to have a moisture model tell you that than me, though.