Suitability of wood wool boards

[skinnymalinky]

Hi all

I've seen these mentioned/sold quite a lot e.g. Savolit Plus branded boards. I see their marketing materials claim that the boards are "Diffusion permeable" and "Hygroscopic" - however they also note that the boards are formed of Portland cement.

Has anyone looked into this? Are people here confident about using these? I am specifically thinking about using them for internal wall insulation i.e. fixed to external-facing walls then plastered over.

Any thoughts?

 

[malcolm]

The wood wool boards tend to be used on the outside of a wall as a base for render. Normally inside you would use woodfibre board insulation which is breathable and insulates roughly twice as well for the same thickness.

I've used both, though the wood wool boards were very much just something to render and I had an air gap behind so breathability didn't matter too much. Inside I've only used woodfibre board which doesn't seem to have caused problems on solid walls after all the normal damp issues had been sorted.

 

[Zebra]

I've used them internally a lot. Not really as an insulator though, more as a base for lime plaster. They are breathable: yes they have cement in but it's only binding the wood strands together, it's not forming an impermeable layer. I can't think whether they're more or less breathable than wood fibre, although as malcolm says, wood fibre is undoubtedly a better insulator.
My upstairs room has fully breathable inside-to-out external walls, with OSB and woodwool on the inside, sheepswool in the middle, then wood fibre on the outside, so back to front from what malcolm is saying (not that I disagree with malcolm's assessment, it's just that my system is external wall insulation, whereas you're talking about internal wall insulation). I'm just describing my upstairs system to demonstrate that I have no concerns about woodwool in terms of breathability.

In short, I think you would be better off with wood fibre than woodwool for what you're thinking of doing.

 

[skinnymalinky]

Thanks both for the input!

I had a local plastering/restoration type company come around and they mentioned that they usually fix wood wool to existing plastered walls before applying another layer of plaster. This was in response to my question to them about insulating plaster products. I would guess they don't get many people asking about insulation and wood wool is their goto solution given its relatively small 15mm depth.

I see you can get wood fibre boards at 20mm which I imagine would be a 5mm well spent vs the wood wool so I will mention this if/when they get back to me with a quote.

In terms of the wood wool breathability question though - thanks a lot for the explanation @Zebra - this clears things up for me!

 

[a twig]

You can get wood wool in a range of thicknesses, Savolit Plus comes in 15, 25, 35, 50 and 75mm for e.g.…

 

[skinnymalinky]

Yeah I just mean that minimal thickness is probably desirable in a lot of typical scenarios hence 15mm board being standard goto option. Esp. when insulation is not a concern.

Where insulation is a concern, spending an extra 5mm on wood fibre is probably worthwhile. Although I dare say most people would opt for a thicker depth of insulation anyway.

 

[johnbanshee]

I have solid stone walls with old messy & cracked lime plaster internally - I seem to read different opinions on how to install Savolit board - they said it needs to be battened to timber.

Questions.:

* Can the boards just be fixed straight onto existing old failed lime plaster (it's all in place but hollow & loose when knocked).
* Can they be battened with a wooden frame
* If battened there will be a small cavity...should that be filled with insulation (sheeps wool etc) or left as air

Thank you!

 

[Zebra]

I believe it can be fixed directly to the wall. I did. No battens, unless you want to do that.