Cork vs Rockwool External Wall Insulation

[jsmac]

Hello PPUKers,

We are planning on installing external wall insulation and rendering with Best of Lime's Warmcote and Limecote on the ground floor of a solid masonry wall. We can only install about 30mm insulation unfortunately because of the window cills and the timber cladding on the first floor so we will also be installing some internal insulation. Since the wall will have a combo-boiler on it has been recommended that we used Rockwool Dual Density slab insulation around the area that the flue will exit the building.

My question now is whether there is any advantage to using

• Cork insulation externally + the Rockwool around the flue

OR

• Rockwool DD slabs over the entire wall?

Both the cork insulation and the Rockwool will take a the render coat so I am wondering if I am overcomplicating this by thinking of using cork with the Rockwool. My understanding is that both cork and Rockwool are 'breathable' and are not hygroscopic so in effect will both behave similarly?

As always your thoughts and recommendations would be most welcome.

 

[jamest]

The reason I would choose cork insulation over Rockwool in your situation is sustainability. Rockwool has a huge carbon footprint as it requires temperatures of 1500 deg C or more to make it and isn't a carbon store. Cork is sustainably harvested from trees, requires only low temperatures to expand it and is a significant carbon store, plus it doesn't shed fibres.

 

[jsmac]

Thank you for your comments. I completely agree that there are sustainability concerns that one needs to be cognisant of. We're using wood fibre for the insulation of a new extension - something that has caused countless quizzical and bemused looks from the tradesfolk around these parts - and Thermafleece for the insulation of the original timber frame section (a solution explored after several excellent suggestions from this forum).

However, if I need to buy a pack of the Rockwool product to use around the flue I can't help but think if I should just buy several additional packs and complete the whole elevation with the same product. All things being equal I would opt for cork EWI but I'm left wondering if a compromise is required in this instance. There are so many spinning plates at the moment with this renovation so I sometimes find myself bouncing between different solutions, while trying to find the best solution, when maybe I just need to get on with the nearest and most effective solution.

 

[jamest]

couldn't you just get a rockwool insulated sleeve for the flue pipe and then do the wall in cork?

 

[jsmac]

I hadn't thought of that. To be honest I wasn't aware that there was such a thing as an insulated sleeve for boiler flue pipes.

Is it the FirePro Insulated Fire Sleeve you're referring to?

Are you suggesting that the Rockwool sleeve wraps around the flue and then the cork insulation butts up against the sleeve? And the exposed sleeve is covered and weathered by the flue collar? I can see how that could work.

Thanks!