Mixing PIR board with Heraklith/Savolit

[Troy Fryer]

Hello,
I’m currently renovating a period property where the architect has speced that the new ashlar wall is to be full filled with PIR boards, then 15 Heraklith/Savolit and lime plaster finish. Currently we’ve full filled the rafters with lambs wool, 15 Haraklith and lime finished them. I’m just wondering what the general rules are with mixing these products, as to my knowledge you shouldn’t mix none breathable and breathable materials.
Thanks

 

[Lacan07]

Hello,
I’m currently renovating a period property where the architect has speced that the new ashlar wall is to be full filled with PIR boards, then 15 Heraklith/Savolit and lime plaster finish. Currently we’ve full filled the rafters with lambs wool, 15 Haraklith and lime finished them. I’m just wondering what the general rules are with mixing these products, as to my knowledge you shouldn’t mix none breathable and breathable materials.
Thanks

I don't know what an ashlar wall is but if your going to lime plaster over PIR board you might as well walk outside and empty your wallet into a drain.

 

[a twig]

Just trying to clarify here, so you’re building a new cavity wall with ashlar?

And the architect has specified PIR in the cavity and breathable finishes (Savolit and lime plaster) on the interior face?

 

[Troy Fryer]

new stud work wall, which they are purposing we fill with PIR board. 11 OSB and then 15mm heraklith with lime finish, thanks

 

[Troy Fryer]

I don't know what an ashlar wall is but if your going to lime plaster over PIR board you might as well walk outside and empty your wallet into a drain.

an ashlar wall, also known as a dwarf wall is the short wall meeting the roof in a loft/attic space closing off the eves. Thanks

 

[Lacan07]

an ashlar wall, also known as a dwarf wall is the short wall meeting the roof in a loft/attic space closing off the eves. Thanks

Looks like these people have a similar idea, but no need for a lime finish here when using PIR as multi finish will suffice with non-breathable insulation.

 

[a twig]

Never seen that called an ashlar wall before - my mind immediately jumped to Bath stone ashlar like I had in the exterior of a previous house in Bath, guess you can take the man out of Somerset…

Back to the question, agree with @Lacan07 - if it’s just stud work with PIR in then fairly pointless to spend extra cash on Savolit and lime plaster.

Can you ask the architect why that was specified? Would be interested to hear the answer

Assume the plan is to make the loft into a usable space / conversion? Otherwise I’m not clear on the benefits of insulating the stud wall if you have already fully insulated the rafters.

If they are needed for support for the roof structure and the rafters are fully insulated wouldn’t you just leave the timbers exposed as they are inside the insulation envelope?

 

[Troy Fryer]

Yeah perhaps it’s a regional thing mate. I’m based in Suffolk and I’ve only ever known them as that.
Yeah I’ll ask the architect and see what he says, I’m also wondering what use there is in filling those with PIR insulation when the rafters are filled with lambs wool? Which doesn’t have the same U value as PIR so will surely loose the heat through the roof? (I’m asking these questions as I’m fairly new too these breathable materials)

 

[Kearn]

Doesn’t quite add up - what are the walls currently? Are they being renovated? Have they being replaced?
It’s a pointless combination, doesn’t inspire confidence. It won’t really work, but it will cost money as you’ve got 2 sets of expensive mats combining modern and traditional.

If it’s a rebuilt wall, then he could be following BRs for new, if it’s existing and traditional, then the U values aren’t as taxing…

The only real benefit of using lime internally over modern muck, is to moisture manage from within. We are doing this on the new external wall of our extension in the kitchen, have to go cavity wall, with reclaimed brick outer leaf, PIR in the gap, but internally I will line plaster as there will be moisture generated in the room space. And also, the new wall will abut two older traditional walls, so there will be moisture transfer, and I’d rather see it and know it can evaporate away.

 

[stuart45]

The dwarf walls are normally called knee walls.

 

[Troy Fryer]

Doesn’t quite add up - what are the walls currently? Are they being renovated? Have they being replaced?
It’s a pointless combination, doesn’t inspire confidence. It won’t really work, but it will cost money as you’ve got 2 sets of expensive mats combining modern and traditional.

If it’s a rebuilt wall, then he could be following BRs for new, if it’s existing and traditional, then the U values aren’t as taxing…

The only real benefit of using lime internally over modern muck, is to moisture manage from within. We are doing this on the new external wall of our extension in the kitchen, have to go cavity wall, with reclaimed brick outer leaf, PIR in the gap, but internally I will line plaster as there will be moisture generated in the room space. And also, the new wall will abut two older traditional walls, so there will be moisture transfer, and I’d rather see it and know it can evaporate away.

Hi mate, thanks for the reply.
It is technically a rebuilt timber wall in place of an existing timber ashlar wall. So maybe it is to do with regs

 

[Lacan07]

Yeah perhaps it’s a regional thing mate. I’m based in Suffolk and I’ve only ever known them as that.
Yeah I’ll ask the architect and see what he says, I’m also wondering what use there is in filling those with PIR insulation when the rafters are filled with lambs wool? Which doesn’t have the same U value as PIR so will surely loose the heat through the roof? (I’m asking these questions as I’m fairly new too these breathable materials)

As twig suggests maybe speak to the architect about the use of PIR with breathable materials, maybe he's got himself a bit confused, it happens to the best of us.

On another note personally I would have suggested using that PIR underneath the rafters with some decent airtightness tape to create a 'warm roof' installation, especially if the loft is being converted into usable space. As you say better U value. I've never been entirely sure of the benefits of using breathable materials in these areas.

 

[paulc]

The only real benefit of using lime internally over modern muck, is to moisture manage from within. We are doing this on the new external wall of our extension in the kitchen, have to go cavity wall, with reclaimed brick outer leaf, PIR in the gap, but internally I will line plaster as there will be moisture generated in the room space.

When I did my kitchen, the ceiling had a layer of PIR nailed up and then covered in plasterboard in an attempt to improve the insulation of the flat roof area. Gave it a skim of lime plaster (after applying a coat of Blue Grit) as I was going for a polished lime finish. The lime plaster will also add slightly to fire resistance.