Reinforcement mesh product to support glass mineral wool insulation batt.

[squealeyhealey]

I'm interested to know if anybody has experience of utilising a reinforcement mesh product to support glass mineral wool insulation batt, in a masonry cavity wall with an air gap?

 

I'm a self-builder planning to build a low energy house with a 350mm cavity between masonry walls comprising 300mm glass mineral wool insulation and 50mm air gap.  I will install a wind barrier to prevent thermal bypass and put  spacers at the the wall ties to hold the insulation in place.  But I'm concerned that he wall ties / spacers will be too far apart to prevent the 300mm thick insulation from, over time, slipping into the air gap.  

 

I'd like to find a rigid-ish mesh that I can use to hold the insulation in place.  I'm aware of the Calsitherm Reinforcing Mesh, but this product does not have BBA certification for this application.

 

Is there a product that has BBA certification for this application, or know of an appropriate solution in the current NHBC standard? 

[Triassic]

If the insulation is likely to sag I’m left wondering if the question should be, “am I using the correct insulation?”

[PeterW]

300mm fibre in a 350mm cavity could be replaced with blown bead for an equal (or slightly better) wall performance and no need for any of the other products. 

[Declan52]

Would you not be better reducing the cavity to 175-225mm and use beads blown in to fill the cavity. Easier and cheaper than what you propose.

[Dudda]

I think it's not a question of the mesh available it's more a question of the quality of the bricklayer. I can't imagine a bricklayer taking the time to put in the mesh well. You also can't check the quality of the installation as it's hidden in a cavity. Also mortar is heavy and any drops into the cavity will compress the foam insulation a lot. If it was a timber frame structure with block outer that's fine but I don't see a need to use mineral wool insulation batt in a blockwork cavity. I'd fill the cavity with beads as suggested above.

[MarkyP]

I'm interested to know the purpose of the air gap? I haven't come across it before in a fully filled context, we didnt have one (dritherm32 batts fully filling a blockwork cavity). The Denby Dales houses spring to mind - they were I think 300mm cavity fully filled with mineral wool batts and again no air gap that I recall.  

[ADLIan]

Modern mineral wool cavity batts are quite dense and rigid so the normal plastic retaining collars that slip over the wall ties will work OK. Why not just fully fill with 300mm (2x150mm) with no residual cavity? Where is the building located? Partial fill normally only needed in very exposed locations (SW England, west Wales & west Scotland) or high rise (18m or more in height) but rainwater penetration is very unlikely across 300mm full fill.

[joe90]

My brick layers admitted to me that they did not Iike a full fill cavity but I pointed out the insulation had a BBA cert for full fill. They were very good and protected the insulation against snots.

[squealeyhealey]

On 26/09/2018 at 12:40, MarkyP said:

I'm interested to know the purpose of the air gap? I haven't come across it before in a fully filled context, we didnt have one (dritherm32 batts fully filling a blockwork cavity). The Denby Dales houses spring to mind - they were I think 300mm cavity fully filled with mineral wool batts and again no air gap that I recall.  

My design aims to achieve a breathable building.  The purpose of the air gap is to prevent the ingress of driven rain as house is in the North West and near an estuary.  You are correct Denby Dale has a 300mm filled cavity, but then Yorkshire is not as wet as this side of the country.

[PeterW]

This makes interesting reading. 

 

BBA report

[ADLIan]

Check product BBA cert. Subject to certain caveats full fill, especially at 300mm, can often be used in any exposure zone. 50mm clear cavity safer still.

[ADLIan]

I was involved in that testing program. No real surprises from this testing compared to standard BBA recommendations, especially hit and miss issues with blown products.

[squealeyhealey]

On 26/09/2018 at 09:35, Dudda said:

I think it's not a question of the mesh available it's more a question of the quality of the bricklayer. I can't imagine a bricklayer taking the time to put in the mesh well. You also can't check the quality of the installation as it's hidden in a cavity. Also mortar is heavy and any drops into the cavity will compress the foam insulation a lot. If it was a timber frame structure with block outer that's fine but I don't see a need to use mineral wool insulation batt in a blockwork cavity. I'd fill the cavity with beads as suggested above.

Yeah, I'm not sure where I'm gona get a brickie to put this toether as I want it - whilst I'm standing over them to check the fit of the insulation!  Insulation batt is to achieve breathable walls & air gap to prevent the ingress of driven rain.  I'd assumed that any mortar drops would fall into air gap - I'd planned to clear bottom of air gaps during build.

[PeterW]

9 minutes ago, squealeyhealey said:

I'd planned to clear bottom of air gaps during build.

 

How...??? This will be 3m down, 50mm wide with ties in the way.... 

 

I think you’re worrying over nothing - are you in an area classed as very severe exposure to driven rain ..??

[Declan52]

Is your outside skin block work??

If so you can add all sorts of waterproof agents to the mix to stop any rain soaking through.

 

[Dudda]

If your using the fibreglass insulation batts to keep the building breathable then you'll have to get special plasters or lime and everything else will have to be breathable. If you put a cement based render on the house that won't be breathable and all the effort to make the rest of the wall breathable will have been in vain. 

While timber frames are breathable, modern blockwork houses generally aren't breathable the way traditional stone buildings were. 

Edited October 3, 2018 by Dudda

[Ian]

@squealeyhealey

even in an area of high exposure to driving rain a full-fill 300mm cavity would be okay as long as you use flush tooled joints in the facing masonry wall.

 

Part C of the Building regs guidance only goes up to 150mm full fill cavity but it shows it’s okay with even zone 4 max exposure sites.

 

Edited October 3, 2018 by Ian

[squealeyhealey]

Yeah, I'm using lime plaster and everything else is gona be breathable'  which results in the very thick wall - thanks for the reminder!

Edited October 26, 2018 by squealeyhealey
Was not linked to previous comment

[squealeyhealey]

On 03/10/2018 at 22:01, Dudda said:

If your using the fibreglass insulation batts to keep the building breathable then you'll have to get special plasters or lime and everything else will have to be breathable. If you put a cement based render on the house that won't be breathable and all the effort to make the rest of the wall breathable will have been in vain. 

While timber frames are breathable, modern blockwork houses generally aren't breathable the way traditional stone buildings were. 

Please advise if blown beads, suggested previously, would be breathable?

[PeterW]

3 minutes ago, squealeyhealey said:

Please advise if blown beads, suggested previously, would be breathable?

Yep very much so..!

 

They are bonded together with glue but still plenty of holes.