50mm Cavity Wall Insulation

[AG1960]

Hi everyone,

We are having the outer leaf of bricks replaced on an old extension and intend to improve the cavity wall insulation from the existing blown-in fluffy stuff.

What type is likely to give us the biggest improvement in a 50mm cavity? I believe this would be the PIR boards but one of the retailers told me that we must maintain a 10mm gap if we use these. Is this true? If so, we could only use 40mm thick boards presumably... would this be better than 50mm of cavity wall batts or rock wool?

Many thanks

 

[Ivor Windybottom]

Your biggest problem is likely to be that it wasn't built in, so there will be hacked-off half-bricks and mortar snots jutting out into the cavity, which will prevent the insulation from contacting it. Insulation with a draughty gap between it and your inner wall will be pointless, as you'll have cold air blowing against the backs of the bricks. Rigid insulation probably isn't a sensible option for retrofit, regardless of its U value.

When rigid insulation is built in, the bricklayer takes care to scrape off the backs of the bricks while building. If there are any lumps when the insulation is installed then they'll be scraped off while the mortar hasn't yet fully set.

Use the batts.

 

[AG1960]

Your biggest problem is likely to be that it wasn't built in, so there will be hacked-off half-bricks and mortar snots jutting out into the cavity, which will prevent the insulation from contacting it. Insulation with a draughty gap between it and your inner wall will be pointless, as you'll have cold air blowing against the backs of the bricks. Rigid insulation probably isn't a sensible option for retrofit, regardless of its U value.

When rigid insulation is built in, the bricklayer takes care to scrape off the backs of the bricks while building. If there are any lumps when the insulation is installed then they'll be scraped off while the mortar hasn't yet fully set.

Use the batts.

Thanks. We have noticed the snots etc on the backs of the inner leaf but think we can get those off with a chisel. That said, you are right about tue ease and flexibility of using the other type, and there will investably be gaps where there is unevenness on the inner leaf, which there is.

Would you fully fill the cavity with the batts?

 

[noseall]

What type is likely to give us the biggest improvement in a 50mm cavity?

Not much really, other than high performance 50mm (wool) cavity batts. Your issues with PIR is, either maintaining the correct air gap which would mean the product would be too thin to be an improvement on the 50mm wool batts. Or with full fill PIR (10mm gap or less), you would need them to have rebated edges (water deflective) but they are unlikely to make them in that thickness.

BC will likely enforce the 25% rule and you may end up having to improve the insulation internally.

 

[AG1960]

What is the 25% rule?

If the cavity was empty to begin with and then filled with wall batts would that be sufficient without having to renovate the room again internally?

The room is already fully decorated and we do not want to be replastering it all again, ripping off skirting boards, replacing them, redecorating. The purpose of the project is simply to replace the outer leaf which is horrible. The structure and room within otherwise remain as they are (save for cavity insulation inprovement) and we are continuing to use the room as work progresses. It simply isn't in the scope of works or budget to be doing any more and the end result will be far better than how it was and could have been left.

 

[noseall]

What is the 25% rule?

In UK Building Regulations, if you are "disturbing" or renovating more than 25% of the total surface area of an external building envelope (or more than 50% of an individual element like a single wall or roof), the entire thermal element must be upgraded to meet strict modern Part L Fabric Efficiency standards.

If the cavity was empty to begin with and then filled with wall batts would that be sufficient without having to renovate the room again internally?

There may be an economic argument that you have improved and in going further would not benefit cost wise - yes.

The room is already fully decorated and we do not want to be replastering it all again, ripping off skirting boards, replacing them, redecorating. The purpose of the project is simply to replace the outer leaf which is horrible. The structure and room within otherwise remain as they are (save for cavity insulation inprovement) and we are continuing to use the room as work progresses. It simply isn't in the scope of works or budget to be doing any more and the end result will be far better than how it was and could have been left.

As above, yes. But it would not surprise me to ask you to maybe improve the roof or whatever.

 

[stevie888]

Are you involving BC? In the real world "owts better than nowt" but BCs do tend to want full compliance if anything is done.

 

[endecotp]

we could only use 40mm thick [PIR] boards presumably... would this be better than 50mm of cavity wall batts or rock wool?

Since no-one else has said it: yes, on the raw numbers, 40mm of PIR is better-insulating than 50mm of mineral wool. (By about 30% I think, so it's not insignificant.)

But as noted above there are other considerations, i.e. the difficulty of getting an unbroken layer of PIR.