50mm Cavity Wall Insulation

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

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.

Ivor Windybottom said:

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.

Click to expand...

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?

AG1960 said:

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

Click to expand...

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.

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.

AG1960 said:

What is the 25% rule?

Click to expand...

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.

AG1960 said:

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?

Click to expand...

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

AG1960 said:

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.

Click to expand...

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

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

AG1960 said:

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

Click to expand...

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.

noseall said:

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.

Click to expand...

Thanks to everybody who has responded, most helpful.

On the above provided by Noseall, it looks as though I am either safe or very close to the boundary.

The works relate to an old extension. Having measured the length of its outer walls, they total 10m in length. The house as a whole though is roughly 45m (detached bungalow), so using this as a basic rule of thumb, I think it's safe to say that less than 25% of external envelope is being disturbed.

The part in brackets may be a problem - the extension has three outer walls. One 5m long but that extended a longer wall, so it is less than 50% overall. The front of the extension wall has a wide patio door flanked by two much narrower piers - I could potentially say the door was never removed, which is feasible and then 50% of that elevation hasn't been affected. The fly in the ointment is the final external wall which is only 1.3m in length. The outer leaf of that being fully replaced!

The inner leaf and roof are remaining exactly as they are.

I think I'm in a bit of a grey area here and it would be extremely harsh for me to be forced to not only improve the cavity insulation, which I am doing, but also renovate with thick plaster the inside of a recently renovated room. On this basis, I will avoid involving BC if I can help it. I've paid them enough fees in recent years to receive what was, frankly, very poor service. This would be yet another 400 quid to come and look at insulation. They don't even provide a duty officer any more so I've had to get advice on here because they won't provide it, apart from pointing me to their website, which is useless in obscure situations like mine. I don't feel any moral compunction on this occasion about just cracking on. The structure will be stronger, better and more energy efficient once I'm finished.

This leaves two final questions: should I get cavity wall batts or Rockwool, or a particular brand? And is it safe to fully fill the cavity?

Thanks again.

Hi all,

Any thoughts on the best non-PIR cavity insulation to use? Batts or rockwool? Particular brand and type you like?

Also, is it sufficiently water repellant to fully fill cavity?

Thanks

Knauf Dritherm 32 is a readily available "high performance" full fill cavity batt. Personally I'd see how wide the cavity is (hopefully a generous 50mm) and try to widen it a bit to squeeze 75mm in - you could step out the brickwork by up to 20mm without issue.

The batts should be squeezed in. Otherwise there's nothing to hold them against the inner leaf, you'll get a cold draught against the back of it.

In new build, with PIR ties with clips are used to hold it against the inner leaf.

Check the manufacturer's specs for yourself, the batts should be specifically designed and rated for full cavity fill.