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?