Is it worth applying a parge coat to the inside of a solid 9" brick wall prior to building a new block wall on the inside to create a cavity wall?

My thinking is that although the new block wall will be wet plastered and therefore hopefully airtight, the cavity in which the insulation will be located may not be so airtight (using mineral wool batts for insulation). Would re-pointing the external lime mortared wall be just as good at making the cavity airtight?

Hi Tony,

In answer to your questions (plus some more questions):

I'm assuming a wet plastered block wall on the inside would be airtight - am I correct? Wouldn't the parge coat help to reduce air movement within the cavity and improve the effectiveness of the insulation? Should I be installing a separate air tightness barrier?

The building will be re-pointed with lime mortar to maintain it's beathability.

Cavity will be 250mm with full fill mineral wool cavity batts. I'm unsure at the moment what blocks to use. I'd like to use unfired bricks but would have to have something stronger in one area to support a 'mezzanine' floor - your thoughts on this would be appreciated.

I'm glad you're saying it's not worth parging - one less thing to do.

I know I'm deviating from the subject slightly but can you advise as to whether it would be possible to use lime mortar for some small single storey extensions to the building. The new extensions will be 4" brick outer skin/250mm cavity/4" block (solid) inner skin. I've read articles about lime mortar having insufficient strength for single brick walls.

I shall delve into my paperwork to see if I can find the references. I seem to recall at least one article that suggested lime mortar had insufficient strength when cavity walls were first introduced. Maybe this was caused by a particularly poor mix. It did say that portland cement became the accepted standard because it's strength and quality was far more consistent and more suited to mass house building.

I've also read somewhere that wide cavities do not behave as a single wall, even with the wall ties. They have to be regarded as two separate walls. Sadly I can't remember where I read this either - this is definitely a problem when using the internet to do your research. So much information you can't print everything out so you're left with just the ageing brain cells to record it!

What sort of lime would you use in this mix Mike?

Hi Pingy, my 1880 terrace has single skin partition lime mortar walls between me and next door, it's still standing. Doesn't make for great sound proofing though!
Tom