Cavity fill: rockwool or EPS?

[tony1851]

There's often a misconception on why cavity walls came about. They were widely introduced after WW1, not specifically to improve thermal insulation (coal was relatively cheap) but to prevent rain-penetration.

Prior to WW1, the vast majority of smaller houses were built with 9" solid walls. If carefully-laid and all joints buttered up, a 9" wall was perfectly capable of keeping rain out except in positions of high exposure.

After the War, there weren't the numbers of skilled bricklayers around to ensure quality brickwork and, at the same time, the government embarked on a large house-building programme.

As both skill and time had reduced, the cavity wall was more widely introduced as a quicker alternative to ensure dry walls. In the old days, it was usual to incorporate vents at the bottom of the wall to help dry out any rain which had penetrated the outer skin (the top of the cavity was usually open anyway so an air flow was acceptable).

Nowadays, because thermal insulation is equally important, residual cavities are not ventilated. It seems to be accepted that some rain will penetrate, so it is guided away by cavity trays and weepholes.

 

[^woody^]

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[MikeStout]

Sorry for being silly, Tony, just to make sure I got you right: full-fill, no gap, yet cavity trays and weepholes? is that right? Weepholes are needed to remove moisture not to vent the cavity?

 

[garyo]

The Cellotex website still seem to be banging on their "keep the cavity" drum

Why is partial fill cavity construction so common?
Using rigid insulation boards in conjunction with a minimum 50mm air cavity is a tried and tested solution familiar to designers, specifiers, contractors and building inspectors alike. It is a popular method for constructing external walls in the UK, and has been used on building sites for the last hundred years.

The clear cavity between the outer leaf and the insulation acts as a barrier to external moisture, stopping it from reaching the inside of the building. Most commonly, the outer skin of the cavity wall is constructed from brick. Blockwork is also used, clad or rendered to provide the desired finish.

https://www.celotex.co.uk/applications/wall-insulation/masonry-cavity-walls

I think I'll just keep stuffing my cavities with rockwool...

 

[garyo]

They do touch on full-fill PIR, but then don't offer a product...

Traditionally, rigid insulation boards were suited to partial fill cavity wall solutions only. Now, more advanced manufacturing techniques have allowed the development of products that overcome issues of water ingress which have typically required a 50mm cavity to be maintained.

The result is a wall where more of the cavity can be filled with some of the most thermally efficient insulation available, giving lower U-values without having to substantially alter the designs that are so familiar to - and popular with - much of the construction industry.

Celotex do not currently offer a full fill product for cavity walls.

@mike, if you choose to go full-fill I'd make sure your BCO is happy in advance, or you have a manufacturer spec sheet that clearly specifies it as an installation method so that you can argue with your BCO