Cavity wall insulation causing cracks on render - Insurer/Installer AWOL

stuart45

Beads are better, but if there's mortar dropping on the ties the mortar can stay damp for longer because the cavity isn't so well ventilated. It's more the fault of the installer if they haven't had the ties cleaned first.

Black ash mortar is the worst for rusting ties.

Chickereeeee

https://forums.moneysavingexpert.com/discussion/comment/81948610#Comment_81948610

There is not a lot of air movement in cavities anyway (at least those with cavity closers, as is the more modern way). Beads would not make much difference. If this are rusting, it would likely be the mortar that is the problem.

stuart45

The house is 1930's, not a modern. Even modern houses have a certain amount of ventilation through the weepholes. Houses with a tray at DPC level have weeps there as well as above the frames. The rusting of ties isn't such a issue now as they are stainless steel. However, with a 30's property with fish tail ties, a certain amount of ventilation is useful. Even then, with any insulation, cavity or internal, the outer skin becomes colder and stays damp for longer. This has 2 main effects on the brickwork. Certain bricks are more likely to spall, and the ties in the brickwork start to rust earlier than the ones next door as the galvanised surface breaks down. This is why you see these issues in older semi's, such as nightfrog's house where next door hasn't seen these issues yet. Once there is a slight crack in the joints the rusting accelerates really quickly as water can enter much quicker.

Bead insulation is also dependant on the quality of the installers. Over or under bonding can cause issues, as will getting voids in the cavity. It's also harder to remove if it's causing issues.

diveunderthebonnet

https://forums.moneysavingexpert.com/discussion/comment/81948610#Comment_81948610

Can i ask a question ,how do you clean the ties in a cavity ,i have watched many houses being built on site ( i used to repair construction equipment for a living ) and tried to imagine how you got into the cavity on a two story house.

stuart45

https://forums.moneysavingexpert.com/discussion/comment/81950068#Comment_81950068

They take a few bricks out at various spots in the corners, and then use a long piece of roofing batten to knock the mortar off the ties. It usually drops down below DPC where it can be raked out with a batten with nails in the end. On new builds you can use a batten from the top to clean the ties before the chippies pitch the roof.

In theory you are supposed to use cavity battens when building a cavity wall, which stop mortar dropping down the cavity, although I've rarely seen them used on site. Never with subbies.

diveunderthebonnet

https://forums.moneysavingexpert.com/discussion/comment/81950226#Comment_81950226

Thanks stuart45 ,i thought you were going to say they worked from the top of the walls were the roof trusses fitted.😁