[Astrid Arnold]

I am working on a whole house retrofit and the client has bought these to fill the cavity in an exterior wall with https://unitylime.co.uk/shop/mrp/bauoth/expanded-glass-lightweight-beads/. The building is having external wall insulation added. My question is if this insulation material is suitable for the cavity. My understanding is that due to the EWI being added, there won’t be any driven rain penetrating the exterior skin of the cavity wall anymore and therefore the risk of moisture travelling to the internal skin via the cavity insulation is eliminated. The dew point of the wall will also change due to the EWI, but my question is about there still being a risk of moisture from the cavity reaching the internal skin of the wall. By the way, there is no retrofit designer or coordinator on this project.

Many thanks for your help!

Astrid Arnold

[George – Retrofit West]

Hi Astrid,

Thanks for your comment. Installing both external wall insulation (EWI) and cavity wall insulation (CWI) can be tricky. Expanded Polystyrene (EPS) is generally a good choice for cavity insulation, especially when it’s bound and secured to prevent movement or settling. This product seems to meet those criteria, but as a more environmentally friendly alternative. It would be worth checking if the product has been properly tested and has BBA certificate.

It’s important cavities are thoroughly filled to ensure airtightness and lock the heat trapped by the EWI from escaping. This will also help minimise the risk of thermal bypass. It’s also crucial the external finish (assuming render?) is sealed properly to create a reliable rainscreen.

Paying attention to details is key. Best practice is for the EWI to be installed below ground level (min 300mm) to minimise thermal bridging and address junctions with the roof and soffit. Often, cavities are left open within the roof void, so it’s best to close them up for added airtightness.

Thanks

George

[Astrid Arnold]

Hi George,

thank you so much for taking the time to respond to my question, I very much appreciate it.

Astrid

[chptr2archt]

Hi Astrid,

I’d agree with George’s response and add that if the cavity is left unfilled, you don’t want it to be a route for air to escape from inside to outside. If air can leak through internal cracks into the cavity then it may find it’s way out at the top of the wall or other hard to reach places that wouldn’t be improved by the presence of EWI. A well installed filled cavity, plus the new EWI is the way to prevent this, I’d suggest.

Alex Bell

[Astrid Arnold]

Hi Alex,

thanks for your reply to this, that’s very useful.

Astrid Arnold

[Author]

Posts