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How to add an insulation cap to internal double brick walls?

Making a Splash

How to add an insulation cap to internal double brick walls?

I'm getting started to insulate our attic. After lots of research I learned that internal double brick walls act as chimneys for cold air - they suck it in from the subfloor and vent it out into the attic. This leads to heat loss in the house.

Unless they are capped.

But I have so far failed to find a good solution for this. I like to add a cap that prevents this convection of cold air, but is not permanent so I can still access the wall cavity, for example if I need to route a cable.

The closes product is a "Baffle block", but I'm not sure if I can just use some normal insulation batt and weigh it down with some MDF or similar. 

I'm thankful for any tips and pointers. Cheers!

Bunnings Team Member
Bunnings Team Member

Re: Adding an insulated cap on top of internal double brick walls

Could you use normal insulation batts and compress them within the cavity @TheHandySqirrel? That would be far less expensive than the product you've mentioned. Perhaps you might like to post some pictures of the top of the walls. That would assist our members in offering suggestions on how you can seal the area. 


My only concern, and if it might not even be an issue, is that double brick walls aren't typically capped. That's likely why you're not finding many products specifically for that purpose. Even the product you've linked to appears to be for reducing sound transmission, not sealing airflow. My concern would be the build-up of moisture between the two walls. However, it sounds like you've done your research on the topic.


Please let me know if you have any questions.




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Re: Adding an insulated cap on top of internal double brick walls

Hi @MitchellMc,

below are photos from above, the second one with flash to show the presence of cables and thus the need for future maintenance.

I have since noticed that only two such wall cavities are accessible from the attic and suspect that the house might have been altered at some stage. All other internal walls are closed.

I agree with your point of moisture as moist air rises. I might just cover the gaps with batts then and see how things progress over winter. Might not be as much an issue as I originally thought.


The (dark) wall cavity between two brick walls.The (dark) wall cavity between two brick walls.


Cables are visible in the cavity.Cables are visible in the cavity.


Thank you for your thoughts!

Re: Adding an insulated cap on top of internal double brick walls

Hi @TheHandySqirrel 


Thanks for sharing those photos of your brick wall. The gap between the brick walls serves two functions, one is to allow any trapped moisture to escape and the second is to provide insultation. Covering this gap might interfere with the walls thermal regulation and could possibly trap the moisture inside causing mould to form. 


However, if you are aiming to improve the insulation in your ceiling, it is still possible to put batts in the ceiling area as long as the gaps are not covered. I suggest speaking to your builder and asking them if it's safe to cover the gap at the top. 


If you have any other questions we can help with, please let us know.




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