Poorly laid cavity wall insulation

[jj4091]

I think you need to understand what the reason for wanting the insulation held against the inner skin is....

My understanding - and I am not an expert! - is that you want it held against the inner skin exactly because you don't want it in contact with the outer skin. So filling the gap between the PIR and the outer skin with vermiculite would be exactly the wrong thing to do. I think the idea is that water that penetrates the outer leaf can run down the inside without bridging across and making the insulation and inner wall wet.

Someone who knows more, please comment.

My thoughts, although like you not an expert, are that you want the insulation held against the inner skin so that it retains heat in that skin and it does not escape into the cavity.
Reading about the properties of vermiculite suggested that it will meet the ops requirement and improve insulation at the same time.

What properties does Vermiculite have? Vermiculite has a vast range of benefits and properties, including:

• Fire Resistant – Its safe up to 1300oC making it a safe choice for home use.
• Moisture Resistant – Micafil Vermiculite has good moisture resistance which helps absorb moisture or spilt liquids.
• Durable – It’s a long lasting, natural material that retains its uses for decades.
• Lightweight – Being lightweight and easy to handle, it make it ideal for a wide range of uses.
• Insulation Properties – Suitable for many insulation situations, such as loft insulation.
• Acoustics – The insulation properties of vermiculite also make it good for absorbing sound.
• Natural and Environmentally Friendly – Occurring for a natural mineral, Micafil Vermiculite is environmentally friendly and safe for use around people, pets and plants.

Perhaps the op should make enqiries of the local BC and the manufacturer before going down that route