Is there a reasonably foolproof way to determine cavity width on existing build without demo?

[YodhrinForge]

I'm starting to get into the weeds of detailing with my architect about my renovation and was just wondering if there's a way to determine cavity size without having someone digging into the walls? I suspect I know the answer but might as well check. There's an extension on the back of the house that definitely has some kind of cavity as there are exterior wall vents, exterior leaf is blockwork(it's just painted so is obvious), inner leaf could be brick? Can only barely see it through a tiny hole in the back of the cabinets at floor level - you'd think it would be block as well if the outside was, but there's also a masonry built detached garage that seems to have been put up around the same time as the extension and that's brick inner block outer(although the block outer might just be a facade? there's a layer of engineering brick visible just above the paving slabs and the block layer protrudes out from the top of that by ~30mm or so) so who knows. The approximate thickness of the extension wall is 320mm, the garage wall is 230mm.

 

Trying to decide on relative merits of different insulation systems & materials but it's all so dependent on if there's a cavity and how thick it is.

[Temp]

Look in the loft or drill a hole through one leaf and the insulation. You can feel when the drill hits the other leaf.

 

If you can measure the total wall thickness it's likely (but not guaranteed) to be about 200mm less than that. eg each leaf is 100mm.

Edited August 28 by Temp

[YodhrinForge]

4 hours ago, Temp said:

Look in the loft or drill a hole through one leaf and the insulation. You can feel when the drill hits the other leaf.

 

If you can measure the total wall thickness it's likely (but not guaranteed) to be about 200mm less than that. eg each leaf is 100mm.

 

Yeah I probably will have to drill it in the end, bloody inconvenient though due to the way the extension was done(hipped roof into middle of existing pitched roof so no easy access from the attic, walls inside are tiled and the roof has that really tough popcorn plaster, probably have to go in from outside and then just jam the hole with silicone for the short term). On the plus side your comment made me double check and I was just being dim with the garage - it's single skin on three sides with solid double on the front with the doors in, they just put some lines into the render on the front side so it would resemble the wall of the extension hah.

[Alan Ambrose]

Little emdoscope cameras that attach to your phone are not the greatest but they may help you too.