Cavity wall insulation

Hi all!
I was trying to measure the width of the cavity in the outer walls of our 1930s semi to work out if cavity wall insulation is possible - by opening up an air brick vent low down in our lounge (seems to be about 70mm but hard to be accurate).

While I was doing that some fluffy white material fell out - anyone know what this is?
Is this an old form of cavity wall insulation perhaps? If so, does that mean there’s not much point in having it re-insulated?

Finally, could it have asbestos in it??

Many thanks


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Hi Peter,
I can answer part of that.

I believe it is cavity wall insulation. We have the same in our 1980s bungalow. We’ve had a corner of the bungalow opened up to add an exrension and this stuff was in it

Does it gave asbestos? Don’t know, our builder didn’t panic so I’m hoping not.

I dont like this type because it is blown in, very light and Ivv bc suspect a lot of friction. They found air pockets on the corner which is irritating to think there may be gaps elsewhere.

I most likely means your cavity is filled, so if you want more insulation you need to do internal or external. My knowledge ends there I’m afraid.

great thanks Robin - very useful :slight_smile:

Interesting photo! Have you got another photo looking more directly into that hole?

Looks like knauf supafil to me, which is what we have in our cavities. And yes the installers do tend to miss bits.

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I’m glad you didn’t lose your phone in there. I’d be tempted to pack that space with mineral wool before covering it over again. It looks reasonably dry in there too so I am not sure whether the air vent was really needed? Can you feel at draft in there when you put your hand in? Does (or did) the room have a gas fire in it or an open fire? Do you still want / need a fire in the room? If no fire then the need for air from this location may no longer be important.
In terms of passive inlet vents the Aereco vents are probably the best. You would probably fit one of those Aereco units higher up, say 1.9-2.0m above floor level.

It you have existing cavity wall insulation, even with minor flaws, the U-value can be improved by adding more insulation either internally or externally. My preference is EWI as that covers any cold bridges, whereas IWI is likely to create new thermal bridges. It is far harder to reach Passivhaus or EnerPHit standards with IWI for that very reason.

If you are planning MVHR then you can start sealing off any air vents.

Hi, new here, hope I am asking in right place, a query on CWI, we had it installed about 14 years ago (fibre stuff not EPS)- then our house was a 3 bedroom semi with garage attached. They installed it in front and back elevations and I thought the gable end where the garage is attached. We Subsequently extended over the garage. Now it looks like (had a look with a borascope) there is no insulation in what was the gable wall and is now the wall between our stairs and garage and stairs and bedroon at first floor. Our stairs/hall is colder than rest of house- I am wondering if installing CWI in this instance would make a difference? It would effectivley have to be installed from the loft I am guessing and from the garage. thanks for your thoughts.

Hi Lisa,

First, welcome to the forum.

At the very least the cavity needs to be sealed. Cold air seems to be getting in somewhere, probably through the garage. You could try making the garage wall airtight by applying brush on Blowerproof or similar, paying particular attention to any penetrations, and then sealing at the top. That would still leave circulating air in the cavity but would be DIY. If you are paying for an installer to insulate the cavity be sure to use rockwool or other fireproof, non melting insulation, due to the garage and associated fire risk.

thanks Tim, hadn’t thought about the fire risk- EPS not to be used then? We also have the main electricity meter etc on that wall, so I guess cables pass through the cavity too- maybe another reason not to insulate the cavity? Good to know we have a DIY option though, I’ve just checked are there are quite a few penetrations. Not sure how accessible the top is?- could I just seal it by stuffing some insulation in the top if I can access it, or expandable foam, or is there something better?

Hi Lisa,

Ideally use something impermeable to seal the top and insulate over the top.

By the way, normal “Blowerproof” dries black. You can specify white when ordering.

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