Cavity wall te replacement

[Gareth Hughes]

A related thing I've been meaning to ask for ages:

We too have a 1910-ish cavity walled house. I read somewhere that in this era cavities were more intended to stop wind-driven rain than as insulation, and a surveyor mentioned that similar houses down the road had very chunky iron wall ties. This suggests that adding cavity wall insulation to such a house would be a bad idea, as it could provide a bridge for moisture, or at least slow the ties from drying out if they ever get wet. Is that right, or were the Edwardians just wrong about the function of the cavity, and it works (dry) just like modern ones? And does the lack of zinc coating on old ties make cavity-filling insulation a bad idea in any case?

PS. I forgot to mention: as far as I can detect, in our case there is only a damp-proof course on the inner leaf of the cavity, and it looks like some sort of tar (applied liquid, then set), definitely not slate. Don't know how unusual that is!

Cavity construction has nothing to do with insulation - its purpose, as you say, is entirely to separate the wall into two leaves, the inner one of which will not be affected by the ingress of water, while the outer one, to some extent, will always be "damp", in contrast to traditional solid-wall construction. Filling the cavity with insulating material, in theory, completely negates the intended performance of the wall construction, by bridging the cavity.

 

[chuckey]

When you fill the cavity, the results depend on what you fill it with. Foam does stop the cavity drying out, also allows wicking down the wall tie/foam interface or if the foam cracks through settlement (extremely unlikely). Foam beads or blown fibre does not suffer from the above disadvantages but its also not such a good insulator and can suffer from settling down. The most cost effective thing to do is to seal the top of the cavity with glass fibre, punched in hard and likewise sleeving around any air bricks. Basically to reduce the speed of the airflow through the cavity. Of course its cheap, but in the end it doesn't save much money. The only cheaper form of insulation is to paste down aluminium cooking foil on your floor boards. draughtproofs and reflects heat back into the room.
Frank

 

[Nemesis]

Not exactly robust though, is it?

How long would it last if walking on it?

 

[DaveBrigg]

I've tried sorting out warped floorboards and ended up foiled every time.