Cavity wall insulation questions

[wooooofudge]

Our house is an Edwardian semi. There is part cellar and part crawl space under the ground floor. There are plenty of air bricks below the ground floor joists and floorboards. My understanding is that it's not always a good idea to put cavity wall insulation in as it can prevent air circulation, trap condensation, etc and cause damp problems. Am I correct?

What about newer additions to the house then? There is a brick built extension and adjoining that an orangery. Assuming they have concrete floors and damp proof courses put in when they were built, are we okay to put cavity wall insulation in these more modern areas?

It's not common for an Edwardian house to have a cavity suitable for CWI, though I have seen it during demolition.

There is an obsessive anti-CWI journalist who gets lots of exposure on the web. Water penetration through the outer leaf can happen if the brickwork, render or pointing is defective, or if the wall is cracked, or if the gutters or downpipe leak. In which case they need fixing, regardless of CWI. I am in a coastal location and in storms, an impressive amount of water runs down the weather side of my house. However the wall is in good condition and I don't get damp. Exposed, storm-lashed walls are not recommended for CWI if there are any doubts about their condition.

Obstruction of ventilation is a red herring because houses are not ventilated through their walls (unless they have holes in them).

Because the walls are warmer after CWI, they are less likely to get condensation on them. However if the house is damp for some other reason, the condensation will now form on the windows where it is more noticeable.

CWI installers can now be forced to remove it at their own expense if they are found to have installed it in an unsuitable wall, so they are now more choosy about their customers. I'd suggest trying your gas or electricity supplier, they may have a subsidised scheme and are very unlikely to run off with your money or go bust. IME the workmen they use are well-trained and neat (YMMV).,

Can I cheekily leap on this thread to ask PJ about putting something like kingspan under the floorboards (above a cellar/crawl space) in our Victorian house? It's typically cold and draughty but I'm nervous of causing problems as I hear these houses 'were built to breathe'...

you can do, but it is easier if you use mineral wool, because it is soft enough to press into irregular gaps without needing precision cutting. In al old house, cold from the (ground) floor is more from draughts. There is no loss of heat by downward convection, but warm air will tend to rise through the house, escaping up chimneys and warming the upstairs ceiling until it can escape into the loft.

To prevent the wool falling out, chicken wire or garden netting can be used, stapled to the joists.

Do not squash the quilt in such a way that you reduce its thickness.

It will slightly reduce noise transmission. Buy the brown mineral wool treated with Ecose, which prevents it shedding irritant dust and fibres.

Take extra care to pack the gaps round the edges of the room, as there will be a gap, concealed by the skirting. Draughts through floors show themselves with black dust stains on the carpet.

Make sure your airbricks are cleared of dust and cobwebs, and that their ventilation is unobstructed by the insulation. Insulate the pipes. Put insulation either above or below cables so that the are exposed to the air on one side, and not encased in insulation. The space under floors is meant to be cold and draughty, like a loft.

Fantastic, thanks very much for the advice PJ and apols to the OP for the hijack!

I'll let you off Pip .

Interesting and thank you PJ. The external walls are really thick we still have the original single glazed leaded and stained glass windows and there is condensation on the two East facing bays (one our bedroom) on cold mornings, though it clears.