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Boring question re: rendering when cavity wall insulation needs replacing

(7 Posts)
greyvix Fri 12-Apr-13 13:49:39

My dad needs to have his house re-rendered due to big cracks and damp getting in. At the same time, it has been discovered that his cavity wall insulation is damp, and will be replaced under guarantee. Should he do the cavity wall first (and risk it getting damp again, due to cracks in the render) or render first, and then have holes drilled in the new surface for cavity wall insulation?
I am clueless, and he is finding it all stressful. Thanks in anticipation.

noeyedeer Fri 12-Apr-13 18:48:31

We're in the process of buying a rendered house. After discussions with the surveyor we'll be having the rendering repaired and won't be putting in cavity wall insulation in. The surveyor gave very good reasons not to get it done, with problems like your dad has encountered being one of them.

Could your dad just have the insulation cleared and then repair the walls?

unlucky83 Fri 12-Apr-13 19:09:27

Also not keen on cavity wall insulation - cavity walls were invented for a reason! If his walls are exposed to driving rain etc which it sounds like it might be - I suspect it would better if it wasn't replaced...might be worth getting advice from a surveyor

I suspect that you need to do both at the same time (god knows how they get wet insulation out!) but remove existing render, remove insulation, replace insulation, replace render ... and try and get it done when the weather is drier (in the summer!) and quickly - insulation in then day after/same say start render .. the odd shower of rain on a suitable bare brick wall waiting to be rendered shouldn't be a big problem

greyvix Sat 13-Apr-13 10:33:53

Thanks for your advice. Good luck, noeyedeer, with your house purchase.

PigletJohn Sat 13-Apr-13 14:06:38

it is extremely unusual to have cavity wall insulation get wet, as you say, the walls are faulty and letting water in so no doubt that is the cause. Pleased to hear that it is being raked out under guarantee. The installers are supposed to inspect the walls for such faults before taking the job on, so they are I hope putting their mistake right at their own expense.

When cavity walls were first invented, cavity wall insulation did not exist, so 1940's building practice is no reason for ignoring subsequent improvements.

Yes, it will look neater if the new render is applied after re-insulation, because it will avoid having filled holes visible in the new surface. Filled holes in exposed brickwork can be filled almost undetectably, but I think your render will look better if it is applied afterwards. Unless the wall is left with unrepaired cracks and holes in it through wet weather, it will not get wet again quickly. I imagine the scaffold will be put up to hack off the old, and the same builders will apply new from the same scaffold, so there will not be a long delay.

OOI, what part of the country is this, and is the wet wall exposed to driving rain, or is there a gutter defect as well?

unlucky83 Sat 13-Apr-13 16:15:44

PigletJohn - the problem with cavity wall insulation is lots of the people who fit are doing on grants/subsidised and the installers work on piece work - the more they do the more they get paid - so they have an incentive to do it when maybe they shouldn't....ie walls are exposed to driving rain etc and there is all kinds of things like it won't be guaranteed unless the whole house is done (so you can't have 3 walls done) and they won't do anything 'difficult'.- also they should pick up on problems like crumbling pointing/cracked render and they don't...
Cavity walls were invented in part to stop water ingress/damp through (relatively narrow) walls (my parent's house (early 1800s) are solid stone walls - almost 2 ft thick) ...modern houses are now built with insulated cavities -a lot have partial insulation- the insulation is attached to the inside of the inner leaf and there is a small gap between the insulation and the inside of the outerleaf...(although pressure on increasing the U values and therefore resulting cavity widths are leading to problem with the walls being too far apart for stability/houses on new build sites taking up too much footage...so not all are partial)

greyvix Sat 13-Apr-13 21:38:43

The house is in South Wales, and is quite exposed at the back. There will be a delay with having the insulation renewed, as the job cannot be fitted in for a while. That's the problem really.

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