Cavity Wall or External Wall Insulation

1 Mar 2011
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West Glamorgan
United Kingdom

I live in a 1960's built house with a cavity wall. I was thinking about having cavity wall insulation until I spoke with some building surveyors who did not recommend having it done as the point of the cavity especially in westerly areas which is where I am (Swansea, Wales) was to help reduce moisture.

Looking at the map I think I am classed in zone 4 and also reading this article from a Daily Telegraph writer

So instead of having the cavity filled would external wall insulation be a better option?
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External wall insulation would be better (you would do both, the external insulation and render would be the new weatherproofing).

Big difference in performance, but also cost.

You could batten/cross batten, with insulation in between, membrane, another batten and then rendar/cladding.

Or go down the more common route of slabs of insulation tied to the building and direct rendar.

The first is cheaper, and a little bit more DIY, and has the advantage of a cavity. The second is the more common method, and absolutely relies on good workmanship (sealing around windows, and other penetrations) to ensure water does not percolate through into the insulation.

Different options, different costs.

with external options you want to look at if your eaves overhang needs extending.
Thanks so would you rule out having cavity wall insulation and go for the exterior wall insulation which would be better as there is still the cavity in place?

What is the cost of the exterior type insulation for an average 4 bed detached house?

The reason I was looking at the cavity wall is that the likes of British Gas are offering this for free
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What is the cost of the exterior type insulation for an average 4 bed detached house?
For something like monocouche or k-rend on a proprietary riblath and foam board fix insulation type system, scaffold, eaves extensions, existing render removal etc you are looking at £10 - £15k.
EWI is the most costly form of insulating about. Expect to pay between £65 and £100 m2 depending on the system used. Unfortunatley as you have a brick external skin you wouldnt be able to get a subsidised or even free install of EWI under the greendeal scheme or ECO :( . In your case it would be most cost effective to have a mineral wool CWI and renew your rendering if you are worried about the penetration of water through the existing surface. Most suppliers of CWI will tell you that the mineral fibre they use will not wick water through from the external facade. If you can get one that will give you a garuntee then you are quids in :D

Ok thanks - I am not against CWI but I have read so many horror stories about it and speaking with an architect friend recently he also said that filling the cavity is not advisable, especially in South Wales because of all the rain we have.

Anyway so the best CWI on the market is mineral wool over the likes of the the beads or foamed insulants? I always thought that the foamed insulants (Polyurethane (foamed-in-place)) offered the best R values over the alternatives
Im not sure as to which of the CWI is technically the best as I have never had anything to do with it other than ordering for it to be installed. I know that most of the firms round my neck of the woods tend to use blown mineral fibre, that is possibly because it may be cheaper than other systems. The resin bonded bead can be used where you have a stone external skin in some instances, usually aslong as they havent used stones to tie both leafs of the wall together. They are bothered about cold bridging in those circumstances which I think they are just using as a get out of liability for if anything goes wrong. After all all those 300-400mm thick window cills on the victorian houses never suffered from more condensation that the rest of the cavity walls.
If you are getting water penetration into the cavity then its a problem with the external render. Even the heaviest of driving rain shouldnt pass through 15-20mm of properly done render and then a skin of brickwork! Anyway Back to the question of EWI a system I have been installing for energy companies recently is sto mix system, its relatively cheap and Ive not had any problems with it. They are owned by sto now which is the largest rendering system manufacturer in the world, but are run as a seperate business. The uk supplier for their gear is Urban West Energy Efficiency Ltd in St helens. Im sure if you gave them a call they would be able to point you in the direction of a local installer to get a price. :D

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