Talking to a builder tonight about our project, and he advised in the strongest terms against trying to put EWI on an existing 30's cavity wall (which has no current insulation).

His opinion was that all your heat goes through the brick and simply warms the air in the cavity up, before escaping. He seemed to suggest that all the EWI firms promise that this won't be the case because the EWI will form a perfect seal, but he suggested that in reality this was extremely hard to achieve. He said he'd take 60mm of IWI over 100mm EWI in such a situation any day ...

Thoughts anyone?!

I 70mm EPS EWI over a rockwall filled cavity wall about 5-6 years ago , so far no signs of negatives.
spend quite a bit of time on sealing top and any routes into it to reduce potential moist air getting in and condensing mid structure.
Empty cavity would, as said, just take the heat away via ventalition unless completely sealed at the top etc..

How about filling the cavity with lean mix or similar and then EWI , suggested by Tom on GBF to give a thermal massive warm wall.

What they all said.
You would be very surprised how much heat an unfilled cavity can leak away even if you think it is well sealed. The EWI will certainly be pretty pointless unless you fill that gap with something. If you are on a very tight budget then just the cavity fill alone will probably be more thermally effective than the EWI.

The main concern with filling the cavity is the possibility of bridging the cavity with something that allows moisture to track across. In that respect Tony's suggested preferences look good to me too.

Hi Scrimper,
I have had cavity insulation fitted to all the houses I have owned. And have never had a problem. CIGA 25 year gaurantee on all of them. Mind you the oldest one was built in the 1960's. I am now looking at moving to our "final" resting place. I am interested in older houses and wonder if there is any particular problems with installing Cavity insulation in them. My last installation was blown in rock fibre took less than a day and cost £200. Have you contacted any insulation companies ? and have they highlighted any problem areas ?

Richard

HR - I'm sceptical of contacting insulation companies direct to be honest because their take on it is going to be somewhat impartial to say the least!

Tony - can't say in detail why they are so hard to fill, but appparently it would necessitate a myriad of holes and then very difficult to ascertain whether it has successfully filled the entire cavity ... and of course, any slumping in the material then leaves gaps at the top where it is most vital!

The other factor to consider is the speed of response to heat input .. which seems to make perfect sense, as with IWI you are going to get a more immediate effect ...

How wide is the cavity? I was told if it is only 25 or 30mm it can't easily be filled.

Where do they think that heat (by air convection) will escape to, from the cavity? into the loft maybe? Either fill the cavity and/or seal its top off from uninsulated loft space. If you're EWIing, unless it's too thin to be worth while, you're bound to be disturbing the eave tiling so you should have good access to the top of the wall and its cavity. If the loft is part of the insulated envelope, then no need to do either.

Whether or not the EWI is airtight (it doesn't claim to be) is irrelevant.

Filling the cavity with insulation is the cheapest and simplest thing anyone can do - many firms falling over themselves to do it. But inadequate on its own - additional EWI recommended.

If it was me, I'd fill the cavity solid with weak concrete, then EWI outside that. That way, you get lots of beneficial thermal mass inside a tea-cosy of insulation.

But one way or another, EWI is def the way to go - avoid IWI at al cost. Spend time to understand the issues and ideas.

Ouch. And just when I thought I REALLY understood it ... !

But thanks for your input.

There is no loft in our house ... 100% flat roof. I would have thought there was plenty of opportunity for the air to escape given that it is a 80 year old brick wall, and I've got no idea how good the cavity trays are at the top.

100m of 6m high 100mm cavities = 60 cubic metres of weak concrete cavity fill - that's quite something! Wouldn't the wall ties possibly be compromised by such an undertaking?

The house has a rendered finish, so aesthetically EWI is a perfect fit ... it's just the frightening cost of it, and the thought of having to deal with the cavities was starting to really put me off. But once we have vacated the property, I shall have the guys really dig around and see what state everything is in within the walls - should make things a lot clearer.

Hi Scrimper,
You would probably get a good performance/cost benefit from cavity insulation. With internal insulation there could be many issues. One of which is where internal walls meet the internal cavity wall giving a thermal bridge. And external insulation is costly. One way is to have the cavity insulated (you will end up with drill holes at about 1 metre grid and extra ones around windows and doors about 25mm diameter). If these are filled and made flush with the current render then you could apply a coat of special "flecked" render coating. This is applied with basically a tray of the special weather proof coating with a rotary brush "flicking" the render coat onto the wall. I had this done one one house. I can't remember the name of this. But it may be worth getting a decent company to give a quote. This will probably give the best cost/benefit.

Richard

Roof/ceiling insulation rates as a higher priority than anything to with walls, you probably only have plaster between you and the outside world!

here's a little heat loss pic.

And a flat roof will be different.

Polystyrene bead (with glue) cavity fill is pretty good at reaching all voids, and has minimal slump. A couple of firms around here (Sussex) will do graphite enhanced poly beads. It's likely to be your most cost efficient way of doing things.

I've used Soudal Flexifoam (I paid £6 per can when buying a box of 12) to seal cavities at the top and side in order to stop any air travelling through the insulation in the past.

Before you get the fill done, take the opportunity to check for cavity wall tie corrosion - it'll be easier to remedy it (if present / necessary) before the cavity fill goes in. Oh, and get a CIGA guarentee too, ask for a few quotes and ask the surveyor some tricky questions and see how they answer.

Posted By: scrimperHis opinion was that all your heat goes through the brick and simply warms the air in the cavity up, before escaping. He seemed to suggest that all the EWI firms promise that this won't be the case because the EWI will form a perfect seal, but he suggested that in reality this was extremely hard to achieve

Coming back to this belief, there's something wrong. We've mentioned cavity warm air escaping laterally into uninsulated loft (or flat roof voids), but he's talking about heat escaping thro an air-leaky outer skin and EWI, regardless of good thermal resistance conductivity-wise.

So it's about heat that may be carried by bulk air movement thro the outer skin and EWI. This assumes an existing air-leaky outer skin, and saying the EWI isn't airtight enough to kill this air movement.

Well, if the outer skin is already that air-leaky, a bit of cavity fill isn't going to cure that at all. Heat escape via bulk air movement will continue just as before - polystyrene bead fill, even with added glue, offers negligible air resistance. Urethane foam fill couldn't guarantee to be continuous either, and I'm not sure it's very airtight anyway.

Whereas EWI, even imperfectly done, will vastly improve the wall's airtightness, compared with doing nothing about it. At best, EWI is well airtight over its open surface, the weak points being at windows etc.

Cavity fill won't even stop the other risk, either - cavity warm air escaping laterally into uninsulated loft (or flat roof voids). To prevent that, you'd have to do something definite e.g.

Posted By: TimSmallI've used Soudal Flexifoam (I paid £6 per can when buying a box of 12) to seal cavities at the top and side in order to stop any air travelling through the insulation in the past.

Hi Scrimper,
Slight mistake there Wall value was u value of 0.3 loft insulation at 270 mm is a U value of 0.16. Part L update in 2010 from :-http://www.homebuilding.co.uk/advice/project-tips/building-regulations/part-l The table in there wall U values were 0.3 and improved to 0.2. Lots of online U value calculators are available.

Richard