Posted By: RexBut he also told me, and this is what I don't understand about EWI, that since the cavity is ventilated, the outer wall is considered external as it has cold air on both sides and does not improve the insulation values.

You're not missing anything. That's why it's essential to full fill a cavity before applying EWI. And change it from being a ventilated cavity to an unventilated one.

Interesting paper with too many numbers for me to understand!!!!

I did find it interesting regarding the density of blown cellulose insulation to prevent slumping. I have Warmcel insulation and the internal 'barrier' was the supplied scrim. the insulation had to be pushed back to fit the dry-lining, so I guess I have sufficient density to prevent slumping.

But....... during the build, it was necessary to do so stuff and I found in a number of locations at the top of the insulation, a 300mm cavity, caused by slumping. Had to get them back to fill the voids.

Personally, I cannot see how a 2400 x 600 x 180 space filled with blown cellulose cannot slump.

I would therefore think that blown cavity insulation on a two story house would slump.

My house is sufficiently warm not to require EWI or cavity filling.

The main reason for using the scrim is that I did not fancy making 100mm holes into each and every vertical drop between studs and then having to fill them all. Scrim was a lot easier.

I did put some scrap timber 'noggins' in as I was doubtful about the non-slumping claims. I actually wanted the Warmcel to be damp sprayed so that it goes like papier-mâché but the original installation company went into liquidation and the alternative said they could not do it with 180mm depth as there was a tenancy for it to just fall out.
I'm certain I could have prevented that happening but it would have taken more time and money.

"With scrim you can also see visually that all nooks and crannies are filled." Yes, but inevitably, the density will be greater at the bottom than the top, hence why I subsequently found gaps at the top where settlement had occurred.

Posted By: Rexinevitably, the density will be greater at the bottom than the top

I wouldn't think so - friction against containing things would initially exceed downward compression by weight. Later on, yes slump may occur, slowly bit by bit.

Welcome to the forum! :) The first thing to do is to find out where the damp is coming from, or tell us if you already know. Is it internal condensation or an external leak or rain?

If it is is condensation then increasing the ventilation or turning the heating up a bit may cure the problem. Insulation is likely an alternative to heating, but not so much for ventilation.

Whereabouts are you (roughly) as local conditions can vary.

If you know for certain it is condensation caused by warm moist air condensing on a cold wall then, with a number of provisos (good room ventilation and a reasonable 'running temperature' in the house among them) you don't need to worry about trapping 'the damp' in, because you know what causes it and you have removed the cause.

''there seem to be so many different products on the market: woodfibre, cork, EPS, XPS foam, rock wool.
I don't really understand the relative benefits of them.''

Woodfibre and cork are less irritating to work with, but WF particularly produces a hell of a lot of 'fluff' (great for wetting and stuffing any open joints (of which, of course, there should be none). Rockwool is, in my opinion, horrible to work with. If you have more than 2 storeys you will probably have to use it is only as a 200mm fire-break, but I hate using it. It is also completely non-combustible, which is really good.

EPS, XPS, phenolic foam are better than the others for a given thickness, but they are plastic. If you do not like plastic you will either not use them, or you will perhaps use them and wish forever that you hadn't. My preferred materials for EWI and IWI are WF and cork.

Thank you that's helpful Nick I was leaning towards cork anyway and that feedback has made me more likely to go with cork.

The ventilation isn't great I suppose in winter but it's not ideal opening windows while the heating is running.

There's no mechanical ventilation. I think the humidity is usually around 50-55. I don't keep a hygrometer in that room - although maybe I will going forward - but when I've placed one in there from another area of the house it usually reads around 52.

Thank you djh, some really good ideas to investigate further there. Really appreciate it.

If I'm reading my psychrometric chart correctly, room air at 50% RH and 20oC would reach dew point at around 13oC, which I guess the wall internal surface temp could be? So condensation from internal air vapour is possible, maybe even likely.

I suppose the humidity & temperature readings might be more useful if Rachel can compare the inside and outside measurements. That will tell us something about whether more ventilation is likely to help much, and might be able to help with U-value estimation. Although more detail about the wall construction is probably best for that.

Cavity fill is or should be non-capillary i.e. unlike masonry it doesn't 'soak' liquid water up (or across).

The older parts of our house have a blown in fluffy CWI which has been in there 20+ years without any damp problems so fair to assume that's non capillary

EWI works in the same way as wearing a high performance coat. If the coat fits you you'll be nice and snug. If the coat is too big and there's gaps all around between the coat and your body, you'll be cold. The gaps allow draughts to carry heat away so the coat does next to nothing. Same with an unfilled cavity.... draughts in the cavity carry away the heat and the EWI does next to nothing.

"Or that you have not got a wet outer leaf..."

Outer leaf is painted render so likely pretty impermeable but it does get wet on the inner face of the outer leaf which I assume is moisture from the interior condensing on the cold outer leaf?? We removed a chimney breast a few years ago an was very surprised how wet it was but no evidence of the condensate wicking back across the CWI.

Posted By: philedgeEWI works in the same way as wearing a high performance coat. If the coat fits you you'll be nice and snug. If the coat is too big and there's gaps all around between the coat and your body, you'll be cold. The gaps allow draughts to carry heat away so the coat does next to nothing. Same with an unfilled cavity.... draughts in the cavity carry away the heat and the EWI does next to nothing.

Love this explanation

Posted By: philedgeOuter leaf is painted render so likely pretty impermeable but it does get wet on the inner face of the outer leaf which I assume is moisture from the interior condensing on the cold outer leaf?? We removed a chimney breast a few years ago an was very surprised how wet it was but no evidence of the condensate wicking back across the CWI.

The addition of EWI will move the dew point outboard of the inner face of the outer leaf which will prevent any condensation here.

The typical EWI (EPS wit thin film render) will not wick moisture through to the bricks.