Send pics!

If it was me, I would open up at least one part of the wall so I could see (and photograph as Tom suggests) the wall construction and cavity. It seems likely that any cavity will be too narrow to be able to provide a reasonable amount of insulation by itself. It also seems fairly unusual for the 1950s, hence my desire to check.

EWI can be finished neatly where your house joins your neighbour. Or another possibility is to persuade your neighbour to join you and insulate theirs at the same time, hopefully with a consequent reduction in cost.

Posted By: alexeixwe can write off cavity insulation

Right!

Posted By: alexeixWhat are the problems with internal wall insulation

Fundamentally, IWI robs the wall of any warming from within, leaves it stone-cold like never befoire, hence liable to intersitial condensation, hence freeze/thaw/spalling risk. That sets a fairly low safe standard of insulation, if IWI, so the wall still gets some warming from within. Apart from that, loss of internal space, fitting around wall objects, insulating wall within floor thickness and abutting partition thickness (impossible if brick partitions, so major thermal bridge). Total faff, very second-best, only if EWI is really impossible. If attempting an internal VCL as part of it, problems double, reliabile continuity almost impossible.

IWI got a bad rep due to some ill-considered computer simulations in the early 2000s, which is a pity as a lot of houses were subsequently not-very-insulated for "safety" and lost lots of heat as a result. More recently, organisations such as Historic Scotland have done real-world trials and found that IWI can be installed to fairly good u-values with no condensation.

A drawback of retrofit EWI is that it cannot be joined onto the floor insulation, making a cold bridge zone all round the base of all the walls. It's also difficult (though not impossible) to join satisfactorily to the ceiling insulation, especially along the gable walls. It's also difficult to do DIY so you are in the expensive market for specialist installers (tight at the moment as everyone is thinking about this). If you have unfilled cavities then cold air will circulate through them inside the EWI, negating it's effect.

Drawbacks of IWI are that it is messy (unless you are redecorating anyway). It's difficult in kitchens and bathrooms unless you take out all the units. It is difficult (but not impossible) to make it continuous from room to room across the internal walls. You don't usually notice the loss of internal space IME, but if there is say a narrow stairwell then you'll notice.

So neither is perfect, you need to weigh the issues for your particular home and make a choice. You can also mix eg IWI on a visible front elevation and EWI round the back. We went IWI on our last home and are leaning towards EWI on the current one.

Posted By: alexeixSincere question - what is the benefit of opening up the wall?

Well if you're considering CWI then you need to know what you're dealing with.

Based on your answers, I guess there’s either a minimal gap or maybe even none at all.
Would rather not create unnecessary mess.

As Will mentioned, you probably need to fill the cavity as well if you're doing EWI. The amount of mess by inspecting will be minimal in the grand scheme of things.

EWI for me, CWI too thin even if it was 50 or 100mm

Like the idea of perimeter insulation, just run the EWI down to top of concrete footings.

(I see this crossed with Will's post above. Nice controversy!)

Or even out and down over the toe of the conc strip footing - considerably helps lengthen the heat path through subsoil, down 150 further + outward 300 further + up 150 further - 600 more heat path for 150 deeper trench!

If of good depth, like 750-900 deep below GL, I'm begining to think this works better than a typical new insulated slab within extg walls, which are a massive thermal bridge at the very line where temp gradient is steepest and most of the heat goes out.

Best as ordinary EPS, which gets wet but drains out again, rather than expensive plastic foam insulations, which are claimed to not soak up water but studies show do slowly absorb it and never let it out again. The EPS can get wet but the trench mustn't have standing water - so make the trench into a french drain, perforated pipe in bottom, sharp clean aggregate fill around. The french drain has to have somewhere to drain out to.

About the fear of rats burrowing into underground EPS (having come up the plastic perforated drain and eaten thro it), I realised recently that the sharp stone fill against the EPS should deter a rat? No need to think of paving slabs stood up against the EPS, or stainless mesh as ratproof protection?

Was it http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=6562&page=1, when an02ew jumped in? He said:

"i desided to to check out the difference between using the wing insulation method of Leca VS simply extending the EWI into the ground as a blade, the later potentially easier so long as the wall extends down onto a found.

i modelled the Leca wing at 2 bags deep extending out from the wall by the lenght of a bag this acheived 1.78W/mK

From modelling these 2 junction types its clear that extending the EWI down the face of the external wall is only equal to the Leca wing insulation when it extend beyond 600mm deep. This may not be acheivable with pre foundation construction."

I agree the horizontal wing (of Leca) is a lot less effective than the downward 'blade' (of EWI), because the heat path length is only the horizontal wing plus end conditions, whereas the blade is both down and up plus horizontal under its bottom. If the blade can be only 600-ish of less, then it has to be supplemented with wing.

Agreed 600dp isn't really a blade, becomes the wing version, which has to extend 6x as wide as the blade is deep (only 1-sided not 2- x Leca only 1/3 the insulativeness of EPS) for same effect. I'm talking

Posted By: fostertomof good depth, like 750-900 deep below GL

I know that all heat paths have be summed, but the inboard ones are plenty long enough already; the blade makes a drastic difference to the outer ones, which as we know account for the vast majority of slab loss. Also I intuit that x-section bunches much thinner at the tight heat-path bend under the bottom of the blade.

Along with http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=17578&page=1#Item_21, it looks like a 3-cornered duel - Therms at dawn! Not clear who an02ew would be seconding.

Prof Little was 100% adamant in 'Breaking the Mould' that the 0.5 U value was only applicable to west-facing walls in Dublin constructed from particular type of concrete block they have there and absorbing a very high % of driven rain into the concrete, insulated with impermeable insulation. He said that if anyone else was insulating a different wall in a different climate, they should not copy that number.

Lo and behold, everyone copied that number!

AIUI he's still involved in this field and has worked with historic Scotland, who are insulating to 0.19.

(Edit: reading back BtM pt5, Little felt that U= 0.27 was "acceptable" if driving rain penetration was not at very high levels, and other numbers for different grades of brick. He was up front that the computer predictions of widespread damp didn't match real world experience, and suggested real world tests for verification.)

As Little and FT said, the 'safe' number depends very much on the particular wall and the local microclimate.

Other considerations are that cold bridges (if present) make it pointless to EWi/IWI to very low U values. Also the embodied carbon of certain insulation and render materials will cause net damage if put on thickly (they will not save enough carbon to payback their manufacturing, before the imminent decarbonisation of energy 'renders' them superfluous). And the economic cost-benefit sweet spot is probably still 0.5.

Tom's farmhouse likely has sections of wall that were never historically heated, eg gable walls above ceiling level, below susp floor levels, porches/outbuildings, pantry etc. A check for damp there might give clues as to effect of IWI heat conservation?

Posted By: alexeixDoes anyone here have experience of such thin cavities

Yes. My first house (1930s semi) had ~9" walls. We assumed they were solid. When I eventually broke in we found the inner leaf was narrow (~70mm) cinder block (and the outer 100mm brick), so there was an approx 30mm cavity. We had already decided to do IWI by then so ignored it.

These days (that was 30 years ago), I'd just say that 30mm of insulation is uselessly little so you need to do EWI and/or IWI anyway. It's a good idea to fill the cavity with something, just to avoid heat loss from air circulation. If doing good EWI then you might want to fill it with something solid and benefit from the thermal mass (one of fostertom's favourite suggestions), but more typically you'd fill it with EPS beads (which flow well). CWI installers can no doubt advise on best fill for narrow cavities.