Narrow cavity wall in new build

29 Sep 2016
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United Kingdom
I'm having an extension built. The wall is made up of an inner block leaf, an insulation layer (60mm ecotherm), a cavity, and an outer block leaf, with 25mm of external render on top.
The cavity was supposed to be 50mm wide but the builder made it narrower (approx 25mm). He insists that it's not a cause for concern, as there won't be any water penetration from outside and the condensation buildup in the cavity is minimal and no issue for inside.
Any thoughts?
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It should be a minimum of 50mm, you’ll find it in Part C page 34 of the PDF and I would expect a BCO with a brain to pick it up. That said, it could very well be fine depending on the exposure and your location in the UK. If the wall was say 0.5m away from a neighbours wall in suburbia I would be less worried than if you lived on the top of a mountain or on the coast.

What do the drawings say?
Location is London, so exposure zone 1. It's not too close to neighbouring walls, these are about 1.5m and 3m on two sides and ~30m on one side (backing into a garden).
Drawings had 50mm cavity with 75mm outer wall which was meant to be stone cladding. Then we changed it to block plus render, and the block was 100mm thick (and the render another 25mm). The builder aligned the block to end where the stone was meant to end, as opposed to begin where the stone was meant to begin, which caused the cavity to be only 25mm. He claims that's more than enough (he says I don't need any cavity) because compared to the previous design, the outer block is thicker (and has some insulating properties) plus there's external silicon render on top.
I'm confused. Are you saying that you have a 85mm cavity with 60mm of insulation and 25mm air, or just a 25mm cavity with 60mm of insulation compresed to 25mm and no air?

Either way your builder is probably not really a builder.
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The drawings had (starting from inside and going out):
- inner block 140mm
- insulation boards 60mm
- empty cavity 50mm
- stone cladding 75mm

We had a hard time arranging the stone and we decided to switch to block and render instead, block 100mm and render 25mm.
The builder aligned the block to end where the stone would end, thus leaving a narrower cavity by 25mm.
Most of these rigid board insulation products can have a 25mm cavity, but there is risk of the 25mm being less in places, and there is a requirement for a very high quality installation.

But it depends on the rest of the specificationas to whether it does conform to the NHBC guides (which building control tend to insist on), the BBA certificate, and other factors.

So it may be OK, it may not be.
But either way, its whether it is desirable to deviate from the design, and if the risk of failure with such a narrow cavity is high or low - and that depends on the builders. Obviously, the builder who is building it is going to say that its all OK.
Obviously, the builder who is building it is going to say that its all OK.
This is why I'm trying to form an independent opinion.

Most of these rigid board insulation products can have a 25mm cavity, but there is risk of the 25mm being less in places, and there is a requirement for a very high quality installation.
Very true, and there are points that it's <25mm especially where there's some excess mortar from the block building (forming a bridge between the insulation boards and the outer block leaf).

The question is what to do now.

One option is to tear it down and rebuild it. I'm trying to establish if this is necessary, as it would be very costly and disruptive.

It seems to me that the builder's point about there being no risk of direct water penetration has merit. Compared to the previous design the block is thicker and has a thick render on top.

The issue would then be condensation inside the cavity which will lead to moisture build up. Could weep holes help in this case?

I also wonder: what if the cavity was fully filled? From the table on p35 of the document above, (if I'm reading it right) 75mm of fully filled cavity it seems that it would be suitable for up to exposure zone 3, so not a problem. Then 25mm of gap inbetween would presumably make it even better.
I'm answering my own question. From Ecotherm's FAQ:

Can you full fill a wall cavity with PIR board?
PIR board is not designed for a full fill cavity and typically requires a minimum 25mm cavity from the external brick / block course to prevent moisture transference.​

Now, how strict is this "minimum 25mm"? What if debris/mortar reduces it at many points?
Yes but the point is that unless your builder (as mentioned already in this thread) is unbelievably meticulous the residual cavity will not be 25mm and the Ecotherm BBA certificate (as do all the other PIR insulation BBA certificates) states that whilst a 25mm air gap is acceptable its good and proper practice to build the wall with a 50mm gap to allow for tolerances and mortar snots etc.

And as also mentioned before you may well also find Mr BCO will not be happy.

If it is less than 25mm the risk of damp crossing the cavity will increase ……
Get the building inspector out.

But tbh, if it was my job, it would be coming down and rebuilt properly at the expense of those cow herders. Cavity issues tend to take a few years to develop.
Yes our wall was built incorrectly too, wall tie and insulation issues in our case, and the builder eventually offered to demolish and rebuild one the SE and architect got on the case. Actually in the end he only needed to demolish the outer leaf and everything else could be fixed from there. So it's possible you could get away with only rebuilding the inner leaf in your case.
We even had steels on the top of the wall, but they just propped them up and got to work underneath.
Get the building inspector out.
Should I also invite the tax officer to inspect my bank account just in case, and my wife to check my web browser history? :whistle:

But tbh, if it was my job, it would be coming down and rebuilt properly at the expense of those cow herders. Cavity issues tend to take a few years to develop.
Before I do that I want to weigh the certainty of extra cost and inconvenience in the short term (extra rent for the additional waiting time and emotional cost from not making it to our new flat this side of Christmas) with the probability of issues at some point in the future. What is that probability and what could happen and at what cost? If for argument's sake the current cost from the delay (not including emotional stress) is say £2k, and there's 50% chance that an issue will arise in 10 years that it will cost me £2k to fix, I'd rather take the chance.
So, can someone please guide me through what can happen in the future?
Is the damp going to come from moisture through condensation alone (in which case it's quite likely to happen) or will it require direct water penetration as well (which is less likely if not impossible due to the thick block+render, assuming good maintenance of the render/cracks repair etc)?
Next consideration: let's say water forms in the cavity, whatever the source. How is it going to affect me? Behind the cavity gap, there's the insulation block, behind which is a damp proof membrane, and behind it is the inner block. The only direct connection is through the wall ties. These are stainless steel and stop at about half the depth of the block. Is the concern that the water will moist the inner block and then it will travel inside?
If that's the case, then can condensation alone, traveling through the wall ties to get to the inner block cause any problems (especially when presumably at that point it will meet higher temperature from the heated interior, which will dry it)?
I agree that the job was not done properly, and the "correct" thing to do is to rebuild it. But before I do that I would like to be presented with a good explanation of what can go wrong and why, and weigh the odds. Otherwise it's choosing to be pedantic over being practical, and it's sort of cutting the nose to spite the face.
Well the last 30 odd years or whatever of partially filled cavity walls has led to millions of pounds worth of research by various well respected bodies such such as the British Standards, BBA, LABC, NHBC and the varoous manufacturers into the long term defects of cavity walls to establish the good practices that are now (or should be) adopted by the industry to ensure long term damp resistance. Rain can penetrate walls around openings or at the top or even through the fabric in extreme conditions, once its through it can track along the wall ties, eventually saturating your inner skin, yes plenty of ifs and buts but thems the facts, as mentioned one needs to take a calculated view on it. Of course maybe the workmanship is awesome or maybe it's a bag of spanners with tin cans, fag packets in the cavity, the ties slope upwards to the outer skin, the outer skin has a ton of air pockets in the mortar, big gaps between the insulation boards and all of the other problems that can blight partially filled cavity walls, we don't know. Bottom line is it doesn't comply with the regs and your builder is either stupid or a liar so why should you take the risk? If you ever get a problem your builder will be long gone, tis your money of course and for the last time Mr BCO may not sign it off.

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