LeDeuxions · notes from twenty years on site · 한국어
You have probably seen it on an expensive building. Every winter, water beads between the stone panels and runs down the face of the wall, leaving mould trails behind. The obvious guess is that somebody skipped the insulation. Usually they didn't. Often there is a good radiant barrier in there, and the wall sweats anyway.
Behind stone cladding you will find something that looks like foil. That is the radiant barrier. It is very thin, and it does not work the way mineral wool or foam does. Those slow heat down by being thick and full of trapped air. Foil does something else entirely — it reflects radiant heat back the way it came.
And reflection has a condition attached to it. Read any radiant barrier spec sheet and you will find a line like "maintain a minimum air gap of 5mm from the finish." The foil needs air in front of it. Without that gap there is nothing for the heat to be reflected across, and the reflective surface stops behaving like a reflective surface. It behaves like what it physically is: a sheet of foil in contact with a wall.
What happens in practice is that the foil gets fixed flat to the substrate and the stone goes straight on top of it. Spec says 5mm. Installed at zero.
The reason is not mysterious. Holding a gap means building a separate sub-frame — more material, more labour, more time. And once the stone is up, nobody can see that layer again for the life of the building. It does not show up on a snag list. It does not fail an inspection. It is the kind of thing only someone who has opened a wall back up knows to look for.
There is a second problem stacked on top of the first. Stone panels have to be held to the wall, and that means drilling through and setting anchors — steel running from the outside face into the structure behind.
Steel is an excellent conductor. Every anchor is a small bridge carrying heat straight through the wall assembly, past whatever insulation is there. This is a thermal bridge, and there are dozens of them across a facade.
Now put it together in January. Inside is warm and carries moisture. The anchor is tied to the cold outside air, so the surface around it sits well below the room temperature. Warm humid air reaches that cold spot, crosses its dew point, and turns into liquid water. That is the water you see running down between the stones. It is not a leak. Nothing failed. The wall is condensing on its own cold spots.
This is the part worth taking away. Writing a good insulation product into the specification does not make a building warm. Neither does a drawing labelled "external insulation." How the layer was actually fixed, and where the steel passes through it, is what produces the number you live with.
If you are having something built, here is one thing worth asking for. Do not only specify the finish material. Ask for confirmation, with photographs, that the insulation was installed to the manufacturer's stated conditions — including the air gap. An honest contractor will send you pictures. Evasion on that question tells you something.
When someone asks me why an expensive building performs badly, this is usually the answer. A material is only a material when its conditions are met. Outside those conditions it is a line item on a spec sheet.
In construction there is almost never an absolute — there is usually a better way waiting to be found. Here the better way starts somewhere unglamorous: in the layer nobody will ever see again.
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Written from jobs I worked on, not from a manual.
Small tools I built for everyday work — pdf300.com
And a few things to play with — ledeuxions.com/games