A thermal camera does not see through walls, it sees the surface temperature of them, and a void behind plaster reads cold in a very particular shape. Ours is two metres by one and a half, on an internal wall.
We had the survey done for a grant. Solid wall insulation, and the scheme requires a thermal survey first so that the money goes where the heat goes.
It is worth explaining what the camera actually does, because everybody assumes it sees through things.
It measures the temperature of a surface. That is all. But the surface of a wall is warmed and cooled by what is behind it, so a cold bridge, a missing bit of insulation, a stud, a joist end, a void, all show as a shape on the wall.
You do it on a cold morning with the heating on, so there is a difference across the wall for the camera to find. Ours was done in the February with the house at nineteen and the outside at three.
The report is thirty pages and most of it is what you would expect. The window reveals are cold. The floor perimeter is cold. The north gable is cold.
Page nineteen is the wall between the dining room and the hall.
It is an internal wall. There is a heated room on both sides of it and it should be the same temperature as everything else in the house, because there is nowhere for it to lose heat to.
There is a rectangle on it, two metres high and about one and a half wide, that is four point two degrees colder than the wall around it.
Four degrees is a lot. It is more than the north gable is, and the north gable has weather on the other side of it.
The surveyor's note says: significant thermal anomaly consistent with an unheated void, recommend investigation before works.
He also said it out loud, at the door, which is why I remember it. He said he had seen blocked up doorways and old flues and a lot of chimney breasts, and that they do not read like that, because a blocked doorway has a lintel and jambs and a flue is tall and thin and warm at the bottom.
He said this one is a box.
What is on the other side.
Nothing, in the sense that there is nowhere for it to be.
The dining room and the hall share that wall. I have measured both rooms and the outside of the house and the arithmetic works: the two rooms plus the wall thickness accounts for the whole depth of the house at that point, with about four hundred millimetres for the wall itself.
Four hundred millimetres is a solid wall of that period. It is not enough for a two metre void.
I did the measuring three times over two years, including once with a laser and once with a tape and a friend, and once by pacing the outside and the inside on the same afternoon because I wanted a check that used a completely different method.
The rooms and the wall come to the width of the house.
The contractors.
The insulation firm the grant sent came, read page nineteen, and declined the job. Their reason, in writing, is that they do not work on properties with unresolved structural anomalies and that we should have the wall opened up and assessed.
The second firm, in 2023, came out, and the man went up a ladder with his own camera because he wanted to see it himself.
He came down and said he could not quote and could not put it in writing why. In the drive he said he had been doing this eleven years and that he had opened up one like it in 2019 near the coast, and that he had put it back and sent the customer somebody else's number.
I asked what was in it. He said the trouble was not what was in it but that it was not the size it looked, and he got in the van.
Opening it up.
I have the tools. It is plaster on lath and a probe would take four minutes and tell me most of what I want to know.
I have not, and the reason is a small and stupid one, so it had better be the one I give.
We had a builder in for the kitchen in 2022 and he ran his multi tool through a section of that wall by mistake, at about waist height, about four hundred millimetres wide, to chase a cable.
It went through the plaster and the lath and stopped on something.
He said brick. He filled it and skimmed it before I saw it and told me about it afterwards, in the way tradesmen do, as a thing already dealt with.
The thermal image was taken the following February and the rectangle is there and it is not interrupted at waist height.
A void behind a lath and plaster wall with a brick face at four hundred millimetres, that reads four degrees colder than everything around it, and is two metres tall.
I have written that sentence out for two engineers and got two answers, and both of them said the survey should be repeated with a different camera, and one of them added that thermal work is often over-interpreted.
I had it repeated in 2024 by a different firm with a different instrument on a different day.
Same rectangle. Four point one degrees.
What we do is live in the house, which we like.
The grant lapsed. Solid wall insulation would have taken about four hundred pounds a year off the heating and we are not going to get it, because no firm will work on that elevation without an investigation and the investigation is a thing I have decided not to have.
The dining room is the warm room in this house, which is the joke my wife makes when anybody mentions the survey, and it is true, it is the warmest room we have, and it is the one that shares that wall.
I have moved the sideboard against it. There is a mirror above the sideboard which was there when we came, and it is screwed to the wall rather than hung, with four brass screws, and I have never taken it down.