Case study · north Essex · solid brick, 1850 to 1918

Mould round every window.
New windows would not fix it.

A recently bought house with mould at the reveals in room after room. Surveyed in heavy rain, every reveal read dry. The cause was the coldest strip of wall in the house, and it would still be the coldest strip with any window fitted.

£350 fixed feeSurveyed in heavy rain, deliberatelyNo damp proofing recommendedReal survey, not a mock up

This is a real Vigilant survey, published with the client's written permission. The client's name, the address and the postcode have been removed, and no photograph shows anything personal or identifies the property. Every reading, temperature, photograph and finding is exactly as recorded on site.

What we were asked

The owners had bought the house recently and found mould around the window frames. In their words: "We were told before we moved in that the windows and trickle vents had not been opened. Since we moved in the trickle vents have been kept open all the time."

Mould at a window is the single most misdiagnosed thing in this trade. It is read as a leak, as a failed window, or as rising damp, and it is very often none of the three.

The building

A semi detached house of 1850 to 1918, solid brick walls one brick thick, pebbledashed externally, with uPVC windows. The master bedroom, bedroom 2, the living room, the porch and the utility room were surveyed, with bedroom 3, which is unaffected, as the comparison room.

The inspection was carried out in heavy rain. That was not bad luck. It is the best possible condition in which to test whether water is entering at a window, and it is the reason this survey could exclude penetrating damp rather than merely fail to find it.

What the readings showed

The pattern was the same in every affected room. Mould sitting on the sealant joint between frame and plaster, and on the plaster immediately beside it. Underneath it, nothing.

Measured	Result
Control readings, internal partitions	Lo, below 6 %WME
Every reveal and adjoining wall, beneath the growth	Lo, below 6 %WME
Room conditions	23 to 24 °C, 59 to 63% RH
Dew points	14.6 to 16.5 °C
Coldest surface each room, above dew point	5.9 to 9 °C
Thermal images of every reveal	No wet patch, no cold spot at the frames
Utility room, crack beside the external door	18.5 %WME, the only elevated reading in the house
Utility room relative humidity	72%, against 59 to 63% elsewhere

Established growth, in the condensation position, with dry surfaces beneath it, and nothing coming through from outside in heavy rain. No condensation was occurring anywhere in the house on the day, with the trickle vents open.

Why it is at the windows, and why the windows are not the reason

In a solid, uninsulated brick wall the strip where the window frame meets the wall is a thermal bridge. It runs colder than the wall around it whenever the room is warmer than the outside, so it is the first surface in the room to reach dew point, and moisture in the room air condenses there before anywhere else.

That is why the growth sits on the sealant joint and the plaster right beside it, in room after room, and nowhere else in those rooms. It is not a coincidence and it is not the windows failing.

The windows themselves are sound. The reveals behind them are dry, and the thermal images show no cold spot at the frames. The mould sits on the strip of wall where the frame meets solid brick, and that strip would remain the coldest surface in the room with any window fitted. Replacing the windows would not address the cause.

The one place where new windows would help is the porch, which has no trickle vents. When those windows are next due for replacement, units with trickle vents would give it the background ventilation it lacks. That is a matter for when they need replacing, not before.

What was ruled out, and on what evidence

Excluded Rising damp: The growth is at window reveals at first floor and above 1.2m at ground floor, and every low level reading is Lo. There is no ground moisture pattern anywhere.
Excluded Penetrating damp, wind driven rain: The external sealant has gaps, so a route exists. But the inspection was in heavy rain and every reveal in every affected room read Lo, with no elevated reading and no wet patch on the thermal images. Rain is not entering at the windows. The gaps are recorded as a defect to make good.
Excluded Penetrating damp, building defect: The dislodged guttering is wetting the pebbledash at the corner and moss is growing there. No elevated reading was found inside behind it, so it is not producing internal damp. Recorded as a defect to repair before it does.
Excluded Interstitial condensation: Solid brick walls, one brick thick, with no cavity or built up construction in which it could occur.
Excluded Plumbing or drainage leak: No pipework serves the affected walls and the readings around every window are Lo.
Excluded Salt contamination: No salting, no ground moisture pattern, no elevated readings on any wall other than the utility room crack.
Supported Surface condensation: The growth in every room sits on the sealant joint and the plaster beside it at the window to wall junction, the coldest surface in a solid brick wall and the classic position for condensation mould.
Supported Thermal bridging: The window to wall junction in a solid, uninsulated wall runs cold whenever the room is warm and the outside is not, and room moisture condenses there first.
Supported Historic residual moisture: Every reading beneath the growth is Lo and no condensation was occurring on the day. What is visible is the residue of past condensation, not evidence of a current source at the windows.

Note what that list does. It excludes penetrating damp positively, on the evidence of a survey carried out in the rain, rather than saying it could not be found. Those are very different statements, and only one of them is worth anything to a buyer, a solicitor or a lender.

The one live problem, and it is not at the windows

The utility room is a small room with no background ventilation, no extract and a door undercut of 1mm, containing a gas boiler in an unventilated alcove immediately beside a washing machine. Relative humidity there was 72%, against 59 to 63% in the rest of the house, and the dew point of 18.5 degrees is the highest in the building.

The wall inside the boiler alcove reads dry. The peeling paint and mould are on the wall to the right of the external door, where a crack in the plaster reads 18.5 %WME. Warm, moist air from the appliances is entering the crack and condensing behind the plaster, where it cannot dry.

That is a moisture production problem, not a building fault, and ventilating the room and sealing the crack deals with it.

The evidence

Fourteen photographs from the survey, exactly as taken.

A section of guttering has come away at the corner of the front elevation. Recorded as a defect to repair, not as a cause. Nothing behind this wall read elevated inside.

The wall beneath it, being wetted, with moss now growing. A solid wall kept wet from outside will show inside in time. That is why it is on the list, not because it is causing anything today.

The sealant between the window frame and the pebbledash has come away in places. The droplets on the frame are the weather on the day: the survey was carried out in heavy rain, which is the best condition in which to test whether rain is entering at the windows.

Bedroom 2, control reading on an internal partition. Lo, below the 6 %WME floor of the meter.

The window reveal in the same room, where the mould is.

Left side of that reveal. Reading beneath the growth: Lo.

Right side, where the growth is heaviest. Also Lo. In heavy rain. Nothing is coming through.

Living room, reading taken at the growth itself.

Thermal image, right side of the living room window. The cold area is the open window, not the frame or the reveal. There is no cold spot at the junction and no wet patch anywhere.

The utility room. A gas boiler in an unventilated alcove, immediately beside a washing machine, in a small room with no extract and a door undercut of 1mm.

The one live reading in the house. A crack in the plaster beside the external door, 18.5 %WME, in the at risk band. Relative humidity in this room was 72%, against 59 to 63% everywhere else.

The porch, above the door. An unventilated single brick box between the front door and the outside, with no trickle vents. Condensation in the same way as the rooms above.

What we recommended

Ventilate the utility room and seal the crack beside the external door, within three months. An extract or a permanent vent.

Clean the mould off with detergent and water or a mould and mildew cleaner and dry the surface thoroughly. There is no need for bleach. The point is to remove the growth, not to kill it. Where the sealant is stained through, cut it out and replace it with a mould resistant one.

Keep the trickle vents open through the winter, as they are now.

Make good the external sealant and refix the guttering before next winter. Neither is causing damp inside today. The point of doing them is to keep it that way.

Give the porch some ventilation and clean its reveals the same way.

And what should not be done

Do not treat this as a damp proofing problem. There is nothing for a damp proof course, tanking or chemical treatment to address. Do not replaster the reveals on account of the mould, because the plaster is dry and the growth is on the surface. Do not seal the trickle vents or block ventilation to keep the rooms warmer, which would recreate the conditions that caused the growth in the first place.

How confident, and what would settle it

The report records this as highly probable on the evidence, not confirmed, and names what would close the gap: the first cold months. If the growth does not return with the trickle vents open and the utility room ventilated, the mould was historic condensation and the matter is closed. If it does return, a temperature and humidity logger in the master bedroom and the utility room over two weeks would show whether the room air is reaching dew point at the reveals. Benchmark: the reveals reading Lo, as now.

Mould round every window.New windows would not fix it.