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104M: Recent purchase penetrating rising damp

Victorian cottage, the owner fears penetrating and rising damp.

Surveyor Tips:

  • Carefully get in to the eaves to see what is going on above a damp patch.
  • Visit the neighbours.
  • Measure the sub-floor humidity through a crack in the floor.
  • Measure extraction rates.

Root cause

Penetrating damp and condensation.


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The master bedroom has a damp spot to the front and front section of the party wall.

Brown discolouration like the stain left by a tea bag is a sign of water passing through building material lor over timber. Here it is clearly penetrating damp from rainwater.

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Looking to the front we can see an unusually large gap. Ventilation is good, but arguably the gap is excessive.

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Looking outside we can see water can easily flow into the property.

There are four issues: 1) The neighbour’s gutter is full of rubble, 2) render has come off the party wall (probably the source of rubble), 3) the flashing has come away from your side of the wall, 4) there is a large gap in the facia. It is possible that all these issues are connected.

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There is dampness on the neighbour’s side.

The neighbour, who is a tenant, told me dampness started a few months ago, November I believe.

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While in the loft I checked the chimney breasts for dampness and found them to be dry.

I checked the party walls just above the damp stains and also found the wall to be dry.

This indicates that the main part of the roof is fine and not the current source of damp.

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There is a damp stain in the middle bedroom, on the rear side of the same wall as the master bedroom.

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The wall was damp on the surface.

It appears that stain block has been used.

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Looking outside we can see a gap for rainwater to flow through.

Render is missing.

There is an argument for joining the flashing from one neighbour’s side of the party wall to the other.

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Description automatically generatedI looked and saw no dampness on the neighbour’s side of the rear wall in their middle bedroom..

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Front airbricks have been painted in and rear decking reduces air flow. However, sub-floor humidity is normal.

If groundwater, the source of water in rising damp, was high then the sub-floor humidity would be high. It was measured at 60%, which is normal, this eliminates rising damp.

Sub-floor air bricks are precautionary, to evaporate away unnoticed water, thereby reducing risk of rot. While the timber floor is exposed, there is evaporation into the house. The benefit of reduced airflow is less heat loss. As an alternative to improved sub-floor ventilation, monitor sub-floor humidity monthly ensure it does not exceed 85%RH for long, if it does improve ventilation such as by opening up the holes in the painted vent.

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The dampness to the front is harder to explain as there is a stain like penetrating damp.

Typically, stains are caused by water passing through a wall, such as leak or rainwater. However, a stain can also be caused by rust, such as from condensation forming on a cold socket. Sockets are prone to dampness as a section of wall is cut out, a metal box installed and covered in absorbent plaster, which exacerbates the effects of condensation, see surveyor.tips/sockets.

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There was a marginally high damp meter reading.

I used a Protimeter damp meter in conductance mode. These meters measures electrical conductance of salts in water, a proxy for dampness. Readings below 20WME are considered dry. The range is 8WME to 99WME. See https://surveyor.tips/dampmeter. Walls measured were largely dry on the surface except where mentioned in this report.

There are calcium sulphate salts on the surface. Calcium sulphate is a key ingredient in cement and other building materials. If diluted in water salts move to the surface. Salts can be removed with sandpaper and decorated. Condensation is the most common cause.

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The thermal imaging camera, where blue is about 5°C colder than orange, highlights heat loss in the corner.

Condensation is formed when the temperature drops below the dew point. From the marginally high damp meter reading, I suspect that the moisture is residual from the previous occupant. A damp wall can take 10 months to dry out.

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Outside, but slightly to the right (when looking from the road), there is a gas meter.

I had a look all around but could not find clear evidence of gaps or render cracks, that could cause penetrating damp. However, metal increases heat loss and risk of condensation.

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A small crack above the front door should be filled. But I don’t think it is causing penetrating damp.

I am no advocate of pebble dash, but own similarly white painted pebble dash house and would not remove it.

The risk with render such as pebble dash is cracking, but so long as you keep it filled and painted there should be no ongoing problem. I use and recommend an external acrylic caulk for filling small cracks and a cement-based mortar for large cracks.

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In the kitchen there is dampness high up a wall on an inside corner.

The corner appears to be supported by an RSJ, that is a steel joist or support. Metal losses heat rapidly causing condensation. The kitchen extractor fan is good, but is it always used? The bathroom extractor fan does not remain on after lights are switched off. Ideally it should last 30 minute, or even be a continuous flow.

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The corner is a natural location for a kettle, which clearly spouts out steam.

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When hot outside the RSJ is relatively warm. However, when it is cold outside the metal loses heat rapidly.

Monitoring relative humidity

I encourage homeowners with property showing signs of damp or mould to monitor the relative humidity. The data can be used to improve ventilation, increase heat balance, insulation and improve airflow through reduced clutter to see the benefits.

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Looking at your data, there is constant heat, but steady increase humidity overnight. This suggests insufficient background ventilation. The simplest solution is to use a continuous flow extractor fan. Consider also using a PIV if you need filtered air, see surveyor.tips/piv.

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