Damp or condensation?

[TR1CKY]

Hi all,

Looking for some advice on this as it's frustrating to say the least!

North facing bedroom. House is a 1950's built semi. Been here 15 years. No major issues with damp in that bedroom during this time apart from tiny (and I mean tiny) mould spots appearing over the coldest of winters in the corner of the bay.

Had a rear house extension in 2022 so moved out to new bedroom and daughter switched rooms into our old bedroom. Queue the madness. Redecorated the room in the summer of '23. Used Dulux Damp Seal undercoat on the plastered walls (and obviously painted over the top with normal Dulux Fluffy Clouds or whatever

Now, as soon as it gets to below about 10 degrees, we're met with steamed windows and dripping walls - just in that room. Bedroom 4 (north facing and adjoining) is clear.

3rd image shows the bay where I painted the 'return' in the damp paint as that's where we used to get the smallest amount of mould in the bottom corner on the skirting in the winters but you can see (just) where it's circled (in images 3 and 4) that's where the wall is actually dripping wet and mould is appearing all under the radiator.

Outside wall was originally pebble dashed and has since been flat rendered over the top of that since about 2013 with no issues or cracks. Windows were done in about 2011. There have never been any air-bricks etc in the room.

Any ideas?! Got a dehumidifier running in there and to be fair, it clears pretty quickly on it's own anyway but it's odd!

Thanks in advance.

 

[blup]

The madness is an example of how tiny changes in conditions can significantly worsen condensation. The wall breathes less because of the sealing effect of the undercoat and the external render, so the damp that is typically found in bays is trapped, keeps the walls colder and the dew point is reached more quickly. Look at chopping out the plaster internally under the windows and replacing with breathable lime based plaster. Likewise removing the external render around the bay, letting the brick dry out and re render appropriately. But from what you describe natural ventilation and the humidifier are acceptable medium term solutions, the radiator under the window should help too.

 

[George_engineer]

Condensation is a primary cause for damp.

Reduce source of dampness (is the room occupied more frequently? wet hair blow drier in the room? clothes drying in the room?)

Solutions are more ventilation, more heating or dehumidification.

Also, check mechanical ventilation in wet rooms is functioning correctly.

 

[^woody^]

The water vapour that leads to condensation is invariably lifestyle related, ie produced by the occupants. That's not a derogatory statement, just a fact. So you need to look at the root cause.

What are you doing differently, what routines have changed, what lifestyle changes have occurred from the time before and after this condensation started?

Bear in mind that you were likely producing a similar levels of humidity before you painted the bedroom walls, and since then you have just shifted it to the windows as condensation.

It's nothing to do with the render. Nor anything to do with the extension - unless that is related to lifestyle or routine changes.

 

[^woody^]

Solutions are more ventilation, more heating or dehumidification.

That's a dated response often touted by landlords and suchlike. These are not solutions, just possible explanations.

More heating? More costs, affordable?
More ventilation? Where does the replacement air come from - it's a cold 87% RH outside at the moment.

The balance between condensation or not is a fine one, and solutions have to be achievable, not text book.

So better [different] use of heating/ventilation and looking at how moisture is produced within the home would be a solution.

 

[George_engineer]

That's a dated response often touted by landlords and suchlike. These are not solutions, just possible explanations.

More heating? More costs, affordable?
More ventilation? Where does the replacement air come from - it's a cold 87% RH outside at the moment.

The balance between condensation or not is a fine one, and solutions have to be achievable, not text book.

So better [different] use of heating/ventilation and looking at how moisture is produced within the home would be a solution.

Not really - fully saturated air at 10C will have a relative humidity of 50% if heated to 20C. This is the whole point of ventilation for controlling humidity (air quality being the other main reason).

My emphasis of what is in your post is exactly what I was saying as the other parts of my post were discussing how has the use of the room changed.

 

[TR1CKY]

Thanks for the replies. Lifestyle change is literally us (parents) moving out of the room and eldest daughter (21) shifting from one room to another. No clothes or wet towels are ever hung over the radiator. The damp seal paint was only used on the angled wall. I've just checked up there and the walls are bone dry now after 3(ish) hours of having the windows 'locked' open but just a little bit of residue on the windows still.

 

[^woody^]

Not really - fully saturated air at 10C will have a relative humidity of 50% if heated to 20C. This is the whole point of ventilation for controlling humidity (air quality being the other main reason).

My emphasis of what is in your post is exactly what I was saying as the other parts of my post were discussing how has the use of the room changed.

Let's say that someone in an exposed or high-rise building, or even next to a busy street was told to increase ventilation. Not practical.

Or that same person could not afford to put the heating on, let alone then let all the heat escape .

That's why it's dated advice, and incorrect advice for many years now.

Let's put aside for now that it is lack of air movement that causes mould, not lack of ventilation. Condensation or high humidity itself is not a problem.

Best thing to do to deal with excess humidity is to deal with the production of it not just the extraction.

Best thing to do with keeping moisture in the air and not on the windows is use the heating differently, not necessarily use it more.

Then there are several other options to investigate before going straight to wasteful extraction and costly heating.

Dealing with condensation and associated mould is not as straightforward as "Open your windows mate, and crank up the heating".

 

[JohnD]

Let's say that someone in an exposed or high-rise building, or even next to a busy street was told to increase ventilation. Not practical.

I'm not convinced that's true.

 

[JohnD]

Let's put aside for now that it is lack of air movement that causes mould, not lack of ventilation. Condensation or high humidity itself is not a problem.

That is certainly not true.

 

[blup]

Windows open for three hours, those suggest levels of humidity you would expect in a kitchen or bathroom. If it's not lifestyle (not kettles boiling all day wfh and clothes drying) maybe mechanical ventilation is worth considering

 

[^woody^]

That is certainly not true.

Enlightening!

But its a true story bro.