1st Floor Condensation under window.

Hello to all! I am after some help and really struggling to find an answer.

I live in sunny (damp) England and a couple of years ago we bought our home, a circa 1810 built Cob Mid Terrace house, it's a lovely home in a great location but we are having terrible problems with mould and condensation under the master bedrooms bay window.

The problem became worse once we had secondary glazing installed as the building is listed we were not able to install double glazed units to replace the single pained sash windows. This has been a god send for noise, I'd argue the 6.8mm thick Glazing is much better than a normal double glazed units!

However this has left the master bedroom being a lot less ventilated at night and as a result e now get a lot more condensation under the window. No where else in the house just the master bedroom where we sleep at night.

I have tried a dehumidifier machine which we leave on through out the day and also have a normal silca gel dehumidifier container in the corner, these do get a lot of moisture out of the room, but the problem persists.

I have also tried using a DPM Paint (Dulux Moisture Primer) on the wall under the window, but that has not helped other than its now easier to wipe the mould and water/condensation of the wall.

So we are now a bit stuck, not sure what to try next.

I had a builder come round last year to inspect some damp  by he front door and he basically said for this issue that as its cob there will be no modern damp roof course and that you will have to live with damp, most people board out between the outside wall and leave an air gap, which is what I have seen people mention, which is what has been done downstairs, but as damp naturally dries after 1.2m or so, if I were to board out under the window in the bedroom would this help with the issue we are facing?

Any input is appreciated on this issue or links to articles that may help also.

Thanks in advanced, I'm off to read some more of this great forum!

Hello Giles and welcome to Permies!

Old houses come with their own problems don't they! We have a stone built house, only from 1920s though, and damp is a constant issue for us - partly roof leaks (the wind and rain here find any crevice), and lack of adequate clearance at foundations leading to damp under the suspended floor. I'm not an expert, just an interested amateur, so here are my thoughts for what they are worth, I hope they help:

I'm assuming it is condensation on the wall surface rather than damp coming through from outside. Condensation occurs when the air temperature drops below the dew point which is higher depending on how much humidity is in the air - less humidity then lower temperatures before there is a problem. So as I see it there are two ways of addressing the problem: decreasing humidity, and raising the temperature.

First: decreasing the humidity:
Have you tried sleeping in another room to see whether it is your breath that is the main issue? If the issue goes away without your breath, then increasing the ventilation locally may help. If there is still condensation, then I suspect there is a general humidity problem in the house - as your builder suggested this could be from the underfloor and foundations, or roof leaks, plumbing leaks. Just a point from my previous career: increasing airflow has more effect at clearing car windscreens of condensation than the temperature of the air from the vents, so a fan aimed across the wall may help, also removing any furniture that slows the airflow locally.

Second: raising the temperature:
This is what putting a false wall in will do: the airgap behind it acts as an insulator (like double glazing does) putting the cold surface of the wall behind an additional barrier. The risk is that the condensation will carry on behind the false wall just hiding the damp problem. I would suggest if you go this route make sure that the gap has plenty of ventilation to avoid issues like rot. There are other ways of insulating the wall, but you would need to be careful not to trap the damp in a hidden area. My understanding is that with a breathable fabric like cob, the outermost surface should be the most weatherproof to make sure the envelope breathes properly. The waterproof paint may not be the best idea.

What intrigues me is why the condensation should occur just in this one place. Is there anything you can think of that is different about the walls in this area?

Like Nancy said "I'm not an expert, just an interested amateur,".

To me. there is a lack of insulation under that window.  This may have happened if the window was installed after the house was built.

If you see this condition on the windowpane that might mean the window is leaking air. What you described does not sound like this to me.

One way to add insulation is with a sheet of styrofoam from a lumber co. I don't like to recommend that product so take that with a grain of salt. I also do not know if that would stop the mold.

I understand the exterior of cob homes so I have to ask is the interior Cob?

That section of wall could be torn away, proper insulation added and then the wall could be restored to look like the original wall.

I hope this has been of some value.

Is the condensation forming on the window and dripping/wicking downward?  Or is it forming on the cob wall?  If you could attach a photo (there's a tab for that just under the reply box) it might help folks figure it out.

Thank you all for taking the time to answer. I will grab a photo tomorrow.

From the looks of everything, the wall is wet with condensation and the window is also, I do not believe it is driping down onto the wall due to the overhang of the window sill internally.