ᐅ Roof window stains in the corner

Hello,

this is a structural issue. At this point, there is inevitably a thermal bridge because the drywall is only separated from the outside by the plastic frame. In my opinion, the only sustainable solution is to use window reveals made from plastic (Roto offers this for their windows). The thermal bridge remains, but condensation can simply be wiped off the plastic.

The upper floor of the house has the highest vapor pressure. Therefore, moist air tends to accumulate in the roof window reveals. Besides heating and ventilating and ensuring air circulation there, there is not much else you can do. With underfloor heating, you should maintain continuous heating anyway. Ventilate at least in the morning and evening before going to bed.

Is this your property or a rental unit?

I would clean the areas thoroughly and remove the mold. Afterwards, the mold-free drywall must be completely dried out. Assuming the rear sealing is intact, you can then paint the reveal with latex paint. Of course, all cracks and joints must be sealed beforehand. Latex paint can tolerate some moisture for a few days without the water being absorbed into the drywall, and it can be wiped off.

If it is your property, in the medium term I would choose a plastic reveal, possibly combined with temporary convector heaters near the affected windows for the coldest days.

Best regards

derdietmar schrieb:

The highest vapor pressure in the house occurs on the upper floor. Moist air tends to accumulate in the reveal areas of the roof windows. Apart from heating, ventilating, and ensuring the air is moving there, there isn’t much you can do. With underfloor heating, you should keep the heating on continuously anyway. Ventilate at least in the morning and in the evening before going to bed.

I keep the temperature consistently between 21 - 22 degrees Celsius (70 - 72°F). I already ventilate in the morning and evening. It’s always tricky after someone showers in the bathroom until the next morning.

derdietmar schrieb:

I would clean those areas thoroughly and remove the mold.

Is isopropanol suitable for this? Or is there a better option?

derdietmar schrieb:

Afterwards, the mold-free drywall must be completely dried out.

Um, how can I tell if that’s done? I only see black spots (about the size of a fingernail) in that one area; the rest of the drywall looks fine. I could try measuring the humidity with a moisture meter.

derdietmar schrieb:

Assuming the waterproofing behind is intact, you can paint the reveal with latex paint.

How can I check that as a non-expert?

derdietmar schrieb:

Before that, all cracks and joints must be sealed properly. Latex paint can resist moisture for a few days without absorbing water into the drywall and can be wiped clean.

Does that mean scraping out the joint and resealing it?

derdietmar schrieb:

If this were my own property, my medium-term choice would be plastic reveals, possibly combined with temporary convector heaters at the affected windows for the coldest days.

It is owner-occupied. Do you have any recommendations for convector heaters? What do you consider cold days? It’s a Velux window—I’ll have to see if they offer plastic reveals.

Attached is a photo showing the painter’s fleece, and at the red arrow in the corner is the fingernail-sized issue.

I found this as well; maybe it will help.

This is the latest version I have; the design (see above) was corrected to 90 degrees

Considering that the bathroom is only ventilated in the morning and evening (= far too little) and there is only underfloor heating, it still looks really good.

Immediate measures:

• Ventilate more frequently.
• After showering, use a squeegee to remove water from tiles/floor/... and direct it to the drain.
• Do not dry wet towels in the bathroom.