New extension – condensation and 'drying out'

We had an extension finished just before Xmas, and have had some issues with condensation around / inside the velux frames, and also in the roof sapce.

Firstly the Veluxes drip water when it gets cold. Condensation is forming on the outside seal / flap of the skylight and then when it reaches a certain point runs / drips down the gap between the window and the frame. No condensation is forming on the window itslef.

At cold points we’re getting water marks on the plaster at the junction of the sloping ceiling and the walls. These are worse at cold periods. It doesn’t appear to be a leak as they don’t appear / get bigger during periods of heavy rain. These aren’t big, maybe one or 2 an inch or 2 across when wettest, but its stopping our decorating as we want to keep an eye on it.

The builder and roofer have been back to look at it. They’ve fitted roof vents to try nd increase airflow and confirm that ther is a lot of moisture between the insulation (Kingspan thats been taped up) and the breather membrane. The space between the tiles and the membrane is dry. There are no soffit vents. The roofer is reputable and neither of them have had this issue with a roof constructed in this manner.

He thinks that it needs to dry out (Velux themselves say that the issue I’ve got can be due to this) but as its cold and damp outside, it could take some time. I’m open-minded to this but have friends that have had similar extensions without this issue. There seems to be something peculiar with ou build that’s resulting in excessive condensation.

Does all of the above seem reasonable?

Seems reasonable to me, it’ll probably be a weather/ time of year issue. Keep the builder and roofer onside and review every couple of weeks..

They’ve fitted roof vents to try nd increase airflow and confirm that ther is a lot of moisture between the insulation (Kingspan thats been taped up) and the breather membrane.

Lots of our building systems are designed to keep all the materials and layers dry. The issue being they are not designed to dry out if moisture gets in.

Would leaving the windows open a smidge help?

Hire a dehumidifier for a few weeks?

Even buying a cheap one is likely to be cheaper than having to start ripping out plaster or frames

You say they fitted soffit vents, if so it’s possible that the ventilation to the roof is inadequate. Should be 25 mm vent strips at the fascia and 5mm at the ridge. A breather membrane can be used and if fitted correctly you need no additional ventilation but they are very rarely fitted correctly.

They taped the joints in the insulation but did they connect this up at the eaves? It’s hard to guess without seeing but that’s faults I see almost weekly from “reputable builders”

Edit. This is worse while drying out and will dry up when frost goes away but if not constructed right it will happen every frosty spell

Hire a dehumidifier for a few weeks?

Shouldn’t be using a dehumidifier near fresh plaster – best to let it dry naturally. (Assuming the extension has plaster in it that is)!

What angle are your sky lights fitted at? There is a limit of 15 degrees. In our extension our roof apex angle was exactly 15 degrees and the builder warned me of this issue. He fitted a kit that canted the angle of the skylight up a bit but I decided to take the risk and go ahead and the builder said if we had any leaking problems he’d do whatever he could to address them. We’ve not had any issues two years on with condensation or anything, so sounds a bit odd in your case you’ve got condensation forming and then dripping into the building.

If your builder says it’s ‘drying out’ then I guess you have no choice at this stage to take him for his word and do whatever you can to address the dripping water to prevent any damage. If after some time it’s still happening then I’d be suspecting an installation issue.

Thanks, that’s reassuring, the plaster is all dry, it was a skim on plasterboard, but then there was render as well. IIRC the taping was done all the way up to the eaves.

Not sure how opening the windows / dehumidifer will help as the condensation is int he roof space, but I’ll give it a go if it will help.

@Wobbliscott – I’m aware of that, thanks anyway. The pitch of the roof is about 20 degrees and its fine in the rain.

Insulation pushed tight into the eaves and blocking the ventilators is a common problem. There should be at least a 50mm air path all the way.
Even with vapour control layers and breather membranes we always spec eaves ventilation as well.