We are planning on doing interstitial insulation in our new construction starting this weekend, after the screed and plaster have been dry for a while. Now I've noticed stains on the subfloor and roof joists. Almost the entire attic is affected. Can anyone tell me what it is and how I can get rid of it?
The procedure after the shell is such that first the roof is completed before water vapor or condensation.
So also before the windows, so it is guaranteed that the moisture can ventilate through masonry or concrete work before.
If the attic were not a heated living space, the thermal envelope would be incorrectly constructed.
I had such a case once, where the vapor barrier under the wooden ceiling was not given with an insulated top floor ceiling.
Mold formed on the foil and the beams. The cleaning brought only optically something, because the foil had pores for the water vapor diffusion.
The agent did not work in the pores and the joint between the foil and the rafter. (The rafter was wider than the base batten).
Since mold bloomed again and again, the foil was later replaced.
Are the rafters drying out? If so, I would wait before covering them.
Couldn't you have laid the insulation and vapor barrier before the interior plaster and screed? When was the interior plaster and screed done? Perhaps there was condensation on the ceiling during drying because it had not yet been insulated?
Take a mold remover (hydrogen peroxide or chlorine is best --> follow the directions for use!) or 96% ethanol (alcohol) and remove the surface mold. Then watch for a week to see if anything else grows.
There are special antifungal (clear) paints for wood.