dry rot?

[jimbo5050]

I am quite concerned about the floor joists on the ground floor of my house extension that was built about a year ago (internal work still on-going!!).
The floor is constructed from 6" x 2" timbers supported by 3 dwarf walls (2 running along the exterior walls and 1 in the middle). There is a void under the floor of about 18" - 24" and ventilation is provided by a total of 8 Ventilators located just below the floor line (4 spaced evenly on two of the 3 external walls) and the floor space is about 80 sq M.
The dwarf walls are constructed from solid 4" wide concrete blocks with no holes in them, save for directly in line with the ventilators. The central dwarf wall has no openings at all.
The space between the joists has been filled with insulation wool.

I hope the above makes sense to someone.

The reason for my concern is that I was doing some wiring work under the floor and I could see that the under side of the floor joists were covered in a fine layer of white/green 'fluff'.

It is dry under the floor and I was wondering if the problem is caused by poor ventilation and what can I do

 

[masona]

Can be a number of reasons, as long it's dry I can't see a problem, use a stiff brush to brush it off & tell us what you see. It could be moisture in the timber that's drying out.

The green colour could be the timber been impregnated with green timber preservative.

A picture may tell us more.

 

[jimbo5050]

Sorry for the delay, had to go off to work.

Underneath, the timber is still fine and it only appears on the bottom face.

I was just worried it was the start of something worse.

I can take a photo of it but I will have to work out how to post it (please bear with me