"IMM" wrote in message ...
"N. Thornton" wrote in message
om...
(xavier) wrote in message
. com...
(Alex) wrote in message
. com...
I have installed rockwool insulation blankets under the floor and I am
now experiencing some worrying problems with condensation as the


snip
MY tuppenceworth would be to suggest you remove the rockwool and
replace it with Kinspan or do away with it all together. Although its
marginally 'helping' your fuel efficiency, its also having the
detrimental affect of lowering the sub floor temperature and thus
increasing the relative humidty. This will present you with an
ongoing condensation problem...

at last, someone who understands the issues. The short story is:
remove the rockwool.

Before doing something so drastic, I would make sure the underfloor
ventilation is clear. If clear then add a few more air bricks and monitor.
This is a far simpler, and less upheaval, than ripping out fitted
insulation.


The situation here is as follows:

There is a cold area (sub-floor space) with a warm area (living space)
above. The vapour pressure differential will be (as is almost always
the case) such that the vapaour pressure in the living space is
greater than that in the sub floor space and thus moistened air will
move from the living space to the sub floor space. Sandwiched between
these two areas is the Rockwool and naturally there will be a
temperature gradient across it - from virtually room temperature at
the upper level to outside temperature at the lower level.

Thus, as the warm air moves through the Rockwool it will cool and
there will be a point within the rockwool where the dew point
temperature is reached. At this point interstitial condensation will
occur. (Don't forget that the sub floor space is now cooler than it
was prior to the installation of the Rockwool and thus Dew Point is
reached more often than before)

To cure this problem the moisture contained in the warm air from the
living space must be prevented from being carried through into the sub
floor space. The only feasible way to achieve this is to insert a
vapour barrier. To me this seems like a very difficult thing to
achieve although I suppose that painting the floor with an impermeable
surface could conceivably work and, if you really want to leave the
Rockwool in place, then this is what you need to try first.

Increasing the ventilation will probably not help the situation. It
will make the sub floor space cooler if anything and thus will
increase the temperature gradient across the rockwool. Its main
effect will probably be to raise the dew point physically higher in
the rockwool band (nearer the living area). The ventilation will have
little if any effect on helping the moisture to evaporate harmlesslly.

This problem is much more than theoretical as far as the OP is
concerned because if it persists (and as winter approaches it will
probably get worse) then his floor joists could be in imminent danger
of fungal attack.

It's one of those 'good ideas' that can have ramifications which are
far worse than the problem you're trying to cure. It's lucky he
noticed it, most people would have nailed the trap door down and
forgotten all about it (until the floor fell in that is).

Either remove the Rockwool or place an effective vapour barrier
between the two areas. Anything else is tinkering )

Xav

p.s. I'm working on the assumption that his UF ventilation was
adequate prior to the Rockwool. He doesn't seem to have had this
problem before then...