[Default] On Fri, 08 Jan 2010 19:55:51 +0000, a certain chimpanzee,
Fred , randomly hit the keyboard and
wrote:

I went into our loft to put the Christmas tree away and the rafters
were wet and some of the cardboard boxes up there were damp. At first
I wondered whether some of the snow had melted and leaked through an
iffy ridge tile but I've been told it is likely to be condensation.
I'm puzzled how the warm air gets up there because there's a good ten
inches or so of insulation (rolls). I've been told that once the cold
spell goes everything will return to normal. I am concerned about the
timber getting wet and all the boxes (and presumably their contents)
getting soggy. Is it likely to just be condensation and nothing worse,
and is there anything I can do to limit it? Would wedging slabs of
insulation between the rafters help?

Does your roof have breathable felt or traditional?

If breathable - these can be used without ventilation (according to
the manufacturers), provided that there is a vapour barrier underneath
and all the rooms producing moisture have adequate ventilation (i.e.,
extract fans). They say that some interstitial condensation could
occur, but that it will evapourate and be ventilated through the
membrane in summer.

If a traditional felt- this should be ventilated at the eaves along
two opposite sides. You can do this with breathable felt too. Make
sure the gaps are free and not blocked by insulation. Again, try and
reduce the moist air entering the roof space by draught-stripping the
loft hatch and using the extractor fans and trickle vents.

As others have said, insulation isn't a vapour barrier. Once the
vapour's in the roof space, you need to allow it to ventilate away.
What you don't want to do is block any ventilation path by putting
insulation between the rafters.

I have seen roofs covered with breathable membranes literally dripping
with condensation. If I had a roof like that, I wouldn't leave
anything up there that could be affected by moisture.
--
Hugo Nebula
"If no-one on the internet wants a piece of this,
just how far from the pack have I strayed"?