In article ,
Fred writes:
Hi,

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?

There are some things to check, but it may be inevitable in this
weather.

1. You need to make sure air is not moving from the house into the
loft. Check for leaky loft hatch, oversize holes around pipes which
pass through the ceiling, recessed spot lights, and any other holes.
Check none of your extractor fans exhaust into the loft space.
A cracked lath and plaster ceiling will let moisture through too.
An ordinary plasterboard ceiling, even without vapour barrier won't
let enough moisture through, subject to 3.

2. Make sure the loft tanks have close fitting lids so they aren't
evaporating moisture into the loft. The CH expansion tank will
get warm it times, and if uncovered in a cold loft, that can
release lots of moisture. If the CH tank is pumping over, it will
get hot, and the pumping over should be fixed.

3. The loft needs to be ventilated. If it doesn't have ventilation,
eves vents can be easily fitted and will create enough air changes.

However, at this time of year, the roof surface will be colder than
even the outside air temperature at night due to heat loss by radiation,
and in this scenario, there's nothing you can do to prevent moisture
forming in a ventilated loft (well there is, reduce both the ventilation
and the insulation, but that's not realistic). This shouldn't account
for large quantities of condensation though, so I would look at the
causes above.

--
Andrew Gabriel
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