5th August 2009
" TRAINS UNDER THREAT
Q I=92ve just reroofed our garage with metal box-profile sheeting (the
old asbestos-cement corrugated roofing was well past its best, and
leaking), and started setting up my model railway layout. Below the
sheeting I put 100mm (4in) foil-both-sides Celotex, with an air gap of
about 50mm (2in), hoping this would insulate and prevent condensation.
But condensation is dripping through in one or two places, on to the
trains! Any ideas as to how I might combat this? I have been quoted
=A3500 (much too much for an OAP) to foam spraycoat (50mm) the underside
of the sheeting. Undoubtedly this would prevent the warm, moist air
hitting the cold steel sheeting, but I was wondering if there might be
a less expensive way to cure the problem. TP, Shotley "
Oh dear, you've got yourself in a right mess. Not only is that
condensation dripping onto your trains, it's certainly also leaking down
into the walls. Not good. ;-(
I'm afraid there is no simple, cheap fix for your problem. Many people
before you have found that turning that lovely empty space, which looks
so inviting without a car, into a train room is not a cheap proposition.
To use a garage for a train room means bringing it up to living room
standards, and that means a major renovation. Some people have found
that constructing a new building is cheaper than renovating a garage.
I would _not_ advise a spraycoat of foam on the ceiling. It will stop
the condensation from dripping through, but there will still be
condensation between the Celotex and the roof, and in the long run that
will cause rot and rust.
What's happening is that warm air from inside the room is leaking up
into the space between the Celotex and the roofing. You must (1) prevent
that leakage; and (2) provide for circulation of exterior air between
the Celotex and the roof. That's easy in principle, but not cheap to do.
OTOH, if done right, it will reduce heating costs quite a bit, though
whether the savings will pay back the cost in a reasonable time is for
you to decide. If the garage is uninsulated at present, I figure you
could save at least 50% on heating cost.
(1) A "vapour barrier" between the room and the wall/roof will block
warm, moist air from coming in contact with the cold roof and exterior
walls. I took a quick look at the Celotex site. They offer a
"plasterboard thermal laminate", which could simplify the job, if one of
the layers is a vapour barrier. But it has to go on the walls as well as
the ceiling. You should also get some advice on a false floor, properly
insulated and with a barrier. An amazing amount of heat can leak out
through an uninsulated floor!
The DIY method is to apply sheet plastic all over the inside of the
room, taking care to tape all seams, and around door and window frames
(the trim must be removed to do this.) For maximum effect, you should
also install a false floor on 2x4 (40x80mm) joists, put foam insulation
between the joists, add a vapour barrier, then a layer of 5mm floor
underlay, and then the floor finish. Apply sheetrock (gypsum board) to
walls and ceiling as the interior finish. If the garage is a wood-frame
structure, fill the wall spaces with insulation first. FWIW, I would
tackle this job as a DIY project. The skill level is relatively low, the
only really tricky bit is taping and sanding the sheetrock seams. A
tedious job, though.
(2) You need air circulation between the insulation and the roof, this
will reduce and even prevent condensation. Over here we use both
whirlybirds (turbines turned by the wind) and passive ventilators on the
roof. They draw air from the eaves up through the roof space to the
outside. Installing these after the fact is not a difficult job, but
does require trade skills and tools. Not a DIY project. A competent
tradesman should be able to do it in half a day (you'd need two whirly
birds or four passive vents for a one-car garage.)
As a long term inhabitant of industrial units with tin roofs I'd suggest
that there are three key factors needed - the roof needs to be pitched at
an appropriate angle, double skinned , and the room must be well
Condensation only really occurs (to the extent that it starts dripping)
when there is rapid change between the inside and outside temperatures -
so any heating inside needs to be ramped up and down slowly. To date a
ply false ceiling has sufficed to keep my stock and computers dry under
such conditions, though in winter printer paper will get just damp enough
to cause problems sending multi-page faxes, but storing it next to a
laser printer left on permanently will suffice for general use.