Model Forum / General / Railroads / August 2007

Once you go the insulation route, it can quite complicated. It's not
nearly as simple as the inexperienced think. We Canucks have decades of
experience with insulation. And Rule One is, you can never have too much
insulation. It keeps the heat inside in winter and outside in summer.
But insulation also traps warm air, and warm air contains moisture,
which has a regrettable tendency to condense when it hits a cool
surface. That means there will be condensation in the walls, as well. So
Rule Two is, provide adequate ventilation. If you think these two rules
conflict, you're right. Insulating a building as much art as science.

Re insulation:
The more the better. You will be heating the shed to prevent damp, which
attacks the trains and also provides ideal conditions for mildew. That
means a minimum year-round temperature of at least 10C, 15C would be
better. I don't know the R factors (insulating capacity) in metric terms
(see footnote), but I would advise R15 as a minimum, and R20 would be
better yet. Your wallet will thank you when it comes time to pay the
heating bill. I doubt that even with multiple layers of foil you will
have that R factor in only 25mm of insulation, so put in at least two
layers of the stuff that Dragon Heat recommends. Insulation is cheap
compared to the cost of keeping warm.

Make sure you seal the edges of the insulating material so as to create
a continuous membrane before you apply the interior wall cladding. The
material recommended by Dragon Heart appears to have a vapour barrier
built in. There should be no gaps whatever in this vapour barrier. You
do _not_ want water vapour to get inside the wall. The fact that it
includes layers of foil means that it will minimize heat loss through
radiation, which can be considerable through a wooden wall (wood is
transparent to infrared.)

Re: waterproof membranes and ventilation.
We call them  "vapour barriers" over here, because that's what they are.
You _must_ have this on the heated side of the wall. Otherwise, the
water vapour carried by the warm air into the insulated spaces will
condense on the cold side of the wall, and that's very bad news. In your
case, it's worse, because you have a vapour barrier on the outside of
the shed already.

It's OK to have an external vapour barrier, but only if there is a
vapour barrier on the inside of the wall, too. Actually, the membrane on
the outside of the house is more of a wind barrier - it helps prevent
the wind from sucking out the warmed air from inside the walls. Thus,
this exterior membrane should _not_ be airtight, as otherwise warm,
moist air can be trapped inside the walls. If it is airtight, make sure
that the wall cavities have openings into the roof spaces, and that the
roof is ventilated enough to allow slow but steady percolation of air
through it. This will help to keep the inside of the walls dry.

Re: floor treatment.
You lose heat through the floor mostly by radiation, so a layer of
foil-backed paper or plastic should do, but you'll have to install
another layer of sub-flooring over it. You shouldn't put the finish
layer directly onto insulating materials.

Re: roof ventilation.
There should be gaps between the rafters to allow external air to drift
up into the roof. I'm guessing that you have a typical shallow gabled
shed roof. If the rafters are merely 3" or 4" deep, you cannot get much
insulation in there, but put in as much as you can. Leave a space about
1" between the insulation and the roof skin, this will permit the
ventilation that Dragon Heat rightly emphasises. If you have the
vertical clearance inside the shed, install an inch or two of styrofoam
insulation inside, and cover it with drywall (gypsum board). You'll need
to install nailing strips for the drywall, however.

Er, that's about it.

Footnote: The R factor is the inverse of the thermal conductance. IE,
the smaller the heat transfer rate, the higher the R factor. In Canada
we use RSI as the metric equivalent. I don't know if the EU does so.
Divide the R factor by 7 to get the RSI factor.

HTH

That would be the blue stuff that builders use, which is safe also good for
scenery. So non wasted