Heikki,
I agree that the ply the aluminum will be laid over should be sealed...unless you choose to glue it on.
If it's glued, the glue will be the sealer.
As for vapor barriers, it is important enough to spent $3.00 for some plastic.
While teardrops are not climate controlled like a house, and don't face the constant difference in temperature and moisture levels between the living space and the outside air that allow condensation to form inside like a house, it still occurs.
In a teardrop scenario, condensation forms, generally, whenever people are inside breathing, and the exterior surface of the tear is slightly cooler than the inside.
The warmer vapor (breath) of the occupants, and any moisture present in the trailer or its furnishings, condenses on the cooler areas of the tears walls, windows, and ceilings.
The average couple exhales a litre of moisture in an 8 hour period while sleeping, more when awake.
The vapor is not stopped simply by the fact that there is a sheet of paneling and some insulation between the occupant and the roof; it will migrate around and through.
When it arrives, it will condensate and worse case it will drip, or otherwise transfer to the insulation, and or wood, and soak in wherever possible.
Agreed, the amount of condensation/damage that can occur is generally minimal since most camp in warm weather, but it still happens even in the summer.
For those who camp in cooler weather, the amount of potential condensation related problems vastly increases.
Some of you may be scratching your heads thinking what is he talking about, Condensation is never a problem here in the dry Southwest...and you may be correct, but for those in more humid and cooler climates, the issue is more than just a passing concern...condensation does a lot of damage.
Too much condensation causes many other problems besides the little drips of water that we sometimes must wipe off the windows and walls when we wake up in our tears.
Once it has taken a hold (besides mildew and mold) the dampness will cause the paint/protection to loose it's grip on the wood you so carefully sealed, so any wayward moisture would not affect it.
But now that the sealer has failed, the moisture has soaked into the wood...
Ventilation is of course a wise practice.
Most leave a window or vent open to help avoid depleting the oxygen in a tear, but on a cool/cold night, it is a balancing act between proper ventilation, and staying warm.
Most open up, or ventilate the excess moisture from a tear in the morning, and a tear that has no insulation or cavities can be ventilated pretty quickly, but one that has insulation in a fairly well sealed cavity, cannot be properly ventilated in 5 minutes.
A vapor barrier however, will contain the moisture to the open living space, where it can be quickly dissipated.
The whole point here is that if one is building a trailer from scratch, the addition of a vapor barrier is a good thing, not something to be dismissed as an impractical bother...
My thoughts...
Rob
Posted: Fri Mar 16, 2007 9:17 am
... Granted I hope the tear will never get that hot nor even close.. 
