ᐅ Insulation in a Construction Trailer: Vapor Retarder Yes or No?

Hello building community,

I recently acquired a construction trailer and now want to convert it into living space.

To keep it from getting too cold in winter, I plan to insulate the trailer with 5 cm (2 inches) of polystyrene foam. Heating will be provided by a wood stove and an electric frost guard. The trailer has an outer shell made of metal, which acts as an exterior vapor barrier.

Now I am wondering if I should also install a vapor barrier on the inside of the insulation. My concern is that moisture might still accumulate in the cavity and then become trapped.

Does anyone have experience with this kind of wall assembly? What would you recommend?

Just fire up the stove even more in the construction trailer

What does "living space" mean to you? Do you want to stay there overnight in winter, or is it just a place to hide out for a few hours?

If it’s the latter, just skip the insulation. It only costs money, requires effort, carries a risk of mold, and ultimately doesn’t make much difference. As long as the stove is running, it will be quite cozy inside anyway, and without a fire, even 10cm (4 inches) of insulation won’t prevent you from getting cold quickly, because a construction trailer is simply too drafty to retain heat.

Living space in this case means that I want to live in it permanently. The wagon will be properly sealed in the end. I will use insulated glass windows common in house construction, and the door will also be insulated and close tightly.

I understand that it can get cold inside on a cold winter night, but I can live with that. However, it should stay frost-free because of the water and wastewater installation inside.

In this case, I would share your concern and recommend insulating from the outside.

Is this allowed??