vapor barrier and rigid insulation

[450Duner]

I am in the preliminary design phase of converting an enclosed car hauler trailer into a toy hauler. Been doing some research on vapor barriers and have seen a lot of mixed opinions. I understand if you place rigid insulation between your studs with vapor barrier between the insulation and interior wall panel this will help prevent water water from getting into insulation and causing problems. My question is what happens when water condenses on the vapor barrier behind the wall panel. Where does it go? Seem like it will run down vapor barrier and pool up at base of wall or start rotting backside of wall panel.

Aren't some rigid insulating panels closed cell and there fore moisture proof? Would it be ok to put the rigid insulation between studs, spray foam any gaps between insulation and studs then tape the seams? How much of a difference does R3 or R4 insulation really make? Seems like there would still be a major thermal bridge through the mteal stud.

Another question is if you do a vapor barrier what happens when you start poking tons of holes in it when you start screwing your interior paneling to the studs. If you used adhesive to attach panels to studs the vapor barrier would act as a bond breaker and the panel would just be attached to the plastic vapor barrier.

....with all the in mind been considering just putting in a few good vents in the walls to help keep the trailer well ventilated as this will be a toy hauler. Usually don't camp in anything less than about 35 degrees. Plan on using Mr heater big buddy for heat.

Any knowledgeable advice or stories of other's experience converting there own trailers and how the final product turned out would be greally appreciated. Pictures welcomed as well!

Thanks

[GuitarPhotog]

If there is not much temperature difference between the interior of the trailer and the surface of the vapor barrier, there won't be any condensation. That said, good 'tween wall ventilation is worthwhile but not absolutely necessary. Remember that the cooler surface has to be below the "dew point" for the temperature and humidity combination for condensation to occur.

Hope this helps
<Chas>

[450Duner]

So in other words the vapor barrier is essentially preventing the interior air from ever condensing?

[GuitarPhotog]

So in other words the vapor barrier is essentially preventing the interior air from ever condensing?

No the insulation is keeping the wall from becoming cold enough to condense.

<Chas>

[MtnDon]

For air borne moisture to condense a surface must be below the dew point (colder than the air) and air must be able to contact the surface. Remove one or the other or both and there will be no condensation. If we assume the exterior skin is going to be colder than the air inside the trailer, the skin is where the condensation will occur. This is assuming the typical CT, metal framework for walls and roof with aluminum skin on the exterior. 3/8"plywood on the inside of the metal framework.

IMO, the very best method to insulate a CT is with spray in closed cell foam. Also likely the highest cost initial outlay. Not always easy to find a contractor who will do a small job and as a DIY project that can be very messy.

Next best would be to carefully infill the bays between the wall or roof ribs with rigid foam sheets, cut to fit and sealed / glued to the exterior skin and the wall or roof ribs. Foam the same thickness as the wall ribs. That leaves the metal wall/roof ribs to act as a thermal bridge between inside and outside. You could still have condensation at the position of each rib even after the 3/8" interior plywood is replaced. That depends on the outside weather,the interior humidity. Many CTC are built like that and work okay.

Better than the above might be the method I employed. My signature line (below) has a link to our CT project. It might be a little long and rambly but there are photos of what I did with our walls and roof. I used a multi layer approach in an effort to rid ourselves of the thermal bridging of the metal studs and metal fasteners. Insulation starts on page 6................

[lrrowe]

MtnDon, why does the foam have to be secured with adhesive? Mine are put in tight and will have sealing tape across the uprights. I know some could budge out a little, but will not the interior siding be enough pressure to hold them in?

Now let me try to answer my own question. If any panels were to move a small amount away from the aluminum sides, then should any moisture find it's way to that area, it could get into that air clearance and create the dreaded condensation. Right?

Now in my particular case where the plywood is on the outside of the uprights separating the aluminum from any interior contact, then that "air gap" issue is not so bad for me. Right again???

[MtnDon]

Yes, regarding the adhesive used between aluminum skin and foam. I should clarify some though. My main adhesive bead for the foam to wall joint is a thick bead of polyurethane adhesive around the perimeter of each wall bay. I also placed a thin bead zig zag fashion in the field of each rib bay. Partly to help eliminate or reduce and "oil-canning" effect of the aluminum skin when going down the road. The insulation panel is cut fairly close but not a real tight squeeze. In theory that bead around the perimeter of each insulation panel insert should be enough to seal if care is taken to get a good continuous bead.

And yes again, regarding the unique exterior plywood of the CarMate where the plywood acts as a buffer of sorts between the interior and the aluminum skin.

[lrrowe]

Thanks.....and to be extra safe, I will put a dab of adhesive on the foam sheets just to hold it there and seal all corners.