How does Bob get the mysterious 1/2" of air in walls?

[user 423]

^
Yes, and I've figured up the cost for insulating a 7x12 cargo trailer with 6'-6" interior height and doing a lot of foam will end up being pretty costly. So I'm trying to plan for a reasonable cost/benefit ratio.

While spending winters in the more arid SW, it would seem that any condensation that collected during the evening/night would be eliminated during the day as it warmed up. As long as the vehicle had some air circulation,I suppose. You're right, everyone will have different travel patterns and needs.

I'm just glad there's so much help available while trying to figure it all out!

 

[John61CT]

I just meant best for the R-value. Too many people overestimate shiny / airgap / bubblewrap effectiveness.

Yes, if you can spare losing the interior space more than the cost of foam, a proper sealed air gap is (a little) better than nothing.

But trying to get an air gap **without** vapor sealing properly, is mostly a waste of time and materials.

If you are striving for good R-value but concerned about trapped condensation rusting body panels from the inside

a concern way overblown, maybe even mythical IMO

one approach would be using a two-part closed cell spray foam kit to "paint" the interior steel, thoroughly but in a thin layer to minimize cost. Surface prep and temperature is critical.

Then put in the board foam to get more R-value, maybe later on, even live in it as is for a while, see how it goes, save up as you can in the meantime.

The irregularity of the foam will force some air gap, and at least the outside layer's complete.

Once all the foam thickness **you find** you need / want is in place, just before installing the innermost lining skin, seal the gaps to maximize what you've paid for, and put in a good vapor barrier without worrying about the "trapped condensation" issue.

Or if your travels avoid extreme temps, you may find just that initial layer's good enough.

 

[user 423]

^
Agreed. For as long as I'll be using the trailer, I don't think I'll be worrying about rust causing any structural problems. I'd like to inspect some 20 y.o. trailers out there to see if they have exterior screws or rivets that are falling out because the uprights were rusted.

 

[Anniesezhi]

So if the Reflectix keeps the heat out, in reverse, how do you keep the heat in when its cold?

 

[John61CT]

With a proper insulation job as described in detail in other threads.

There is no one right way, for those following nice weather maybe hardly any needed

But essential for those going through 20# of propane every few days.

 

[Scout]

Osmosis, foam, balsa wood, ...?

Unless something has structural integrity, I do not see how it will keep from being deformed with insulation, etc...

All I see is hundreds of references and not a single explanation or a pic.

I noticed in his build video he didn't bring the insulation totally flush with the ribs. It would take 1 1/2 inches to accomplish that and i think he did 1". Then he put his paneling over the ribs so maybe that is what is being referred too. Also i he didn't use reflectix so i think he was just saying if you did you would need a gap.

 

[John61CT]

If the air gap is actually sealed properly all round

aka "vapour barrier"

with say 6mil plastic, nothing to do with Reflectix or any shiny

then it has some (small) value as insulation, anywhere in the layers.

______
For Reflectix (or other shiny) to work as a radiant barrier, its air gap layer must be on the outermost layer and shiny must be on the inside of that air gap facing out.

Another use is as part of your window insulation panel facing out, ideally pressed up tight against the inside of the glass.

With any other configuration you may as well not use any shiny at all, money wasted.

And the bubble wrap part just gives some physical support, no insulation value.

 

[gp]

^
Agreed. For as long as I'll be using the trailer, I don't think I'll be worrying about rust causing any structural problems. I'd like to inspect some 20 y.o. trailers out there to see if they have exterior screws or rivets that are falling out because the uprights were rusted.

I would be interested in hearing results from reflectix placed against the metal causing rust too. Sometime back, I read about someone's dad who put a foil backed material on the underside of their camper van ceiling, and it eventually rusted the entire roof. Has anyone who has done this had a rust problem over time? It would seem that heat being reflected back onto the inside of the metal wall (with or without an air gap) would cause any condensation to turn to rust pretty quickly, like a sauna. I have seen roofs completely rusted out and wondered if that was the cause of it.

 

[John61CT]

Air gaps, even with the complete vapor barrier needed to be effective, are not a space-efficient technique relative to their benefit, compared to a decent foam layer.

Especially for ceilings, where most people would want minimum thickness of insulation layers for maximum interior height.

But as a separate issue, only a tiny proportion of trailer makers that use steel do a decent job preventing rusting of interior and chassis framing.

Just like the electrical wiring, that is one area I would take care of immediately after delivery, or at least before insulating.