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Full Version: 1" or two 1/2" polyiso
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Getting ready to insulate the CT roof and walls. I asked Rmax folks about dead air space between foil side and trailer skin and someone replied that it's not necessary but 3/4" could improve radiant barrier. Now before I buy the stuff I'm wondering if two 1/2" inch boards stacked with 2 foil sides facing out instead of 1" with 1 foil side would improve radiant 2 lines of reflective defense rather than one and still have R6 value from the thickness. Does that make sense to anyone? Also, if I install between the metal studs/ceiling ribs, I would still have heat from the studs/ribs to deal with. I'd rather not mess with compromising the limited interior space but will if necessary. Anyone have a way to deal with this.....reflectix spaced opposite each stud/rib on luan or plywood interior wall maybe before installed? Would like to get this right the 1st time. Thanks.
I assume you understand that a radiant barrier is good only for preventing solar heat gain in a hot climate, and is useless as far as keeping you warm in a cold climate.

And the radiant barrier needs an air space in order to do any good at all.

That said, the extra foil provided by two layers of half inch polyiso vs a single layer of one inch polyiso accomplishes exactly nothing. The only reason to use two thin layers instead of one thick one is that it's easier to bend the thin layers to deal with curved surfaces.

If heat gain is your major concern, I'd run 1 inch thick HORIZONTAL furring strips along the walls every two feet or so, and fill in in between with one inch polyiso, leaving the one inch dead air space to make the foil effective as a radiant barrier. This will also provide a thermal break between the ribs and the inner plywood wall, which is screwed to the furring strips.

This will only cost you one inch on each side.

Thanks John. That's a good plan.
John gave you good advice. The one thing I would add is that most Cargo trailers have very curved roofs and for that you are better off with two layers of 1/2 inch instead of 1 layer of 1 inch because it will bend better to the cure.

I haven't used Polyiso yet on a curved surface, so I don't know that for sure. But I did use regular stryofoam on my very curved trailer roof and the 3/4 inch bent very well to conform to it. I used three layers of 3/4 on my roof and one layer of 1 inch on the flat walls.

You could use 1/2 inch inbetween the ribs and 1/2 an inch over the ribs and minimize headroom loss and heat loss through the ribs. My guess is your ribs are every 2 feet so only half of them will be at a joint. I started mine so that none of the ribs has a joint over it.

I just used sheetmetal screws with large washers to screw up into the ribs to hold them up in place. There is no weight so they don't want to fall down. It's worked great for 4 years now!
Bob, my roof is flat so no need for 1/2" for curve. Ribs are spaced around 24". If I installed the 1/2" insulation between the ribs, that would give me 1/2" of dead air space which is good, so do I just cut them to fit tight between ribs given the 2nd 1/2" layer will act as support being over and fastened to the ribs? Since I'm concerned more about keeping AZ heat out, would having the 2nd inner polyiso layer foil side facing inwards (for winter) compromise the radiant barrier created by the outer layer foil (facing out of course) during summer? I plan to buy the ceiling insulation/hardware before I head to Quartzite (Eherinberg) next week and do the installation there.....ceiling 1st, walls after my Jan 1 pension check hits. Hopefully any guidance will be close by if needed. Thanks. Chris

I'm not sure you would keep the 1/2 air space. As the trailer bounces down the highway, the inner layer might migrate towards the wall, leaving you with no air space between the inner layer and outer wall, and a 1/2 inch space between the two layers of insulation.

Consider sticking a few spacers of 1/2 inch wood to the metal walls with bits of vhb tape to keep the inner layer from moving inward.

I see your point. I'll use spacers. Would 1/2" bubble wrap in the entire air space serve the same spacer purpose plus add some R value in winter, or would the plastic interfere with the foil's job? Thanks again. Chris
Chris, I think you have a very good plan and John's idea of the spacers is very good. I don't know enough about bubble wrap to answer your question. To be honest I wouldn't do it, too little gain for the extra work and expense.

The reflectix pointed up toward the ceiling between the ribs and pointed down over the ribs is an excellant plan.
Ok, looks like I'm good to go with the right plan. Thanks Bob and John for all the help. Chris