Opinions wanted for best insulation method

I like foam board myself and would probably go for it.

Fiberglass is fine in all but the floor but will absorb water if it gets wet, contributing to rot. I've done a lot of walls with it and/or used the original if clean. Most trailers had some kind of vapor barrier, either inside or outside depending on weather where it was built.

I have yet to see a better insulation material for a "stick and tin" constructed trailer. The closed cell foam will not absorb water and condensation, and the rigidity of the foam provides a hard backing surface for the thin aluminum skin.

If you are careful installing it, you could even make the argument that the plastic skin they use to make it could serve as the vapor barrier, just use HVAC foil tap to seal the seams from air flow.

Assuming your wall frame thickness is 1 1/2", makes me ask why would you use 1 1/2" thick foam? Thicker foam does not bend easily, thinner foam is much easier to shape, cut, install. If you use a single edge razor blade to lightly cut through the skin on one side in "one inch" strips the foam can be easily cracked yet stay together and you can form, it into the radius roof/ceiling areas. If you have a 1 1/2" open wall cavity, you would do better by using a two-sheet installation. Use a 1" sheet and insulate around all of your wiring, gas lines, water lines, 12V DC and 120V AC wiring inside the walls. Then use a 1/2" sheet on the outside to finish off the cavity space. I have done this with "two layers" of 3/4" foam on several trailers with great success. Unfortunately, both my local Lowes and Home Depot have dropped this 3/4" thickness size from their inventory. They also sell the HVAC foil tape in their ductwork section.

I think your going to find that your estimated number of required sheets is on the high side. You are simply thinking linear footage. But you have a lot of space taken up inside the wall/roof/front/back with wall studs and framing, plus the four foot wide sheets are eight feet tall, and your trailer is only six feet tall, your over by 25% right there. At $45 per sheet you want to buy only what you need and can use.

The best thing I have found to make for cutting without the mess of small white balls getting tracked into the house is to use a jigsaw with a homemade blade. I use a old hacksaw blade, and grind the teeth off leaving a "sort-a-sharp" edge. The foam cuts like butter, and "no mess".

John Palmer

Thank you, John!

It’s possible my memory failed me and Mobiltec did not suggest a reason not to use rigid foam board. I’ll have to fact check that, apologies if I misspoke here!

I explain very well why I use fiberglass and not the foam board in my classes. It's up to you to decide after hearing pros and cons. But in this thread the popularity contest seems to go to foam board. 

I don't like to argue a point too much. I just put my feelings on a subject out there and let you decide for yourself. But here is what I've found over the years, especially after going back into a couple of the trailers that I built up to 12 years later....

I can not find a bad reason for using fiberglass. That being said, I can find reasons not to use foam board. One it's difficult to cut and kinda makes a mess while doing so. Best way to cut that stuff is with a hot knife or hot wire. If you saw, it's messy. Two, you never really get it into all the little spaces and cracks. Three, foam board once heated up does nothing for insulation. As a matter of fact, it transmitts heat all the way through to the inside once heated. And it takes a VERY long time to cool down. That really sucks at night. So if you dont have a way to cool it down inside, you may as well sleep outside at night.

Larry... What about when water gets in there and gets the insulation wet? I say to that, build your trailer so it doesn't leak and then perform maintenance on it in a regular fashion to make sure it doesn't leak in the future. There are many things you can do to maintain the water tightness of a trailer over time but that is another subject. 

Someone mentioned the "Puff Factor" and you won't get that with foam board. 

Finally, cost..... Simple as that. 

Here's a video on installation of fiberglass insulation....   

Video Link:  youtu.be/_-sgRP981iQ?si=cMX8rUUd-XLa16AP