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Full Version: Polyiso: 2.5" or 4"?
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I've got leads on two piles of reclaimed polyiso panels. One involves an inconvenient drive to get 4" panels (apparently whole) at $25 each, the other is a very local source for 2.5" panels (mostly partials) at $20 each. Total cost including gas is about $400 vs. $320. This would be going into a step van with nice flat walls, with spray foam to fill any gaps. Plans for the van involve multi-pane skylights, sealable floor vents, no side windows, possibly fans, both a propane and a wood stove, and AC. At first, just the skylights, vents, and propane. The intended climate is montane/continental: both cold and heat (mostly dry heat) will be problems, but more the former.

I have cardwelling experience, but have never worked with insulation like this. How much extra trouble will partial pieces be? I'm guessing not much, but please share your experience.

As a personal general guideline, overkill is better than underkill if it costs about the same. For instance, I shoot deer with a 7.62x54R to ensure they're killed quickly - that is, humanely. Minimized suffering of the animal is worth not having much of a skull left to mount.

My inspirations for this specific task are positive and negative.

Negative: 98% of houses in the US are underinsulated in my opinion (frame of reference: Iceland and Norway). I don't consider it acceptable to torch hundreds of dollars worth of fossil fuels or multiple cords of wood per winter, to maintain reasonable temperatures indoors. That's the product of a wasteful and ignorant culture.

Positive: Passivhaus and R-2000 standards, the apex of which is buildings heated - in proper winter weather, mind - by nothing more than the coils of a refrigerator and body heat. The impression I get is that as total R-value increases, so does the importance of ventilation and airtightness. Bob's article on insulation suggests that too much is a bad thing in hot weather, but based on some houses I've lived in, I think that problem can be solved by good ventilation control: wide open at night, sealed tight during the day.

Am I likely to notice the difference between 4" and 2.5" (R30 and R18, approximately), given two factors?

1. The van will never be airtight, because it's old and I'm not a magician.
2. The roof probably can't be more than 4" thick because of height restrictions. So with 4" panels, the walls/roof/floor would be perhaps 4/4/2 (while 4/7/2 or so would be ideal); with 2.5", 2.5/4/2. I already have a few 2" panels to work with.

The difference is 3" of width, about 3.7% of 7 feet (and even less of the length).

Conventional wisdom is that 1" is plenty. I'm not interested in conventional wisdom - it also tells me to borrow several times my net worth to buy a  shoddily-built sticks-n-bricks box, among other things. But I am interested to hear from people who 1. boondock in properly cold climates, 2. who've worked with insulation in comparable settings (vandwellers!), and 3. share the same mentality regarding resource consumption that I do.
Having lived in Alaska in a box van for 6 years, I am a very big believer in insulation! But, it wasn't clear to me how cold will it be where you spend the winter? I knew I had to regularly face -30 below zero F so I put 2 inches of styrofoam on the wall and 6 inches on the roof. It worked very well. But I never had to deal with heat, it' very rare for Anchorage to get over 80.

Go with the 4 inches but put it in two Fantastic Fans and several windows with good openings. Ideally you will have enough solar to run an air conditioner for brief periods. Then the insulation will pay off winter and summer instead of being a penalty in the summer.
Well, I'm in Montana now (common extremes between -15 and +40C, night never more than a dry 18C, pretty dry overall actually), but would like to be able to handle something like Whitehorse (lows of -40, occasionally worse).
Having worked on insulating a Morgan Cargo Body, the track saw system I used to cut the sheets did a fine job giving me straight and clean cut edges. 2 1/2" or 4" thick panels is beyond it's capability.
First, welcome to CRVL!

As to your questions, do you like puzzles? Partial sheets of insulation are a pain to work with. You may not agree right now, but you will by the time the project is finished!

I haven't read Bob's article that you referenced in your PM, but as to not needing much insulation in the summer, I disagree strongly. In a vehicle with a highly conductive metal shell, I don't see how you could have too much insulation - you'll run out of room before you can put too much insulation in!  This is particularly pertinent if you intend to use an air conditioner - the more insulation you have, the less the air conditioner will have to work to keep things cooled off.

The most important area to insulate is the roof - that's where insulation should be the thickest. It is also important to make the van as airtight as possible. For anyone dealing with cold climates as well, the floor should be just as well insulted as the roof. 4 inches in the roof and floor and 2.5 inches in the walls would be a pretty good combination.

If I was building out an old step van for my own use, I would 1) frame out what I wanted for doors/walls/windows, then 2) attach awnings/solar/etc tat needs to be fastened to the skin, ten 3) run my wiring/plumbing/etc. and then 4) shoot 2 lb. closed cell spray foam.

That would give you the best insulation coverage, the best air seal, plus the added benefit of minimizing resonance in the body structure. (Step vans tend to be pretty noisy going own the road!) Polyiso sheets are good insulation, but no matter what you do, they won't adhere to metal walls as well as sprayed foam, so you won't get the same structural strength/noise reduction.

As serious as you say you are about insulation, I'd recommend you have the van sprayed. I suspect you'll be disappointed with anything less than the best. Polyiso boards would be my only other option. If you go with them, don't use "great stuff" or any equivalent cheap single component spray foam in cans to seal gaps between the sheets.  Get a little two part closed cell foam kit for that. There will be plenty of places where it was a pain to get the board to fit, so you'll have no problem using up the material.
Well, as of last night, I'm the proud owner not only of a 1973 Ford P400, but also about 500 square feet of 4" polyiso - 10 full 4x8 panels, 12 4x4s. This is on top of about 100 square feet of 2" stuff. The decision to go with this thickness was made by a combination of measuring the box interior on the spot - 7'2" width, 6'8" height and that's before I remove the inner ceiling panels and gain perhaps 1.5" - and seeing that typical passive houses range from R30 to R70 walls/roofs. 4" polyiso is somewhere between R24 and R30.

The walls will be 4" for sure, but how should I distribute 8" between the floor and roof? 4 for both? 6 on top and 2 on bottom? External skirting is probably out of the question.

At the moment, I'm fretting/researching about transmission noise in this particular van (NP435 gearbox). I've driven it about 100 miles on the freeway and yeah, no kidding, noisy! But the vast majority of the noise is from the front, and the body seems pretty solid, so the structural and acoustic benefits of spray foam in the back seem pointless, and more trouble than just putting in these rigid boards.

I'll look into two-part foam, thanks.
Warm air rises, and will accumulate at the roof, the cool air falls and will accumulate at the floor. The greater the difference between the inside and outside temperatures, the greater the force for heat to pass through. Insulating the roof is 100 times more important than the floor. The roof is critical, the floor is relatively unimportant.
It makes sense that more insulation on the roof and less on the floor would promote low heat loss from the space as a whole, but also exaggerate the temperature gradient from floor to ceiling, while uniform insulation would be less effective at slowing total heat loss but more effective at keeping floor temps closer to the ceiling. My personal physiology really likes my feet and body warm and my face cold; I can't do saunas or steamrooms, but a hot tub outdoors in the winter is bliss.

My impression from research on RVs and houses is that R24+ is quite a lot of insulation, and if I can get the space reasonably airtight (yeah right...), it may cost next to nothing to heat. In that case, if it's a tradeoff between, say, $5/month for a large vertical heat gradient or $15 for uniform temperatures, I'd go for the latter. What do you think?
You're right the whole question is theoretical, the fact is if you put 4 inches on the roof it will be incredibly well insulated!! In all but truly extreme cold that thing is going to be super easy to heat.
My 1992 Ford E250 Cargo Van came already insulated with 2" of Polyiso on the sides, top, & back end.  Candles are sufficient to keep the interior temperature in the high 50º's - low 60º's.  The back end was sealed off 20" from the back doors to provide space for tools/cargo.  That wall separates the living/insulated area from the back end of the van.  Three thick curtains on the bulkhead separate the insulated area from the driver/passenger area.

I do have a Mr Heater, that I light first thing in the morning, to take the chill out of the interior.  Sometimes on a cold night, I'll light Mr Heater & heat the interior for 3-4 minutes before crawling into my bed. 

The prior owner, lived in the van for 5 years, he did the insulation project; he lived & slept in it in PA, for 5 long cold winters.
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