Polyiso PMF instead of XPS or EPS

[Tonto]

I a newbie to the site and to foamie builds. My proposed design will be a trailer foamie with a lift-up roof and folding sides to make its height lower when pulling it. it. I was planning to use the PMF foam as structural support with polyiso since the price of XPS foam cost has shot up.

I have read conflicting reports about using polyiso on this site - some have said it is crumbly and won't work, while others seem to like it. I'd appreciate peoples thoughts and also the experiences of people who have tried polyiso - particularly using the the PMF approach. My concept is to build a lift-up roof, with sides that then flip up to support it, similar to the UK Gobur Landman (www.goburcaravans.co.uk). While I am very aware of the need to prevent wobblies with that approach, my initial worries are whether the PMF walls would provide enough structural support using polyiso and how to handle the aluminum vapor barrier backing on each side of the 4 x 8 panels.


Some questions:

1. Who, if anyone has tried using polyiso (or know others who have tried it) and what have been your (their) experiences with it?
2. Would you do it again?
3. Build question #1: Would I need to sandwich 2" polyiso between 1/8" plywood, or would PMF canvass provide adequate strength both for walls and for the ceiling? (I was thinking of building SIPs with canvas on both sides that I would then assemble, instead of building the frame with the insulation panels and then covering the entire structure with canvas.)
4. Build question #2: If I just use canvas without plywood over the foam, do you suggest peeling off the aluminum barriers on each side, or is there any glue that would adhere as well as Tite Bond II does to foam that does not have a vapor barrier? Would perforating the foil without removing it be adequate?
5. Build question #3: Any suggestions how to remove the aluminum?
6. What other considerations should I have? What am I overlooking?

I have read that polyiso loses R value as the exterior temperature drops, but I am in a generally warmer area in the US, so staying cool in the summer is more of an issue than staying warm in the winter. No reports yet of glaciers making it into the Southeastern Conference yet...

Many thanks. I am excited to join the forum and benefit from your experience.

[QueticoBill]

I can't speak from experience except instead of foil faced I've used fiber face and it's fabric texture and cellulose content would seem to make it more suitable for pmf over foil. Go to a roofing place - not a big box store - the kind of vendor who sells to roofers doing schools and warehouses.

I also found the sell polyiso pre attached to plywood and OSB skins. That was new to me.

Sealing is important. It absorbs water much more than XPS or EPS.

Welcome aboard and best wishes for a great build!

[Tonto]

QB, many thanks for your thoughts. I will experiment with that.
Any others out there?
Tonto

[Pmullen503]

I would frame your panels with wood. It will be more durable than foam, whichever foam you use.

I would get a sample of polyiso and see how tough it would be to take the foil off and how stiff it is once the foil is removed.

Also, with any folding design you must account for the wall thicknesses which can add up quickly with the relatively thick foam/pmf system once you start folding them on top of one another.

[Ottsville]

I have seen sheets of polyiso suck up water with age. It wouldn't be my choice, but there are others more knowledgeable than me.

[Tonto]

Many thanks to you. I pan to test poly I so for strength and all to see how the Pmf appears to protect it from water. But before that, my initial test will be peeling the aluminum off. Additionally, I will also build a test panel with 1/8 plywood on each side and compare strength.

One thought I had was to cut 4’ strips 1/8’thick x 1 1/2” wide from 2x4s and place them in the foam about 4” apart to add additional strength to the panels.

When having water absorption from polyiso, it it because the canvas/paint covering was punctured, wore off over time, or other breaches of the barrier? That truly concerns me.

I may have found a 16’ trailer today. If that works, I will be doing well.

Many thanks, and keep those suggestions coming!

Tonto

[ghcoe]

I find that Polyiso is not very structural. XPS is used in construction as a base for stucco. XPS has a strength rating in pounds where I don't find any such rating on polyiso. Do the google and see what you come up with.

[tony.latham]

I find Polyiso foam board's compressive strength rather amazing. I know this because I ran over a half sheet of 3/4" with my Tacoma. I could barely see the tread impressions in it.



Tony

[Pmullen503]

I find Polyiso foam board's compressive strength rather amazing. I know this because I ran over a half sheet of 3/4" with my Tacoma. I could barely see the tread impressions in it.



Tony

Was that with the foil on it? I have an oven made from foil faced polyiso for molding xps foam for my RC aircraft. I know it's heat resistance is superior but the foam itself isn't any stiffer that 250 lb/sq ft XPS at room temp and it's kind of crumbly on the exposed edges. Of course, it's been subjected to 210F temps many times.

Perhaps polyiso also comes in different compressive ratings?

A test is the only way to be sure.

[QueticoBill]

so many little differences between EPS, XPS, and Poli-iso. It appears similar compressive strengths to XPS. Soaks up a little more water than XPS but much less than EPS. Much more fire resistance. First choice for roofing - probably for fire resistance. Certainly XPS is least environmentally friendly. I think for tear drops, adhesion is the deciding factor and I don't know which is any aren't good enough. I wonder if poli-iso is compatible with epoxy or polyester resin. And it's all over shadowed by sourcing. I'd guess poli-iso is more universally available because of its roofing use.

[tony.latham]

I find Polyiso foam board's compressive strength rather amazing. I know this because I ran over a half sheet of 3/4" with my Tacoma. I could barely see the tread impressions in it.



Tony

Was that with the foil on it? I have an oven made from foil faced polyiso for molding xps foam for my RC aircraft. I know it's heat resistance is superior but the foam itself isn't any stiffer that 250 lb/sq ft XPS at room temp and it's kind of crumbly on the exposed edges. Of course, it's been subjected to 210F temps many times.

Perhaps polyiso also comes in different compressive ratings?

A test is the only way to be sure.

It was foil backed.

Tony

[Tonto]

will test...

[GPW]

Give it the bend test , and the burn test ( don’t smell the fumes ) …

For more info … https://www.ecohome.net/guides/2254/pol ... to-use-it/

[Tonto]

Because of some family health issues, I have been silent recently, but we hope they will resolve over the coming weeks. BUT, I found some 4x4 poly iOS panels for roof under-layers which are triangle shaped with paper/fiberglass facing. I plan to glue them together (Titebond II) to make 4x4 panels 2” thick, and will test. I am not sure whether I have to remove the facing or not - it does not come off easily.

I then plan to use an exterior sheathing with 1/8” PVC panels that I have found. My logic is that will add to weather resistance, add a touch more insulation, and provide a smooth exterior for painting. These 1/8” 5’x 10’ weigh about 10 pounds each. They bend easily, so by themselves they do not add stiffness.

So some additional questions:

Does anyone have experience (first or second hand) using PVC sheets/panels? Are there drawbacks to using them the way I am thinking?

Does it make sense to have 1/2” x 2” pine or plywood strips around the edges of the Pmf (inside the canvas) to provide additional structural rigidity (a skeleton)? Are there negatives to that, other than additional weight?

I am furthermore thinking about making my polyiso pmf panels 12” (1 foot) wide x 4’ highso would place them side by side to create the 8’ long panels to be faced on the exterior with the PVC panels. With each 12” x 4’ panel itself having the wooden skeleton, flexing should be reduced. Then I would need a second panel, about 3’ x 8’ as on top of the first to give me a 7’ ceiling. (My base design will be a rectangular box, not a teardrop design for Release 1.0). Thoughts?

Should I post these questions in another sub-forum?

Many thanks. This is very exciting!

Tonto