Foam shell OVER framed walls?

[JohnRock]

Hi,

Just recently got the DIY trailer fever and started planning a squaredrop build.

Overall plan:
1. 5x8 Westbrook utility trailer
2. 5x8 base storage platform with drawers, about 12" high
3. 5x9 normal trailer design on top, about 48" high (extra foot from angled back, thinking of going a bit wider the better protect base drawers from water)
4. Requires roof rails (kayaks, bikes, etc.), so walls need to be able to support typical car roof rack limits
5. Planning to skin with PMF

I initially was attracted by the foam-only walls, but came to the conclusion that I need some wood given the larger size, roof support requirements and just basically making it easier to attach things to the walls.

My next idea was to frame it all in 2x3 lumber, and cover it all in say 1 inch foam so I keep the benefit having rot-proof. I later realized that having walls thicker than 2" would complicated installing pre-made doors and windows.

I did see that HD sells 0.75 inch XPS, and even 0.50, which I wasn't aware of, so I still think it's doable.

Has anyone built walls that have foam on the exterior (minimal wood under PMF), but still with a wood frame (easy to build, can support roof rack, plenty of attachment options), that isn't too thick (to attach pre-made doors and windows)?

[JohnRock]

How does this wall configuration sound?

Layers from inside to outside:

1. 1/4" ply
2. 1" stud framing (3x2 ripped to 3x1") with 1" foam
3. 1/4" ply
4. 1/2" foam
5. PMF

Would this be solid enough to support a roof rack? Does the ply need to be glued to the studs, or would screws or brad nails be enough?

Thanks

[RJ Howell]

How does this wall configuration sound?

Layers from inside to outside:

1. 1/4" ply
2. 1" stud framing (3x2 ripped to 3x1") with 1" foam
3. 1/4" ply
4. 1/2" foam
5. PMF

Would this be solid enough to support a roof rack? Does the ply need to be glued to the studs, or would screws or brad nails be enough?

Thanks

In a word, soft.
I like the ply interior, studding (don't like wood) and ply exterior. Why the additional layer of foam? You winter camp?
I also am in the camp of fiberglass vs. pmf. PMF done right costs the same as fiberglassing and FG is so much stronger. Hey, if you have the materials PMF can be inexpensive and if done right works very well. I've done both... I am fiberglass guy.

That last layer of foam over makes the skin soft. I'm not seeing the benefits over the cons...

Glue and screw!

Support comes from the structure, not the skin. Foam over is soft and will deform if mounting item to it. Another reason for losing the foam skinning.

If you really want that thermal break, do it on the inside. Still creates issues do be dealt with..I think easier to overcome.

Like how you're thinking before diving in, asking before spending $$ & time.
Hope this helps and will be watching progress.

[Pmullen503]

If you are going to use studs and ply anyway, I'd fill the cavities with foam and glue and screw the inner and outer ply to the framing. PMF or epoxy is your choice. I prefer epoxy/glass over wood but PMF is fine too.

I did a foamie with roof racks for boats. I made post for the corners, (2) 2"x1.5" wood uprights with 4" of foam between and then 1/2" ply plates, 6"x8" plates routed into top and bottom ( inside and out). After joined to the wall foam and covered with PMF, I bolted channels to them and used Yakima gutter mount racks. The idea was to carry the weight down to the floor.

Lasted ten years and counting.

[JohnRock]

Thanks for the feedback!

- Additional layer of foam: I do live in Canada, but it's not for winter camping, rather extra rain protection. I like the idea of making this as rot proof as possible and having a layer of foam wrapping to whole thing seems like a good idea: little to no wood directly under the skin, extra insulation, cheap.
- PMF vs Fiberglass: I don't have a preference, but I have a bias for simplicity/speed at reasonable costs. I want to keep this project fun, especially given my dad is going to be doing a lot of the work. I'll read more on Fiberglass to make sure I don't dismiss it too fast! Do you have a good reference to learn more about it?
- Soft: my understanding is that once you have foam wrapped with PMF or glass it is hard enough, but from what you're saying, if I'm going to attach artificial raingutters, I need to be directly on wood? I couldn't have screws going through the foam and attaching to the stud beneath?
- Glue & screw: I have a very good dowel system... thinking I might use this to attach the framing together, then use the screws and glue for the layers as you suggest.
- Foam inside: I've read that foam inside will make it too wet from condensation. I also want "easy access" to studs to attach things inside; I want to make it as modular as possible for changing needs.

Seems like it's 2-0 for Epoxy/Glass over PMF, and plywood over foam... I'm re-considering my foam exterior for sure.

[JohnRock]

If you are going to use studs and ply anyway, I'd fill the cavities with foam and glue and screw the inner and outer ply to the framing. PMF or epoxy is your choice. I prefer epoxy/glass over wood but PMF is fine too.

I did a foamie with roof racks for boats. I made post for the corners, (2) 2"x1.5" wood uprights with 4" of foam between and then 1/2" ply plates, 6"x8" plates routed into top and bottom ( inside and out). After joined to the wall foam and covered with PMF, I bolted channels to them and used Yakima gutter mount racks. The idea was to carry the weight down to the floor.

Lasted ten years and counting.

10 years and counting is definitely convincing.

Do you have a sketch of your build technic? I can't visualize it, but seems like you had the same goal as me. Did you have 4" thick walls?

[tony.latham]

Has anyone built walls that have foam on the exterior ... but still with a wood frame

There's a much better way:



You might take a gander at my build log.

Tony

[RJ Howell]

- PMF vs Fiberglass: I don't have a preference, but I have a bias for simplicity/speed at reasonable costs. I want to keep this project fun, especially given my dad is going to be doing a lot of the work. I'll read more on Fiberglass to make sure I don't dismiss it too fast! Do you have a good reference to learn more about it?
I didn't find PMF to be easier or cheaper or quicker, but I did use quality paint and heavy weight cloth (duck cloth. Paint takes much longer to cure than FG

- Soft: my understanding is that once you have foam wrapped with PMF or glass it is hard enough, but from what you're saying, if I'm going to attach artificial raingutters, I need to be directly on wood? I couldn't have screws going through the foam and attaching to the stud beneath?
You'll crush the PMF. Same can be from the FG! Yet the FG is stronger and resist punctures better. For patching I trust FG over cloth & paint everyday!

- Glue & screw: I have a very good dowel system... thinking I might use this to attach the framing together, then use the screws and glue for the layers as you suggest.
Glue & screw plywood to studs, studs to studs. Dowel with you wish, yet still screw. A lot can be said for adding mechanical fastening.

- Foam inside: I've read that foam inside will make it too wet from condensation. I also want "easy access" to studs to attach things inside; I want to make it as modular as possible for changing needs.
Still requires a finish and not much different than placing XPS between the studs and ply over. Fastening to is the other issue I hinted at. Again, do not see the real need to add the additional layer. If concerned about insulation, use 1.5" XPS, heck go 2".

Tony's system is tried and true. Reading up on what he's done (doing) would be probably the best resource for you.

Have fun with the project!

[KCStudly]

KISS - Keep It Simple, (not) Stupid.

Don't over complicate things. This will always be a big enough project as it is and adding multiple layers will just make it a bigger one.

Condensation will occur and needs to be handled by ventilation, regardless of construction method and materials.

My walls are 2 plies of 6oz FG weave/epoxy over 1-1/2 inch thk foam with 5mm (3/16) okume inner skins and minimal framing... the only real vertical framing in my walls is 2x2 cedar surrounding the door openings (which doesn't go all the way to the roof) and 1x4 let into the foam at the galley bulkhead. My ceiling skin (also 5mm okume) sits on top if the wall foam and is capped by 1-1/2 inch foam and cedar spars, so adds strength at the wall to roof juncture, which is also reinforced by overlapping the FG from the wall onto the roof over a generous radius. But the only hard (wood) edge on the wall top is where the galley opening is, which extends forward a little past the hatch hinge spar. If I had used this same construction and had made that hard wooden edge go all the way fwd, or had just put dedicated hard anchor points near the tops of the walls, I would have confidence in installing a roof rack. Maybe not so much if I had used PMF inside and out, but with the SIP affect of hard skins, no problem. Four vertical 1x4 "studs" let into the wall foam on the flat prior to skinning would be more than adequate to support a roof rack, and would still provide a thermal break. Yes, I have (and continue to have) to deal with custom fitting my doors and windows, but I wanted the 1-1/2 inch foam, and unless you nail the exact trim rings you need for your wall thickness, fiddling with shim rings and/or flanges is almost always needed to get good fit and finish, IMHO.

[JohnRock]

So at this point the foam over frames walls is out! Thank you for all your insights.

After further review, I think PMF will probably be my skin of choice, if nothing else because I'm confident I can do it and it has proven it does the job.

I'm currently planning the build with 2x3 because I'm used to building with wood so it's easier for me to design and iterate, but once it's taking shape, my logic is that I can always streamline the construction to another technic that would either make it lighter or simpler to build.

My next post will be in the Traditional Designs or Build Journal (?) to start sharing my plans.

Thanks!

[CanuckShooter]

I built 7x14 standup, it's posted in the foamie forum if you care to take a look.

wood framing/luan/ 1 1/2 foam/ pmf over it all. That was almost 3 years ago and it's still going strong. 1/4 in ply twice is overkill and totally unnecessary imho.