Camper made from SIPs - advice?

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Camper made from SIPs - advice?

Postby JacksonCBall » Sun Jan 04, 2015 6:42 pm

Hi all - Newbie to the forum here (besides secretly spending all of my time on here for the past month). I'm looking for feedback on my design, especially it's strength structurally. But any other feedback is welcomed! For the record, I have zero background in construction, carpentry, or teardrops. Just an outdoorsman looking to build a camper. :thumbsup:

Here is my current design:

sPanel#1.jpg
sPanel#1.jpg (148.21 KiB) Viewed 4419 times


sPanel#2.jpg
sPanel#2.jpg (128.81 KiB) Viewed 4419 times


I am planning to get precut 2.5" structural insulated panels (SIPs) from a supplier. SIPS will make up the side panels, roof, front panel, and back panel separating the cabin from galley. In the Sketchup images, the green, red, blue, and gray panel on the front will be made from SIPs. The floor and galley door will be plywood. The SIPs are 7/16 OSB sandwiched around 1.5 inch foam. I will attach each panel to the utility trailer and themselves using 2x2s w/glue and screws. My reasons for getting SIPs is due to their strength, insulation, ease of assembly, etc.

My main question is whether this stucture should be sturdy enough for road use? (without any extra framing besides the attachment points w/ 2x2s) Also I've seen people saying OSBs are very susceptive to water damage - If I coat the OSB with "the mix" per GPW and then skin w/ aluminum, will it be good to go? I will seal all the joints of course, maybe with silicone caulk.

For the record, if you were wondering about the curved front.. it will be covered in marine vinyl or topgun fabric, insulated with flexible closed-cell duct insulation foam. I will insulate the floor of the cabin with XPS foam.

Thanks for any advice or thoughts.

Cheers :beer:

-Jackson
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Re: Camper made from SIPs - advice?

Postby KennethW » Sun Jan 04, 2015 8:44 pm

The SIP would be plenty strong but a little heavy with the 7/16 OSB.
It would make it a easy build. I personally don't like silicon caulk(when dry you can sometimes pull it off). I would go with polyurethane caulk a lot sticker, but harder to work with.
I am going to try making a stress skin panel camper with 1" foam core with FRP on one side and canvas Titebond on the other side. :twisted:
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Re: Camper made from SIPs - advice?

Postby KCStudly » Sun Jan 04, 2015 9:43 pm

I don't think the extra weight of the 7/16 OSB is worth the ease of construction. I'm not even sure that the construction would be that much easier than traditional stick building, given the need to reinforce and overlap all of the edges.

I'm thinking that the panels aren't that cheap, either, unless you have a connection in the industry.

If you do decide to go that way, it will be interesting to follow along, but there is a big weight penalty with the OSB.
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Re: Camper made from SIPs - advice?

Postby JacksonCBall » Sun Jan 04, 2015 11:02 pm

Thanks for the responses guys. Kenneth - I'll look forward to seeing your foam build! and will avoid silicone caulk... KC - The SIPs estimate was $450 w/ the cutouts so certainly pricier than DIY.

Unfortunately 7/16 seems to be the standard size for SIPs so if I go through a manufacturer I'm probably stuck with that. I doubt they would special order something else for a project this small. I was surprised they agreed to do 2.5" as SIPs are usually a minimum 4.5" thick.

Weight is definitely a concern so I'm glad you guys brought it up. I've got a Subaru Outback with quite limited power and listed 2000# towing maximum - but I'd like to stay under 1000 lbs. Maybe somebody can tell me if I'm vastly underestimating how heavy the thing will turn out:

I was quoted 3 lbs per square foot for SIPs, which is pretty much correct from what I've looked up. 7/16 OSB is 1.4 lbs/sq.ft. Multiply that by two for the SIP and get 2.8 lbs per square ft. The foam adds very little to the total weight from there. So let's say the sides are 24 square feet (a quick estimate, but 4x8=32, minus door and window cutouts), plus the other panels, would equal around 85 square feet.

So ~255 lbs in SIPs.

Then here is the rest of my weight estimates, admittedly a lot of guesswork:
Maybe 30 lbs of 2x2s.
Some .040 aluminum + trim = 40 lbs.
A Northern Tools 5x8 trailer: ~270 lbs.
5x8 plywood floor @ 3/4 inch (coated w/ automotive undercoating or something along those lines): 80 lbs.
Back door: 40 lbs.
Doors and windows: 30 lbs.
Marine vinyl, extra insulation: 10 lbs.
Then finally, the paint thinner / polyurethane waterproofing mixture, caulking, screws, etc. Not sure how much those intangibles will add. Plus some minimal shelving in the galley. Let's add in another 50-100 lbs that I haven't accounted for,

That comes out to being in the 800-850 lb. range. If that's true (which is what I was banking on), then the trailer would be in the weight range of many others. But perhaps I'm being too optimistic...
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Re: Camper made from SIPs - advice?

Postby KCStudly » Sun Jan 04, 2015 11:58 pm

You can cut your panel weight in half by making your own using 5 mm ply (3/16) inside and out, and 1/2 inch ply for the floor would be enough.

Another thing to consider, though it does add cost, is using select cedar for any structural framing and/or edging. It is lighter and more rot resistant than many other species.

My walls are a Benroy-esque profile, 50 inches tall x 9'-8" long, 1-1/2 inch foam core with minimal 2x2 cedar framing along the sill and around the door opening, and 5 mm ply interior. I set 3/4 inch (1x) blocking into the foam where needed (some inside, some out). With the door (but w/o the window, door hardware or galley interior skin) one wall weighed 45 lbs.

That was before stain and poly on the inside, and the painted canvas exterior is yet to come.

Have you seen any of the foamie or foamie hybrid builds?
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Re: Camper made from SIPs - advice?

Postby JacksonCBall » Mon Jan 05, 2015 1:33 am

I appreciate the tips KC. I have looked at the foamie section some, but will have to delve into it in more detail. Guess I was trying to find the easiest and quickest way to trailerdom, now that I've gotten the bug. Living in an apartment puts some space/time constraints on the project unfortunately... but I'll see what I can manage on my own, or beg the SIPs guy to make me some thinner panels.
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Re: Camper made from SIPs - advice?

Postby Shadow Catcher » Mon Jan 05, 2015 7:34 am

Water resistance is #1 marine grade plywood and #2 OSB, this is because of the resins required to hold it together. This is an aircraft you are building not a tank so yes less is more.
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Re: Camper made from SIPs - advice?

Postby pchast » Mon Jan 05, 2015 8:33 pm

You would be surprised how quickly a td comes together, either foamy or
skeleton build. Stick built frame takes longer. The biggest time cost is in
wiring and finishing. The box is relatively easy IMHO.
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Re: Camper made from SIPs - advice?

Postby JacksonCBall » Mon Jan 05, 2015 9:02 pm

Shadow Catcher wrote:Water resistance is #1 marine grade plywood and #2 OSB, this is because of the resins required to hold it together. This is an aircraft you are building not a tank so yes less is more.


OK thanks. Great way of looking at it.

pchast wrote:You would be surprised how quickly a td comes together, either foamy or
skeleton build. Stick built frame takes longer. The biggest time cost is in
wiring and finishing. The box is relatively easy IMHO.


Good to know! And I'd say my design, simply being a box with some radius cuts, is relatively straightforward.
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Re: Camper made from SIPs - advice?

Postby JacksonCBall » Wed Jan 07, 2015 9:18 pm

I lined up some SIPs with 1/4 inch OSB. Not quite as thin as 5mm ply but I think it'll do.

I'm trying to figure out whether I should have the floor made from the same SIPs.

KC, you suggested 1/2 inch ply for the floor - Do you think it would be better to just do that myself, w/ foam and 1/8 inch topper? Or what about going with the 1/4 inch SIPs - would it be too thin for the floor as far as protection from rocks etc? Would the polyurethane and/or another type of waterproof coating on the bottom strengthen/protect the floor to a significant extent?
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Re: Camper made from SIPs - advice?

Postby KCStudly » Thu Jan 08, 2015 8:45 am

My floor is a built up homemade SIP, if you will, 5 mm top and bottom, 2x2 (1-1/2 actual) cedar rim frame with 1x2 (3/4 x 1-1/2 actual) xmbr's on edge. (Due to trying to maximize plywood yield, and the fact that I chose to build 64 inches wide, I ended up with more seams, and therefore more xmbr's and blocking than might otherwise be needed. Probably compromised the concept of a SIP somewhat by interrupting the unity of the panels using simple butt joints over the frame members.) The frame is fully biscuit joined and glued with TB2. The voids were filled with foam board glued in with 3M 30NF "Green" (water based) contact adhesive 100/ct coverage on both sides.

I prefinished the floor prior to assembly, and have kept it covered with rosin paper during the build. During the bulk of the heavy construction (before the front wall went on) I kept a partial sheet of 1/2 ply (to become my galley counter later) laid over the rosin paper so that I could step in, stand, walk on and work with out concern. Once that came out there have been a few occasions where I have had to kneel, and although I did this cautiously, I never really felt like I was doing any harm; only felt the slightest of give, kind of like walking on that padded Pergo flooring only less so. Once the rosin paper comes out and the mattress goes in I won't think twice about it. 8)

As far as rock damage is concerned (I assume we are talking general road debris here, and not off road rock crawling), I think that is more of an issue for the forward facing surfaces, the front wall and cabin corners, and the tongue box. Most people report that the undersides of their cabins require very little maintenance, if any at all. Some people don't even skin the underside, just fasten insulation with long screws and fender washers.

My suggestion of using 1/2 inch ply was for if you were doing a simply single panel floor. In fact, I would be tempted to go down to 3/8, depending on wall attachment method. However, if you are going with a SIP or built up floor, 1/4 inch top and bottom is more than enough, IMO. We are talking TD, not standie, right?

One last thing, somewhat unrelated, because I will be taking TPCE on extended forest service road trips, with potential for washout conditions, boulder fields, fallen trees, etc. I purposely built my cabin to sit fully on top of the trailer frame with no wall skirts or edges hanging down. If anything bottoms out it will be steel first. Most people like to cover the frame by extending the walls down, but on a canvas wrapped outer skin it is better, IMO, to pinch the bottom of the canvas between the box and frame.
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Re: Camper made from SIPs - advice?

Postby JacksonCBall » Thu Jan 08, 2015 11:12 pm

I plan on standing and moving around on the floor somewhat. Part of my design is the idea that I can lift up the marine fabric with telescoping poles to convert it into an open air tent with more headroom to stretch out in. Continued rain protection but less claustrophobia. (I have an IKEA folding foam mattress that can be moved out of the way or used as a couch). I think the SIPs will be strong enough to handle that and I will be padding the floor with something like puzzle-piece gym mats.

My primary purpose for the trailer is to get off the beaten path so many of the trips will be on forest service roads like yourself. The SIPs at least will be provide rigidity for such applications (whether cheating like me with prefabbed ones, or homemade). I will also be building the cabin directly on the trailer frame w/ no overhangs. Thanks again KC for your wealth of knowledge. It has been greatly helpful in clearing things up. Time to get moving and hopefully have the thing rolling this spring.
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Re: Camper made from SIPs - advice?

Postby KCStudly » Fri Jan 09, 2015 7:39 am

Do you have a link to the ikea mattress? I would be interested in that.
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My Build: The Poet Creek Express Hybrid Foamie

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Engineering the TLAR way - "That Looks About Right"
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Re: Camper made from SIPs - advice?

Postby dales133 » Fri Jan 09, 2015 8:24 am

I've gone a similar route on the floor but 12mm on top with a miranti 2x1.5 actual frame 35mm foam core and 3 mm bottom.
I figured the strength of the floor ply was better on top and the underside is just to seal it up.
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Re: Camper made from SIPs - advice?

Postby JacksonCBall » Sun Jan 11, 2015 2:23 pm

KC, here is the mattress: http://www.ikea.com/aa/en/catalog/products/00204587/

It's not great for a 5' wide teardrop because it only measures 4' wide but I got the mattress/frame for $20 from a friend a while ago. It looks like they have other folding couch/bed mattresses that might be wider. I plan to section it and re-sew it so that the upper part will be ~5' and can be turned sideways in the cabin as a couch.

dales133 wrote:I've gone a similar route on the floor but 12mm on top with a miranti 2x1.5 actual frame 35mm foam core and 3 mm bottom.
I figured the strength of the floor ply was better on top and the underside is just to seal it up.


Good idea. Thanks.
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