Plywood camper fabrication questions (thinking of idaSho)

[rruff]

I think you will find that they don't list prices because... if you have to ask, you can't afford it... ? The lightest one will be plenty strong, especially with wood in the core.

As an example the lightest one is .031 W/m-K... And we have 3.28 ft/m and 1.8F/K, so .031/3.28/1.8 is... .0053 W/ft-F. Divide that by the thickness, and multiply by the surface area and delta T, and you'll get heat flow (or power) in watts.

Or 1W=3.415 BTU/hr, so .0181 BTU/hr-ft-F... * 12in/ft = .217 BTU-in/hr-ft^2-F. Or inverted is 4.6 hr-ft^2-F/BTU-in ... which is R value per inch... I think.

Or in other words, 1/[k (in W/m-K) * 7] = R value per inch.

 

[ReluctantTraveler]

I think you will find that they don't list prices because... if you have to ask, you can't afford it... ? The lightest one will be plenty strong, especially with wood in the core.

As an example the lightest one is .031 W/m-K... And we have 3.28 ft/m and 1.8F/K, so .031/3.28/1.8 is... .0053 W/ft-F. Divide that by the thickness, and multiply by the surface area and delta T, and you'll get heat flow (or power) in watts.

Or 1W=3.415 BTU/hr, so .0181 BTU/hr-ft-F... * 12in/ft = .217 BTU-in/hr-ft^2-F. Or inverted is 4.6 hr-ft^2-F/BTU-in ... which is R value per inch... I think.

Or in other words, 1/[k (in W/m-K) * 7] = R value per inch.

thanks! That math just broke my brain a little bit, but I downloaded the spec sheet from the company that you went with, and all of the values for the least dense panel on mine are pretty comparable.

I’ll report back here once I get pricing information. It took two weeks of nearly daily calling and emails to even get the spec sheet, so they can’t want business that bad.

 

[ReluctantTraveler]

I guess the good news is that I don’t have the money to actually start my build yet, so it’s not a big deal if it takes a little bit of time to get information.

 

[rruff]

thanks! That math just broke my brain a little bit.

It was nauseating for me too, but I couldn't find a direct conversion and I don't use R value... ever. The number came to what I expected, so I hope it's right!

I hate British units, especially for energy. Even the British don't use them anymore!

 

[ReluctantTraveler]

It was nauseating for me too, but I couldn't find a direct conversion and I don't use R value... ever. The number came to what I expected, so I hope it's right!

I hate British units, especially for energy. Even the British don't use them anymore!

What was the number, if you don't mind me asking? I literally cannot figure it out.

 

[rruff]

4.6 per inch.

 

[ReluctantTraveler]

4.6 per inch.

Excellent! So 2” walls would be about R9, not including the plywood and glass.

Thanks!

 

[rruff]

Don't forget the wood in the core...

 

[ReluctantTraveler]

Don't forget the wood in the core...

I assume you're referring to the about 16"-on-center verticals I'm planning to add for mounting things like cabinets?

 

[rruff]

Yes.

 

[ReluctantTraveler]

Yes.

Alright, just heard back from the vendor on pricing. Got quotes 13k just for the foam for my build!

That's $8.55/sq. ft. for 2" thick, and $15.80/sq. ft. for 3" thick, which is apparently a "custom thickness" for this vendor, and thus, nearly double the price despite only being 50% more material.

The vendor you used, even with their now increased pricing, is substantially less expensive.

 

[rruff]

$13k for foam would be >1,000 sq ft...? Are you sure that's right? Seems like too much. $8.55/sq ft isn't that bad.

 

[IdaSHO]

Sorry must have missed it, what type of foam sheet are you referring to? XPS? or something different?

My camper is built using 1-1/2" XPS foam. Easy to find, great insulation properties, and inexpensive.
And in my opinion, for the small volume of space of a camper, anything beyond 2" is massive overkill and a waste in both money an weight.
Our camper with just 1-1/2 foam easily maintains whatever temp you could want in sub zero temps with a tiny 6kBTU furnace

Typically in temps below freezing and below zero we track a bunch of snow into the camper, so moisture is an issue, that requires air exchange to minimize condensation.
Even in those temp, with the windows open to provide air exchange, we still don't have problems heating it to our normal cabin temp (between 65 and 86degrees F)

 

[ReluctantTraveler]

$13k for foam would be >1,000 sq ft...? Are you sure that's right? Seems like too much. $8.55/sq ft isn't that bad.

I need about 750 sq feet (300ish of 2" and 450ish of 3")

 

[ReluctantTraveler]

Sorry must have missed it, what type of foam sheet are you referring to? XPS? or something different?

My camper is built using 1-1/2" XPS foam. Easy to find, great insulation properties, and inexpensive.
And in my opinion, for the small volume of space of a camper, anything beyond 2" is massive overkill and a waste in both money an weight.
Our camper with just 1-1/2 foam easily maintains whatever temp you could want in sub zero temps with a tiny 6kBTU furnace

Typically in temps below freezing and below zero we track a bunch of snow into the camper, so moisture is an issue, that requires air exchange to minimize condensation.
Even in those temp, with the windows open to provide air exchange, we still don't have problems heating it to our normal cabin temp (between 65 and 86degrees F)

Kenny! Hope you've been enjoying your winter camping, friend!

So, my concern is less insulative value and more structural strength.

My camper design is substantially bigger than yours because we've got a family of 4 plus a dog. I also plan to periodically have it off the flatbed just on jacks while occupied because we do more campground camping than you, and want to travel off campgrounn without breaking camp.

I've got a 13' box on an 11' flatbed, PLUS a king-sized cabover. 8' tall walls in the main living space to accommodate an insulated basement for water, batteries, etc.

Between the large span of the roof and substantial cabover, I really want to make sure this thing is rigid as hell when its not on the flatbed.

I had originally looked at the cheap green/pink stuff you'd buy at Home Depot (Owens Corning XPS or whatever), but it doesn't seem to be structural. As a result, I pivoted into the structural foam core you'd find in marine applications.

My cousin in an architect (houses, not RVs), and was concerned about structural integrity of box store XPS without steel or aluminum frame.

Quick aside: why did you choose to frame and then foam over building panels and then combining them?