Insulation values of core construction and interior vs exterior temps

[chris_the_wrench]

I have been planning on building my own hardsided popup truck camper for over two years, and I have the sketched out the floorplan,along with most of my components spec'ed. Watching several of the recent camper builds such as Pods8 and Homeskillet's has really peaked my motivation.

I have long planned on using foam sandwich construction, and have recently been thinking very hard about thermal insulation values of such cores. This past winter I made a trip to the Yukon and Alaska in my Lance camper, and while there we experienced camping in -33f temps. Going into this trip we knew our current camper was not designed for these temps, so I insulated every nook and window I could. We managed to keep the cabin at slightly above freezing for most of our trip by relying on the factory propane heater and a small electric heater where we had plug ins. However the coldest nights eventually overpowered our propane heater and our fresh water, grey water and black tanks eventually froze solid(not too fun) but fortunately no real messes, except a cracked hotwater heater, and we worked around these issues.

Now, back to the current question as we continue to finalize our next camper settup and determine wall thickness. We want to be able to travel into those types of temps, and not have to worry, as much, about our new rig freezing.

The big question:
Can someone explain to me(and how to calculate) in layman's term how much insulation(r-value) is necessary to keep the interior volume of a camper(without worrying about leaks through windows, seams, and etc) at a certain temperature while the outside temp is a specified temp and a heater that is capable of producing 2700-10500 btu's(planning on using a Wallas diesel marine heater)?

Basically I 'm trying to determine if 2" thick walls with a core r-value of 10 will be sufficient to keep up with -40 temps outside, or if I need to bump them up to 2.5" or even 3".

Thank you for any input, and hopefully my rambling makes some sense.

-Chris

 

[thebrassnuckles]

tag

 

[dwh]

There are lots of places on the net to find the information about how to calc the losses as regards R-value. Here is one:

http://www.pexuniverse.com/content/calculate-heat-loss


From the real world, Stephen Stewart build a camper box on a Unimog and drove it over much of the world. He built his walls 30mm thick. Here is his page on insulation of the camper:

http://www.xor.org.uk/unimog/mymog/insulation.htm


Somewhere on his site, he mentions that it was not enough and that he would have been better off with 60mm walls, at least around the bed. That is reflected by the last line on his insulation page - in red.

It is also notable that after a trip to Iceland, he added a radiant floor heating circuit to his hydronic system:

http://www.xor.org.uk/unimog/mymog/heating.htm

On this site, westyss built his camper box out of a composite sandwich which has an R-value of R-7 per inch. the raised top is made of 2" material, the bottom of the box is also made of 2" material, with a 1" layer added to bridge the gap between bottom and top. I.e., the bottom walls are actually 3", but the section between the top and bottom is only 1" when the roof is raised.

He says it's nice and toasty in the camper in the cold. His build thread is here:

http://www.expeditionportal.com/forum/threads/48351-out-with-the-old-in-with-the-new

 

[FusoFG]

The big question:
Can someone explain to me(and how to calculate) in layman's term how much insulation(r-value) is necessary to keep the interior volume of a camper(without worrying about leaks through windows, seams, and etc) at a certain temperature while the outside temp is a specified temp and a heater that is capable of producing 2700-10500 btu's(planning on using a Wallas diesel marine heater)?

-Chris

BTUs required to replace heat loss = (surface area of walls, ceiling, floor, etc. times the difference between the inside and outside temperature) divided by the R value of the insulation.


You could make a separate calculation for each surface using a different inside temperature / R value and add the results. Thats why houses have more insulation in the ceiling than the walls and ice boxes have more insulation on the bottom.

The more surface area the more heat loss. The greater the temperature difference, the more heat loss. The bigger the R value the less heat loss.

That number will just keep up with heat loss in ideal conditions ( no air leaks, etc). Raise that number to allow for raising the room temperature from a cold starting temperature, door opening, duty cycle of the furnace, etc. I doubled my number.


You might want to attach a flexible skirt around the bottom of the camper when parked in very cold temperatures to prevent wind blowing underneath and increasing heat loss.


You might want to use a hot water furnace like an Espar or Webasco instead of a hot air furnace. Then you can plumb it into your engines cooling system and use engine heat to keep the camper warm while you drive. And if done right, you can preheat the engine for easier cold weather starts.

 

[charlieaarons]

Propane heaters in typical N. american pickup campers are usually at least 30000 btu/hr, sometimes 60000 (5-10kw). The problem is not heat output, it is poor insulation. Typical pickup campers have about R 2.5 walls (or worse) and single pane windows.
Expedition campers with typical (for them) 60mm foam walls (actually 55mm foam, the rest is the fiberglass cladding) are about R13, and they have double pane windows. So they do fine with 5kw hydronic heaters.
My camper warms up from -20F. to +70F. in 8-12 hrs with its' 5kw Webasto. And it stays warm.
Your 10500btu/hr Wallas is IMHO insufficient to warm an average camper at subzero temps even if insulated like a Unicat. That is only 3kw, it will be running full blast all the time at -20F IF you have R13 walls and double windows. OTOH with single windows and R2.5 walls it will be a "drop in the bucket".
Unicat's R13 is in metric 0.44w/meter squared-deg K-hr. Double windows are about 3w/m.sq-degK-hr. Likewise, R2 walls are about 3w/.....
You multiply heat loss factors for each type of surface by surface areas to add up to the entire camper; then multiply by temp gradient to get wattage requirement. Multiply by at least 2-3 for overcapacity to bring temp up, avoid running at 100%, air leaks etc.


Charlie

 

[Bogo]

Good info on the heat loss calculations. http://en.wikipedia.org/wiki/R-value_(insulation)

There are foam insulations that have higher R values than 5, but they aren't as radially available. As of a few years ago I'd seen R-7.4 per inch for one spray in foam. This article mentions R-8 on a foam board, no idea of manufacturer. Foam board insulation overview.

If you have the $$$,$$$ you could consider using Vacuum Insulated Panels but they are expensive. R25 to R50 per inch can be done with them. Great for refrigerators and freezers in small spaces, but to expensive for anything bigger. Prices were around $800 to do a 2 cubic foot cooler with R-38 panels.

Peninsular Glass's Motion Windows division has double pane RV windows. http://www.motionwindows.com/ Look at the 1800 series. Wall thicknesses from 3/4" to 3 1/4". Note, walls less than 2" thick will have the frame stick in more than the interior clamp ring does. Unfortunately they are aluminum framed so they will conduct heat via the frame. Their double pane sealing doesn't last forever, but then no double pane RV window manufacturer's does. If it gets condensation in between the panes, just have them rebuilt.

Good structure design will help allot. Do your best to eliminate places where metal goes from the inside to the outside in one path. Spots like that will conduct allot of heat out fast. Also if the interior and exterior skins are AL, then that will magnify that effect because the skins will provide more radiating and collecting area for the heat transfer. If you can't design out a heat conduction path, consider capping it on the inside with a foam padded cover. Something like a truck frame rail that goes from uninsulated to insulated can be covered with spray on foam when inside the insulated area.

If you make your own exterior panels, you can incorporate structures into them for mounting interior walls and cabinetry. Vacuum bagging can be used to provide the pressure when gluing up the panels. The biggest hassle will be having a large enough flat surface for doing the vacuum bagging. Most floors are not flat enough. The second biggest hassle is getting the skins. Pre-finished aluminum and fiberglass skins are available from the RV repair industry. That is likely the most realistic source. Fiberglass, kevlar, and carbon fiber skins are also available, but rarer and more expensive. Each has their own advantages and disadvantages. The biggest problem with them is finding a supplier that will sell in small quantities at a reasonable price.

Another thing is to be careful of allot of plastic parts made for exterior RV use. It is likely they can't handle very low temperatures. Avoid plastic flex hinges as one use a -30 will likely break it off. Most exterior fittings will be available in metal versions for a bit more, but then you have heat conduction issues. It is all trade offs. I use a plastic frame around a storage compartment hatch, but make sure the hinge and latch are all metal.