This may just be an exercise, or it may get put into practice.

The situation: I'm building an ICF house in upstate SC while living in the barn. The barn is big, drafty, and uninsulated. With winter coming, I'm thinking about building a very small 'sleeping room' that I can heat with a small heater. If I use a corner of the barn, two walls would have R-19 insulation. The two remaining walls I would frame and staple up insulation. I would insulate the ceiling, and line the entire room with foil-lined foam panels, and tape the seams. Looks are not important!

The question is this: If I were to build the room 6 X 8 feet, and 7' high, for a volume of 336 cubic feet, what would it take to maintain say a 30 degree temperature difference, given roughly R-19 walls and ceiling? I have limited power, so I could only run a single 1500 watt heater, which are usually rated at 5120 BTU. This whole arrangement would be strictly temporary, being used for about 3 months this winter. Normal night time lows in our area are about 30 degrees in January, although it can drop into the single digits on occasion. The floor is an uninsulated concrete slab, and I might even throw foam panels on the floor and put plywood on top. I would use a generic exterior door rated at R-3, which I could use on the house when I was done with it.

Right now, I'm living in a pop-up camper, set up inside the barn. Last winter, the electric heater barely helped, raising the camper maybe 10 degrees above the barn temperature. Of course, the camper is bigger than my proposed sleeping room, and has zero insulation. Thoughts?

Makes sense to me if you insulate the floor and make sure the whole thing is well air sealed.

I'd take the pop-up inside a barn over a firetrap "sleeping room" any day.

30F? 5000 BTU of heat in a pop-up? That's verging on luxury.

The Green Solution is to upgrade your sleeping bag.

The problem isn't so much sleeping- it's after you get out of bed! Last fall, I actually slept with a ski cap on my head some nights.

Posted By jdebree on 29 Oct 2012 05:10 PM
The problem isn't so much sleeping- it's after you get out of bed! Last fall, I actually slept with a ski cap on my head some nights.

Now I'm sure thats good motivation to get the house done!!! We routinely use sheds that size to protect water system and have no problem with 1500 watts. Ususally only an infra red heatbulb.

Just guessing, the size you are proosing, a 1500 watt heater will run you out. The taping/airsealing will do wonders.

I would probably drape a sheet of plastic over the camper. If that isn't enough put some batt insulation on top as spacers and put on another sheet of plastic.

The volume of the room has no bearing on the heat load, nor does the square footage of floor. It's all about exterior surface area.

In a barn the roof and two exterior walls will have a bigger heat loss than the walls & floor that face the somewhat-warmer barn. Windows & doors make a difference too.

You can estimate this using U-factors based on the whole-wall R values and surface area. For now let's assume the barn door is open and the barn stays at the outdoor temp.

A typical 2x6 framing with R19 cavity fill comes in at about R13 after framing. If your adding an inch of foil-faced iso, add another R6+, call it R20. The U-factor for R20 is 1/R, which is 0.05 BTU per hour per square foot per degree F.

A 6' x 8' x 7' room has about 200 square feet of wall area, but let's assume you have one 10 square foot window, and one 20 square foot door call it 180 square feet.

The wall losses with a 30F delta would then be 190' x U0.05 x 30 = 270 BTU/hr

With R19 and 1" foam on the ceiling it's the same U-factor but only 48 square feet so your ceiling losses are 48' x U0.05 x 30F= 72 BTU/hr

A cheap double-pane or a single-pane + storm window has a U-factor of about 0.5, so your window loss will be about 10' x U0.5 x 30F= 150BTU/hr

The R3 door has a U-factor of 1/R3= 0.33, so for 20' of door the losses are 20' x U0.33 x 30F= 198BTU/hr.

So everything not including the floor you're at (270+ 72 + 150 + 198=) 690 BTU/hr.

With an inch of XPS on the slab you can pretty much ignore the loss out of 48' of floor, but for yuks lets give it a fudge-factor of 1 BTU/foot, bringing your total up to about 750 BTU/hr.

A sleeping adult human puts out about 250BTU/hr, bringing the heat load down to ~500 BTU/hr, which is about 150 watts, a full order of magnitude less than the 1500W space heater's capacity. If if you DOUBLED the heat load to account for ventilation factors, you'd still be just fine with a 300W heater.

That could be supplied by IR lightbulbs if you like, but put it under thermostatic control so you don't roast yourself.

If you skip the wall & ceiling foam but keep the floor-foam the load rises to about 1000BTU/hr (not counting the sleeping human), which is still only about 300W. As long as you have a pretty good air-barrier like sheets of housewrap stapled to the studs on both sides of the assembly, crummy R19 batts and no-foam would do just fine for this temporary "way better than just camping" sleeping quarters room.

Short of actually conditioning the space to particular air temp, an electric blanket does wonders. In Japan 6'x 6' electric-blanket technology rugs are commonly used for lying-on-floor-watching TV comfort too, which are fine even at room temps well below what 'mericans call comfortable.

Sounds like it would work- too well, in fact! There will only be the one door- no windows. I do have a smoke alarm, so I'm not too worried about fire danger. Since the walls will just be studs with foam board and insulation, a panicked man could exit just about anywhere on the interior walls, or even the ceiling.

I've thought about trying to insulate the camper, either inside or out, but there are a lot of built-ins in the way, and the floor is terribly cold, and would be tedious to insulate. I could put foam on the inside floor, but that does nothing for all the built-ins. I think I can slap together a rectangular room faster, and re-use more of the materials. I wish I could do it in the basement of my ICF home shell, but then I'd have to take it apart and hide it when the inspector was called out. Once the house is fully sealed and weatherproof, the basement should stay above 55 in the winter, but by the time I get that far, it will be springtime!

The major heat loss is up so insulating the camper may be as easy as putting a couple foam boards over the top and covering them with a tarp. On winter camping trips we shovel snow over a small backpacking tent. It is surprisingly effective. A fleece beanie is more comfortable than a stocking cap. Online stores like rei or campmor sell goose down booties that can trump any cold floor. And, hey, you'll have stories. Last night, with Sandy howling around the house, the missus asked me 'remember how Irene rocked the camper?'Onward and upward eh?

Perhaps you should continue the small sleeping room concept into your main house. Depends on how quickly the remainder of the house can cool down/heat up.

Posted By toddm on 30 Oct 2012 06:36 PM
The major heat loss is up so insulating the camper may be as easy as putting a couple foam boards over the top and covering them with a tarp. On winter camping trips we shovel snow over a small backpacking tent. It is surprisingly effective. A fleece beanie is more comfortable than a stocking cap. Online stores like rei or campmor sell goose down booties that can trump any cold floor. And, hey, you'll have stories. Last night, with Sandy howling around the house, the missus asked me 'remember how Irene rocked the camper?'Onward and upward eh?

In a typical camper the major heat loss isn't up, it's out, since there is typically something on the order 4x more wall area than roof area.

But if you buried the camper in snow the sides would be as insulated as the top, eh?

Gotta be there to truly appreciate the phenomenon, but even with an electric heater at floor level the temp gradient between floor and ceiling can be 20 degrees. So, yes, a higher delta t means more heat loss up than down. That also explains why you want down booties. In a camper van, you don't want to get too close to the glass either. Dunno if foam boards would slow heat loss enough but it's worth five minutes to find out. My van was tolerable through two winters in Pa with a half inch of polyiso under the floor and perhaps a bit more in the high top. The first winter, without electricity, I had a 5kbtu catalytic heater that I turned off when I turned in.

The ceiling heat loss of an uninsulated camper is more than loss through the floor, but nowhere near the heat loss out the sides.

The delta T of the walls would follow the same gradient so, if a cap isn't sufficient, buy some duct tape and hang boards down the walls by four feet or so. Another $3 and maybe 20 minutes.

Actually, I got to thinking about those booties and I'm in for the tiny room, too. Could you could throw down some xps with hardie backer over it and put some kind of radiant heat in the middle? I'd want camper's amenities, though, such as they are. The life is tough enough without retreating at night to a cell.

With insulation on both, 3F is a more typical floor/ceiling differential. So insulating a floor (that has outside airflow beneath it) is almost as important as the ceiling.

Here is an idea: Build an igloo around the camper with square bales of hay or straw. To reduce the weight on top, cover the top with fiberglass batt insulation or several layers of rigid foam board.

Posted By arkie6 on 01 Nov 2012 03:36 PM
Here is an idea: Build an igloo around the camper with square bales of hay or straw. To reduce the weight on top, cover the top with fiberglass batt insulation or several layers of rigid foam board.

That actually seems cheaper that building a room! Great idea too!

I am thinking now about trying to insulate the camper. Last fall, I closed off the wings on each end with rigid foam, and it did help. The roof is rather irregular, but I could lay batts across it, and then either drape batts down the sides, or put up some rigid foam. The floor is a problem, as are things like wheel wells, the door, etc., but if I can slow the heat loss from 75% of the camper, maybe the heater can keep up.

Yesterday morning, we hit 31 degrees outside, but the camper was 64. Part of that is solar gain- the barn faces southwest, and on sunny days I open up the big doors to let the sun heat up the concrete floor. That doesn't work so well on cloudy, drizzly days, though.

I lke Arkie's idea too. Even if the bales only came up to the floor, you'd have a shot at keeping your feet warm. Catalytic heaters are clean and safe from a CO standpoint and super efficient. They consume oxygen though. Most people crack a window but I won't go to sleep with it operating.

Hang in there. It gets better. I'm about six months away from actual retirement.