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I have a cap over the back of my truck and spend many nights a year camping and sleeping in the back rather than setting up a tent. I really like sleeping in it, saves the tent setup effort and gives my truck an aspect of multi-purposeness and self-sufficiency. The cap is fiberglass and the bed has a plastic liner over it.

The only problem is that it's not at all thermally insulated (save the sleeping bag you're in) and my favorite camping is in the mid Appalachian mountain area where the temps at night go freezing as early as now and often get down even much lower. I have spent a night in around-zero weather (30ish give or take) in my 15 deg bag and a fleece bunny suit and I didn't enjoy it too much and I'm not even that cold sensitive.

So I'd like to go and camp a few more times this year until maybe early December. Is there a portable (so it's not permanent) solution to add some thermal insulation to the inside walls of my truck cap to make it sustain down to about 20 deg outside weather while keeping it around 40 inside. Since the space is not all that large (6' bed on a Tacoma) and the roof of the cap is regular, I imagine my body could warm up the area as long as this prospective insulating solution was in place.

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Is the bed metal? What is the cap made of? Metal is a fairly good heat conductor, and as such a pretty bad insulator. Heat proofing a truck bed would probably require lining up both the bed and the cap with some heat insulating material. Wood boards would work well, but you could give foam (yoga style) mats a try as well. – Nisan.H Oct 21 '13 at 14:25
just posted an update: "The cap is fiberglass and the bed has a plastic liner over it." – amphibient Oct 21 '13 at 14:27
offtopic, but I used to have a favourite rock that I would keep in the campfire and put in the bed of my truck when it was time to sleep, gives hours of extra warmth. – furtive Oct 28 '13 at 23:08

I'd probably go with a foam roll mat under you, as your biggest heat loss well be conduction through the metal bed.

As the space isn't that great, the cap shouldn't need insulation, but this will depend on your sleeping bag.

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the bed has a plastic liner -- no direct contact with metal – amphibient Oct 21 '13 at 21:20
@amphibient: a simple plastic liner is still rather poor insulation. – Michael Borgwardt Oct 22 '13 at 11:43

You might want to look at Reflectix, an thin, well-insulating material used for car windshield shades, among other things. You can get it at any hardware store.

My only concern would be that it's not breathable, but I suppose neither is your cap.

For the floor, conventional camping sleeping pads are probably the best bet.

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