How come tents don't come with radiation reflective surfaces on the inside of the tent?

For example:
http://cdn3.volusion.com/moqgn.ooprv/v/vspfiles/photos/117-3.jpg

On a regular (non-emergency) tent, the inside of the rainfly could have a reflective surface, and also the flooring.

My guess is that you would be in your sleeping bag, there is nothing to radiate. all your heat will be in the insulation

Thermodynamics would disagree with you. Radiation is always happening, as long as you aren't at absolute zero Kelvin, which I think is impossible. The insulation will also be emitting radiation. More so, as it warms up from you being in it. I think you end up emitting more radiation the warmer you are, so you'd emit more radiation if you were in a warmer bag. The warmer bag would start to warm up, and at steady state, you and the bag will emit radiation at a higher level, than your cold skin without the bag. If you die of cold, you will still be emitting radiation, but you will also be absorbing radiation, so the net loss to radiation will be zero, assuming the ambient temperature is remaining the same. Anyway, I hope I got all this right.

The heat transfer will decrease due to the thermal resistance of the sleeping bag.

If you have surfaces that reflect radiation back to you, then that helps with the net loss due to radiation. I guess you can say the same thing of conduction, on a molecular scale?

Bill Stephenson, what's the best material you've found for the super shelters you've been working on through the years? What would you use that's durable as a ground cloth? I've been looking at Stansport, that has a polyester backing, and Grabber, that has a plastic backing. Would you go lighter than that? I'll be back to search more extensively.

Ok, I'm back...
In your video, you said you use "bubble wrap flooring". I also see you've used "Reflectix" bubble foil insulation. Wouldn't reflectivity on only one side be preferable? Do you use the Reflectix bubble foil as your only sleeping pad?

Have you ever had a situation, where it is cold, and you couldn't start a fire because the wood is too wet (in regard to your ?

Well this is heat transfer... not thermodynamics (but close enough for layman discussions!). Radiation is certainly always happening. The question is whether it is a significant part of the heat transfer away from the person sleeping in the tent. The equation is:

q" = epsilon*sigma*T^4

Epsilon is a number between 0 and 1 and it is what you are changing by using a reflective surface.

Sigma is the Stefan-Boltzman constant and is a really small number (5.67 x10^-8 W/m^2-K^4) and it means that radiation heat transfer is negligible unless temperature differences are really large. Notice the temperature is raised to the 4th power!

Radiation heat transfer travels through line of sight, so the most important thing you need to do is block your line of sight to space (which is really, really cold). That is why a tent wall keeps you quite a bit warmer. Your tent wall will be near atmospheric temperatures (instead of space temperature). Your sleeping bag will be a bit warmer than outside temps but not significantly so there won't be much radiation heat transfer between the two bodies.

Bill's Bakers tent works well with a reflective surface because he uses it with a high temperature heat source (fire). The reflective surfaces give more line of sight paths for radiation heat transfer to occur over.

hope that helps

Yeh, what he said

I once had a Brooks-Range Rocket tent that was made with a reflective inner (well, it was a single wall shelter, so reflective toward the inside). Was a bit smaller than I liked, and it had problems with delamination of the reflective interior, so it was discontinued. Significant condensation on the one winter weekend I used it.

I'm neither an engineer or a physicist, but I do know that for a heat-reflective coating to be effective, the tent would have to be kept tightly closed. Unfortunately, keeping the tent tightly closed greatly increases condensation. I'd rather have a well-ventilated tent, a few more ounces of insulation, and, above all, dry insulation, thank you!

Thanks BZH. Any idea what percentage of heat is lost to thermal radiation, at 35F with the doors open?

If I had a 35F night and a human having slept in a sleeping bag for a few hours, and a pyrometer, I could measure the surface temperature of the sleeping bag. Let's say, for kicks, the surface of the sleeping bag is 40F (277.6K). 2 meters squared of sleeping bag surface area. Epsilon = 1 to get top estimate.

2*(5.67*10^-8)*(277.6^4 - 274.8^4) = 27 Watts

Average male generates about 75 watts sleeping.

JustWalking, I found this review (and a couple others): http://www.larsonweb.com/shelter/id9.html
However, no one really mentioned being that much warmer from the interior reflectivity.

Ha ha... I'm just now getting around to it! (Bill's post number 184509)

Thank you! I have wondered about such things and have always wondered if so called "space blankets" were mostly hype or not. It seems to me than any opaque plastic would work as well.

It seems to me, that if they had followed your suggestion, the astronauts may be subject to a lot of solar radiation within the space stations. Although what percentage of reflectivity an opaque plastic would have, I am not certain. I hope not a black opaque plastic bag, as you will surely burn up as the black absorbs all the radiative heat.

Granted, I'm talking about the reflective surface pointed outwards, in this example. However, it is still pertinent for the inside of the tent, if that's what you want.

Sorry about the slow reply.

I use Reflectix brand bubble foil insulation. They make "Double Layer" and "Single Layer" (of bubbles) and I've used both. I ended up using the single layer because it's folds and rolls into a much smaller parcel and there's not a huge difference in performance that I noticed.

You can get it with either one or both side made of the reflective material and I've used them both as well. I placed it so the reflective side was towards my sleeping bag and I didn't notice any real difference, they both worked well.

I really don't think you can do much better than the bubble foil. It's light, durable, waterproof, and multi-purpose. You can fold it so it's several inches thick and it make a great pad to sit on.

I am a huge fan of the "SuperShelter" design and the small "Baker's Tent" style I used for my tent. I made the Baker's tent because the design was simple and easy but I've grown to love the design for it's comfort and ease of getting in and out of.

And honestly, used with campfire that design is perfect. It's what a Baker's Tent was designed for so even without reflective material they'll still warm up inside. But you can use a lot smaller fire and a lot less fuel and get them a lot warmer when you use the reflective material.

Without a campfire I wouldn't say my tent gets warm with just me in it, but I have sat with two friends in it on cold mornings with no fire and it warms up pretty nice inside then. And it will warm up fast with just me and a stove going inside too.

I would encourage anyone who does cold weather camping to make one and give it a shot. You won't be disappointed. In fact, you'll probably be amazed. I was and still am.

I often buy tent from the Enkeeo store, their tent's quality is good.

That's a pretty nice looking tent in the photo.

OM is right about condensation. I closed mine up pretty tight one night while sleeping on a gravel bar next to a creek (no fire going) and it got really foggy overnight and the ceiling of the tent was dripping wet by morning. Had there been even a slight breeze it would've been raining inside.

I figure it be worth adding some small triangles of bug screen material to the very top of both the front and back and a few small squares of it near the bottom of a tent like that in the photo to provide some ventilation. I'm not sure that would keep it completely dry but it would have to help. You'd certainly lose heat doing that but you'd stay drier and more comfortable.

I've mentioned this before but it's worth mentioning again. You can fashion an emergency blanket into a box (reflective side in) and use it to dry wet clothes with either solar or a fire. A small fire is all that's needed and you can dry wet clothes in about the same amount of time as you can with your clothes drier at home (under an hour).

I learned that by spilling a beer on a fleece jacket inside a shelter I made for testing this material on a snowy day and was surprised at how fast the jacket dried. Fleece dries pretty fast compared to cotton but that jacket was holding almost a full beer so it was pretty wet.

For that purpose alone I believe it's worthwhile to carry one of those. I know you can't have fires everywhere, but you can set a stove inside to dry clothes that way, and while I've never used solar to do that I'm sure it would work faster than just air drying them.

A single layer of bubble foil insulation in or on the underside of a hammock makes a huge difference in warmth and it's waterproof, weighs about the same or less than a down quilt, is multi-purpose, and it costs less than $10.

I carried one of those silver emergency blankets in my pack for years and never even took it out of the package. When finally decided to play with them it took me awhile to understand how they work and how to use them correctly. When I finally understood how to use it I was amazed.

I know I've repeated that a lot, but I've also used them a lot now and that tent of mine has made cold weather camping so much more comfortable it's hard not too want to tell others about it.

Interesting for the emergency blanket in a box dryer; I never would have thought of that. I've heard of one being used to help reflect heat from a fire back onto a person, but never attached that idea to drying out wet clothes.

Please tell me though that the beer spill was accidental and not a planned part of the test. I would have cried if I had to sit and drink a bottle of water waiting for beer to dry out of a jacket instead of the other way around.

Yes, it was accidental, and yes, I cried

At least I learned something so it wasn't a complete loss, but it still saddens me to this day to have paid so dearly for it.