single vs double wall in frigid conditions

[David Gardner]

Very cool.

The “usual” height of such tents seems to be about 42″ or 1.07 meters.

I was thinking the exhaust vent controlled the flow too.

Dew point? Condensation?

[Roger Caffin]

Interesting stuff.

In a storm I would suggest that the inlet area should be about one tenth of what you have, about 10 squ in. You do not want the storm hurtling into the tent!

Actually, in a bad snow storm you might reduce the inlet area to 1 squ in (or less) and rely on the flapping of the tent fly to pump air in and out of the exit. That works quite well for us. It does make the calculations a bit more difficult though.

Cheers

[David Gardner]

The vents in my design can be variably opened from and closed from about 300 sq. in. to zero. The idea I’m pursuing with Ben (Ben, please verify) is a tent with an airtight double impermeable wall that will create a temperature differential between inside and outside such that even in zero wind conditions the occupant(s) body heat will be enough to power ventilation sufficient to avoid any interior condensation. If we can make that work, then, combined with controllable ventilation to maintain the interior temperature above the dew point, I think any kind of breeze or wind will help drive humid air out even below wind speeds that pump humid air out.

[Roger Caffin]

I will argue against having an impermeable inner tent. You need that inner tent to breathe. We leave a tiny inlet area in bad weather – it has a good ‘hood’ over it to keep rain and snow out, and we pretty much seal up the inner tent against any wind coming in through the inlet. That still gives us air circulation so we don’t suffocate, plus it lets the moisture we breathe out go through the inner tent to make frost on the inside of the fly.

In this case I think the inlet was closed – completely. There was a bad storm outside.

Often have to give the whole tent a good shake when pulling it down, but that usually gets most of the frost to fall off. Not all of it, I grant you. The shaking can be a bit perfunctory in a storm – well, tough.

Well, OK, sometimes it is cold enough that frost also forms on the inner tent. Such is life. Just don’t brush you head against the frost and you won’t get it down the back of your neck.

Cheers

[David Gardner]

I am not advocating or arguing for two impermeable walls in a double wall tent. I agree that with existing designs and materials a permeable/impermeable double wall tent is the way to go in frigid conditions.

But that does not have to be the end of the story. Ben and I are exploring the possibilities. Could it be better? Can condensation be eliminated entirely, instead of merely relocated from inner to outer wall, using only the body heat of the tent occupant(s)?

The calculations Ben is doing are to see whether that is at least theoretically possible. If it is, then I would be very interested in prototyping it and testing it in real life to see if it actually works. The inner tent only needs to breathe if you assume there is no other way to illuminate interior condensation.

If it works, another benefit inherent in the design is that it keeps the interior measurably warmer than it can be with traditional tent designs with no weight, cost or fuel penalty. There might even be a weight and cost savings if the interior can be kept warm enough to reduce the amount of insulation required for sleeping bags or a quilt.

And the colder it is outside the better the design works, because of the greater differential between the same amount of interior heat generated by the same number of occupants vs. the colder outside temperatures.

I don’t think it’s impossible. Given the potential upside it’s worth exploring. Anyone have any ideas that might make such a design work, or work even better?

[Roger Caffin]

Obviously I have some reservations, but equally I have never tried it myself. So – I would love to hear and see the results.

One thing I will mention is that you will need a taut pitch for both the fly and the inner tent, otherwise the flapping can leave the inner tent stuck to the fly, by either condensation or frost. This is from experience with earlier models.

Just make sure you don’t seal the inner tent too well: you would not want a build-up of either CO2 or CO. And I suggest you will need a vestibule space inside the fly for gear and cooking in bad weather.

Cheers

[Ben H.]

Hey all,

Trying to catch up on these posts. I’ve been plugging away at the analysis but it is sufficiently complex at this stage that I spend a considerable amount of time troubleshooting errors I introduced while adding more needed complexity. I am using an engineering tool (EES by f-chart software) and it was getting hung up by relative humidity over 100%. Instead of relying on built-in subroutines, I’ve had to program them myself to make them more robust. It takes a little time but it will help me in the future when I try to develop a simpler tool anyone on here could use (excel spreadsheet).

The idea I’m pursuing with Ben (Ben, please verify) is a tent with an airtight double impermeable wall that will create a temperature differential between inside and outside such that even in zero wind conditions the occupant(s) body heat will be enough to power ventilation sufficient to avoid any interior condensation.

Yes, I am trying to develop a relatively simple analysis tool that will help you explore this space. Currently, I am having a hard time getting condensation to occur in the model. That sounds like a good real-world problem to have but it also indicates the model is not conservative enough because we know condensation occurs in tents. I think the issue is on radiation modeling (I haven’t added any). In the current model, if you shrink the double wall thickness down to zero (a single wall tent), the tent wall temperature would be the outside ambient temperature. The heat-induced flow and temperature rise in the tent is enough to reduce the relative humidity even though moisture is being added. I think in reality radiative heat loss to space is enough to cool a single wall tent down below the dew point and cause condensation. In other words, the single wall temperature could be less than the air outside ambient temperature.  I’m still exploring the analysis.

[Roger Caffin]

Hi Ben

Yes, radiative heat loss is more significant than you think. Treat the night sky as being at -70 C. As a first approximation, the tent wall will halve the temp differential, but that is in dead still air. As soon as you have any air movement there will be conductive heat loss to the air flowing past.

Also read up on katabatic air flow down valleys. The effect can be very significant – as we know from a few valleys we have camped in!

Ir is NOT a simple problem.

Cheers

[Paul McLaughlin]

It seems unlikely to me that you will get to where you want to be, given that the human occupants are generating humidity as well as heat, and that airflow to remove the humidity involves heat loss. Factor in radiative heat loss, and convective heat loss via exterior airflow over the tent fabric, and it seems like there just isn’t enough heat generated to go around. That isn’t to say that well designed ventilation isn’t going to help – just that the fabric temperature is not going to be that big of a factor, other than in a narrow band where you are close to the dewpoint and thus a small change in fabric temperature can have a big effect. More productive, probably, is controlling the moisture input in the first place via VBL – though there you can easily get into tremendous fiddle factor maintaining temperature in that very narrow band where you don’t sweat inside your VBL but you are warm enough.

[Roger Caffin]

It seems unlikely to me that you will get to where you want to be
Exactly.
The rest of the wishes are dreaming.

Cheers

[David Gardner]

Yeah yeah yeah.

And the Earth is the center of the universe, ships made of steel can’t float, things heavier than air can’t fly, a human can’t run a mile in less than four minutes, a plane will fly apart if it goes faster than sound, and so on.

You guys are thinking inside the box. 50 years ago who would have believed a person could safely and comfortably go out in the wilderness for a week with only 8 lbs of gear?

Use your imagination. I can see an almost rigid “frameless” tent with air filled bladders for structure, internal baffles/cells to limit air movement so the tent walls act like insulation combined with internal radiant barrier and maybe even some down like a DAM to achieve a 10°C differential between outside and inside temperature, and a simple self-adjusting system that accounts for temperature and humidity.

I am sure there are many technical barriers before that can be achieved. Maybe it can be achieved and maybe it cannot. But one thing is 100% certain: it can never be achieved if it is never even attempted.

[Roger Caffin]

almost rigid “frameless” tent with air filled bladders for structure, internal baffles/cells to limit air movement so the tent walls act like insulation combined with internal radiant barrier and maybe even some down like a DAM
You know, it sticks in my mind that someone has already attempted that.
Was it Nemo who tried air-filled bladders for tent poles? I don’t remember the marketing success there.

Cheers

[David Gardner]

Pre-DCF, among other things.

Marketing schmarketing.

I have no doubt at all that a 3 lb 2 person alpine tent that achieves the criteria I set would  sell like crazy at $2,000.

[David Gardner]

As a thought experiment, the Locus Jhedi tent weighs 1000 g, Of which 650 g is fabric and 350 g is the poles. Eliminate the poles and double the fabric you get 1300 g. Add 100 g for air bladders to 1400 g/49.3 oz.

Better yet, develop a self-healing waterproof and airtight coating for the fabric (like the fuel tanks on fighter jets so that if they are penetrated by bullets they don’t leak) and eliminate the air bladders entirely.

[Roger Caffin]

All true I imagine.
But the air poles didn’t work in bad weather. Not so good.

Cheers

[David Gardner]

Agreed, not good.

Then it wasn’t designed properly. US military has developed and deployed air supported structures for years. And I don’t mean the kind with the fan that continually fills the interior to create positive pressure and keep it inflated.

A proper working design would withstand winds at least as well as a tent with poles. One advantage to an “air pole“ structure is that the shape is not limited by what poles can do and it can be made much more aerodynamic to start with.

[Roger Caffin]

develop a self-healing waterproof and airtight coating for the fabric (like the fuel tanks on fighter jets so that if they are penetrated by bullets they don’t leak) and eliminate the air bladders entirely.
What a nice idea.
But would the result weigh as much as a jet and cost as much as a jet?
Trade-offs . . . .

Technically: you can do that with a fuel tank because it has a large volume and a rigid shell: you can poke a spray head inside and wave it around. But you can’t do that with either air mats or air poles. A long-standing problem, especially for air mats.

Cheers

[David Gardner]

You’re right Roger! It’s impossible to do, and always will be. And even if could be done it would cost more than a jet. What was I thinking?

Let me sleep on it.

On a 6 oz mat under a 14 oz quilt in a 5 oz pocket tarp after boiling 16 oz of water for dinner with a 2 oz cook kit using 7 g fuel.

Impossible. Right.

[Roger Caffin]

In all seriousness, I would not say it is impossible. Just that we haven’t figured out how yet. One of the problems with the air pole concept is strength.

* The strength of the air pole against collapse is a very small fraction of the strength of a (say) carbon fibre pole. That is for ‘reasonable’ pressure. The Nemo ones buckled easily.

* The strength of the air pole tube against ‘explosion’ is limited by the fabric, the sewing and the coating technologies – and the weight of the result. You would need very high pressure.

* The weight of the pump needed to blow the tube up to an adequate self-supporting pressure (ignoring explosions for now) is in the several kilogram range. You would need more than a push-bike pump to get there.

OK, all of this is for a mountain tent able to handle a storm. If you want a beach tent: easy. And yes, I have been in bad weather where I had to crawl.

Cheers

[David Gardner]

In all seriousness, I would not say it is impossible. Just that we haven’t figured out how yet.

On that we definitely agree. And that the strength of the supporting structure is a crucial issue. Some things to be explored:

• If DCF is taped into tubes to function as air bladders, which are further constrained in DCF tube-like structures within the canopy’s double walls, how much pressure can different diameter tubes of different DCF weights take? Properly designed and optimized I bet you can go to 200 psi/1379 kpa or higher.
• Better yet, make “spiral wrap” tubes using Dyneema threads (so it remains flexible) the way CF golf shafts are wrapped (before being made rigid with epoxy resin) to really optimize the strength to weight ratio.
• Is a “tube” even the ideal shape? What about an array of internal pentagonal and hexagonal cells reminiscent of geodesic dome? How about “dimples” like some air mats, which would have an added aerodynamic advantage like the surface of a golf ball?
• How about combining the two so that the center of each pentagon and hexagon (or maybe at the corners where the shapes meet) is a point of adhesion between the inner and outer walls?
• I have 200 psi pump for my mountain bike’s air shock, not in anyway optimized for weight savings and including a large dial gauge, that weighs only 6 ounces. Trade the gauge for a tiny peak pressure blow off valve and make the tubes from CF and that can be cut in half at least. Since DCF essentially does not stretch you could use a schnozzle-type bag for 99% of filling the structure, and only use the pump for the last bit to get the pressure up to where you want it.

All this for a tent that may be lighter, stronger, easier to erect, more aerodynamic, eliminates condensation AND keeps you warm enough to reduce the amount of insulation in your sleep system.

We’re not that close yet but it seems like a great goal to work towards. To paraphrase what JFK said about the US endeavor to reach the moon, we choose to do it not because it is easy, but because it is hard.

And who knows what other amazing things we’ll discover or develop along the way?

[Roger Caffin]

I wonder what such wrapped fabric tubes would weigh IF you made them strong enough?
The three carbon fibre poles for my blue summer (single skin) tent weigh 297 g.
The four carbon fibre poles for my red winter tent (double skin) weigh 370 g.
There’s your challenge.

A second challenge for the concept: When you accidentally get a gorse spike into the fabric of a pole, how do you repair it? I doubt very much an external patch would hold for more than a few hours. Vulcanised patches on inner tubes rely to a large degree on the outer tire to hold them in place.

A spiral wrap – good idea. That is how the best arrow shafts and tent poles are made. But you would need a curved mandrel (matching the tent) for this, and getting the mandrel out afterwards is a real problem. We haven’t solved this for CF tent poles yet.

Challenges!
Cheers
Roger

[Paul S]

We do a fair bit of winter backpacking/snowshoeing trips. We like a tent with a mostly fabric (i.e., not all-mesh) inner. It is warmer by anywhere from 4-10 degrees even when ventilated enough to keep condensation mostly at bay. True, a warmer sleeping bag with a single wall tent would be more efficient weight-wise in terms of warmth while sleeping. But what about when you’re not sleeping, just hanging out in the tent? Then, the solid inner means less breeze blowing through, so it’s warmer, less spin drift. A more comfortable place to be all in all. Altogether nice  from our (my wife and I) point of view.

[Roger Caffin]

Then, the solid inner means less breeze blowing through, so it’s warmer, less spin drift. A more comfortable place to be all in all. Altogether nice from our (my wife and I) point of view.
Keeping wife comfortable is very important.

Hot soup I think.

Cheers

 

[Eric Blumensaadt]

Well folks, I’m going to try my TT Notch LI (Dyneema) solo tent this winter. It has the”solid” inner tent and generous upper mesh for ventilation. THAT is the question, ventilation: will it ventilate as well as my similar TT Moment DW? Likely since it has similar lower an upper vents.

One thing for sure it will need every guy line and the use of all 4 of the stake loops I have sewn on the fly hem using Dyneema tape and Tenacious tape reinforcements. Given that I think I’ll be fine. I can use my avy shovel to clear away the walls of snow if it snows in feet instead of inches. Dyneema is very strong so no worries of it tearing or sagging under snow weight. I have TT pole handle adaptors B/C I use my poles “handles up”. Maybe I’ll have to cut stick poles so I can use my hiking/ski poles during the day. Hmmm…

BTW, What Roger Caffin said, all of it

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