sealed cavity double wall tent

Sounds right. When are you going to do a prototype?
I think the only way to prove out the concept would be to do the doors,vents and body and try it. Then reduce what is doubled.
My thoughts only . . .
jdm

Posts like this are why I love this site! I am a tinkerer by nature, and I absolutely love to see somebody take a good knowledge base and a desire for something different and make something happen! Your tent idea sounds awesome, by the way. I would put a lower intake vent and peak exhaust vent to keep any water vapor moving out instead of camping on your inner- any wind that may manage to enter the lower vents should be fairly negligible, considering the material used for the inner.

The outer is half made and I stopped because I could not decide how to design the inner flexibly. This concept has perhaps made me able to finish the outer. The inner needs more cuben buying.
Yes the inner would have ventilation to match the outer at both ends and to the high vent of the outer. I also think the door to the inner would be mesh with a cuben cover, this would match the door of the outer which could be left open.

Sounds good! Any ideas on when you might be able to get the materials and put one together? I would love to see how it turns out and hear how you like it.

Good luck!

Please keep us updated.

This sounds like a $1000 tent if put into limited production! ! !

What do you think????

I like the concept and generaly agree with the logic. A single wall tent would be warmer and have less condensation if the fabric were warmer. A second tent placed over it provides an insulating dead air space and minimizes the heat losses associated with wind and wet rain/snow. The warmer tent wall will surely reduce condensation -but I rarely find condensation a problem in really cold weather anyway – its a dry sort of condensation. However, at temperatures below about 10 degrees F the double walls adds a significant bit of warmth particularly if you use a candle to add a bit of heat. The radiant heat will also dry the air significantly and "burn" condensation off the inter walls. My suggestion would be too make two similar tents and use them together when its really cold. For snow use I would prefer having skirts to provide a good seal. On a mid it seems this could be accomplished simply by installing a second set of guy line loops about 8-12 above the bottom seam. A separate shaped ground sheet makes it much easier to pitch mids with 6 or 8 sides. Thanks for sharing your project ideas and good luck!

I have some reservations about this.

When the inner tent is breathable the moisture does go through to condense (or freeze) on the outer. One reason it does this so well is that the gap between the inner and outer has very low humidity: the moisture has condensed out. The vapour pressure gradient across the inner tent becomes quite high. This happens despite the inner tent being fairly cold.

But if the moisture can not escape through the fabric, it may condense on the inner. After all, if it is -15 C outside the tent, it is extremely unlikely that the inner tent will be all that warm. Yes, you could close up the inner to try to keep the warm air in and the cold air out, but that does present certain survival problems.

Can one get the right balance between ventilation and cold? I doubt that one can do so over a broad range of conditions. However, it might work under some conditions, so we will be very interested in hearing how it goes. Do let us know!

Cheers

Will you be sewing, adhesive-bonding, or taping this shelter?

I really like your idea and have been hypothesizing about the same concept for some time as well. Based on my math, the potential is truly there for a comfortable and storm-worthy sub-kilo 2 person alpine tent. Do keep us posted please and share any photos that you can as well.

I also cannot help but wonder if there is some way of making the lightest Cuben fiber breathable, yet still able to block wind – perhaps 50 cycles through a top-load washing machine?

Here are a couple of Cuben mountain tent designs that i have been working on (the latter uses reflective barrier cuben that aids in heat retention, with a translucent top section for daylighting and passive solar gain:

Yeah, a range of conditions will be difficult. I was thinking a small two person tent supported from outside to prevent the outer from collapsing on the inner.
I am guessing that at 0C, you could get the inner film warmed to about 10C or so. Whether this would be enough to cause the vents to disipate the moisture? I don't know. Under very cold and dry conditions, it might.

As far as washing, goes, I sorta doubt that would do anything for breathability. The plastic would likely just wear and split, leading to localized breathing, but not really enough on the scale of a tent.

My thoughts only . . .
jdm

the recent experience that reignited this idea was 2 of us spending a just below freezing still night in A borrowed Stephenson's Warmlite 2R. This is basically an impervious waterproof bag, 2 hoop tunnel tent sewn all round to a silnylon groundsheet. It is ventilated at the foot and the top and bottom of the head. Between the hoops it has a double layer impervious on both layers but the cavity is a little ventilated to the outside, but not downwards, along both bottom edges. Stephenson recommends their vapour barrier clothing to reduce moisture.
The single skin ends were running with moisture onto the groundsheet except where it turned to ice. The double skin portion was a little wet but no ice and not enough to run down.
Comparatively My outer/inner tent will have more end ventilation, about the same top ventilation, but being a mid the top vent will be higher for more chimney effect. I think I see what Roger is saying, A side porch outside the inner with a large door in the inner (which could be left completely open in still conditions) would provide a large moisture sink away from the sleeping area and without puddles on a groundsheet.

Hi Derek

The problem is that, without a bit of wind, the ventilation still may not work too well. My blue tent has very good through ventilation, but even so, I have known it to collect lots of condensation (or frost) directly above our heads while the foot end is almost dry.

I suspect the problem is that the amount of heat escaping from our quilts is not really enough to drive significant air movement through the tent. The warm air rises to the roof, cools, and stalls. Sigh.

So I sit up and wave a 10 g sponge around while Sue lies there waiting for breakfast. :-)

Cheers

In reference to the ventilation issues – I 've been having a good time playing with an inexpensive 5'x 6' single wall dome tent. The only ventilation is a wee bit of net at the top covered by a beanie sized fly along with a slice of screened window at the top of the door. Needless to say, with the door and window buttoned up it is a stuffy, damp and miserable thing. Ah! but add a 9 hr candle lantern and it is surprisingly comfortable. True the candle adds 10-20 degrees f but the thermal syphon also makes it easy to control the ventilation. Since the top vent is fixed open – controlling air exchange is a matter of cracking the door zippers apart – usually a couple of inches is plenty. I open the zippers about a foot off the ground to avoid drawing in the cold damp air which lies on the ground. Another big plus is the ventilation stays consistent regardless of wind speed. The radiant heat off the hot lantern keeps things fairly dry. Of course if the candle goes out – well it quickly reverts to stuffy, cold, damp and miserable.

A UCO 9 hr candle lantern is 5 oz plus 2 oz per candle. Somebody must have some clever ideas for a light weight MYOG lantern?

Roger,
Much agree. I had a modified Eureka Crescent with netting under the fly at one end, and uncoated nylon under the fly at the other end. When I slept with my head at the netting end, the condensation was all on the fly inner where it couldn't reach me.
When I slept with my head at the nylon end, there was usually a lot of condensation on the nylon inner, even though uncoated, that got me and my gear wet when I moved about. This made me a believer in always having netting at both the head and foot ends; not just to protect from condensation, but also to have plenty of venting at whichever end I chose to lay my head.

Roger,
Yes. The ventilation is a problem. Like most cold weather camping, it requires constant adjustment to maintain the optimum balance between heat retention (comfort) and dew point (and consequent condensation.) Loss of heat, and you get a puddle.

Under extreme cold/dry conditions, a double walled tent works well. As evidenced by the larger mountaineering tents out there. As it warms up, the dew point starts building condensation. No longer can it sublime off. You add ventilation. This drops the internal temperature, too. The human bodies in the small tent loose the battle between heat production mantaining a favourable dew point and humidity production from respiration (and insensible perspiration)…you get condensation. Those inbetween temps that really cause a lot of condensation issues are also the temps most of us camp at.

This approach is interesting, but requires a really good thermal barrier as the skin. Typically, silnylon is not enough. Under some conditions it is. Under others it fails miserably. I do not believe it has been tried with cuben, though. With a good thermal barier, it can be made to work, this I know. Whether it can be made light enough for a typical UL tent? This is the question. I hate the thought of lugging around a 12pound tent to save a half pound of condensation (if that.)

Cuben fabrics do not bleed heat as silnylon does. A fabric bleeds moisture, it will cool itself by evaporation to get cooler…more condensation. Cuban provides a 100% barrier against this cooling. Soo it WILL be better. How Much Better?…another question in my mind. Maybe it will require a three layer wall? Maybe cuben plastic is not good enough, like silnylon. It may not supply enough thermal resistance to make a good ventilation scheme work, like silnylon. Maybe it will condense some, but, stop at a small amount, like mountaineering tents. I don't know.

But, this has really good prospects of answering some of my questions and I am interested in hearing more…

jdm

Hi James

Technical debate: forgive me if I am a bit 'to the point'. I tend to be that way.

> Those inbetween temps that really cause a lot of condensation issues are also the temps most of us camp at.
I am not sure about this whole argument. We get plenty of those conditions too. My experience has been that the dominant factor has always been the amount of air movement – driven mainly by wind. Really still nights make condensation (or ice); breezes mean a dry tent.

Getting (or trying to get) a balance between heat production and moisture production to avoid condensation seem a bit unlikely in the field. Especially so as conditions change during the night and I am asleep! So we don't bother.

Technically, the energy difference between evaporating liquid water away as in condenses on a tent and subliming the ice off are not necessarily that great. As the water freezes it gives up some energy to the fabric, warming it. But I suspect that the whole energy balance thing hardly matters compared to the effect of air movement – inside and outside the tent.

Is there a difference in the thermal insulation properties of silnylon and Cuban? Probably, but I have severe doubts about the difference having any significance at all. In fact, I doubt that the thermal insulation properties of either fabric are significant in this context. The fabric is so thin that the temperature drop across it is probably going to be less than one degree. I do know that difference between the inside and outside of my single-skin tent is rarely more than a few degrees even on very cold nights, and most of that is due to the trapping of air.

I can get a bigger warm-up inside my winter double-skin tent while cooking dinner. But that requires that I stop most (not all) of the air movement through the outer tent and block all the easy air movement into the inner tent. That can work moderately well. But then we spoil it by opening up the ventilation a bit when we go to bed.

An analogous argument has taken place over aluminium and titanium pots. The argument put forward by some is that, since titanium does not conduct heat as well as aluminium, it won't heat water as well. But the thinner wall thickness of titanium pots means that the temperature difference across the two sorts of pots is actually similar, and it's under one degree anyhow.

> Cuben fabrics do not bleed heat as silnylon does. A fabric bleeds moisture, it will cool itself by
> evaporation to get cooler…more condensation. Cuban provides a 100% barrier against this cooling.
Sorry, but I don't think this makes any sense at all. I don't understand the idea of fabrics bleeding heat – unless you mean conducting heat. I don't think either silnylon or cuban bleed moisture. At the pressures we are talking about here, both are 'waterproof', and neither absorb any significant amount of water. The idea that silnylon and Cuban will differ in their self-cooling by evaporation of water seems lacking in any physics. Sorry, but.

Cheers

Roger,
Ha, ha, I certainly agree…technical debates are always pointed and blunt.
I will respond to your arguments one at a time, finishing with a speculation.

> Those inbetween temps that really cause a lot of condensation issues are also the temps most of us camp at.

>>I am not sure about this whole argument. We get plenty of those conditions too. My experience has been that the dominant factor has always been the amount of air movement – driven mainly by wind. Really still nights make condensation (or ice); breezes mean a dry tent.

>>Getting (or trying to get) a balance between heat production and moisture production to avoid condensation seem a bit unlikely in the field. Especially so as conditions change during the night and I am asleep! So we don't bother.

Exactly my point. "Like most cold weather camping, it requires constant adjustment…" from the previous paragraph. No disagreement here! I don't stay up all night watching dew form on the walls of my tent, either. I have way too much respect for you, I guess, I made the statement and backed it up in two paragraphs…my fault, I just confused the issue trying to be clear.

>>Is there a difference in the thermal insulation properties of silnylon and Cuban? Probably, but I have severe doubts about the difference having any significance at all. In fact, I doubt that the thermal insulation properties of either fabric are significant in this context. The fabric is so thin that the temperature drop across it is probably going to be less than one degree. I do know that difference between the inside and outside of my single-skin tent is rarely more than a few degrees even on very cold nights, and most of that is due to the trapping of air.

Yes. I agree again. But we were not discussing single walled tents. Rather sealed cavity double wall tents.
>>I can get a bigger warm-up inside my winter double-skin tent while cooking dinner. But that requires that I stop most (not all) of the air movement through the outer tent and block all the easy air movement into the inner tent. That can work moderately well. But then we spoil it by opening up the ventilation a bit when we go to bed.

Yes. Provided you can keep the CO down to acceptable levels as you have outlined in some of the stuff I have read, here. (Well done, BTW.) As always, a certain amount of ventilation is required to maintain proper oxygen levels. Even more so, with sealed cuben tents, since they are really plastic films, not breathable fabrics like silnylon. This is the context of the discussion.

I think we can skip the Aluminum vs Titanium debate, here. We have had that discussion.
> Cuben fabrics do not bleed heat as silnylon does. A fabric bleeds moisture, it will cool itself by
> evaporation to get cooler…more condensation. Cuban provides a 100% barrier against this cooling.
>>Sorry, but I don't think this makes any sense at all. I don't understand the idea of fabrics bleeding heat – unless you mean conducting heat. I don't think either silnylon or cuben bleed moisture. At the pressures we are talking about here, both are 'waterproof', and neither absorb any significant amount of water. The idea that silnylon and Cuban will differ in their self-cooling by evaporation of water seems lacking in any physics. Sorry, but.

I disagree. Not conduction, I am ignoring that. Evaporation will still occur in both cases. It will happen even at 100% humidity as part of the vapor pressure numbers liquids are described with(or even solids for that matter). In a fabric (such as silnylon,) that vapor is free to pass in both directions. It is a breathable fabric. We have a net loss of heat by allowing this to happen to the outside. This is a pretty clear case of what meteorologist’s term "evaporative cooling." And, in my experience, this seems to be true, subjectively speaking. In the case of an impermeable film(cuben,) water vapor is not free to pass through it. Hence, it will remain at equilibrium. Evaporation and condensation will roughly cancel each other out…no net loss of heat. So, it will stay warmer since it is not "bleeding heat" through evaporative cooling. So, I suspect it will be slightly less prone to developing severe condensation under the double walled tent conditions we are talking about. Of course, I am assuming a steady state system, not a dynamic growth of humidity as is the norm with small tents…more to illustrate what I was referring to. But I think it is pretty clear that increased humidity will only exacerbate the problem and the temp differential between the two materials.

I would speculate that it would take more than the simple films we are discussing to eliminate the condensation through a chimney effect in small tents, though. A super insulator at the film thicknesses we currently use *might* do it. Or, a double walled tent made up of a couple inches of down insulation between two layers of cuben. Exorbitantly expensive, but, feasible given todays weights of materials. Probably no more weight than a good -40F bag + poles and stakes… Ahh well, nice to think about…
jdm

>In a fabric (such as silnylon,) that vapor is free to pass in both directions. It is a breathable fabric.

Haven't run across any of the breathable silnylon yet. Profabrics sells a breathable PU coated nylon, though.

Hi James

A pleasure to debate.

> In a fabric (such as silnylon,) that vapor is free to pass in both directions. It is a breathable fabric.
Ahhh … Can you cite or reference any measurements for this claim?

As far as I know, silnylon does not transport moisture vapour to any normally-measurable degree. Having pressure-tested many fabrics, I know that every silnylon I have seen so far is quite waterproof up to about 12 kPa.

Are you perhaps confusing eVent or EPIC with silnylon?

Cheers
PS: a down-insulated double wall tent … hum …

Sam, Roger,
Breathable is not really the correct term, sorry. Again, my imprecision in terminology is tripping me up, again. I am not an engineer, rather a scientist(now retired), by training. (Also, as is obvious, an incoherant word smith.)

Silicone is hydrophobic to water. It will not allow water to pass through it for any reasonable value we are looking at. It will: in rain, or, under pressure. That's why it makes poor tent floors and dry bags. I think it is safe to ignore that, other than to point out that it is permiable to some degree and why I mention it.

Water vapour is sneaky stuff. It is NOT water. If it can escape through the fabric, it will.

I have enough experience with silnylon it to say that it holds back some portion of it. (Like any fabric or permiable membrane.) But, it will allow some to pass. So, it will cool. This cooling will cause more condensation…. and so on. Hmmm, I supose it would be possible to measure the temperature of the water condensed on the inside, the temperature of the inside air, and the temperature of the outside air to confirm this one way or another. Never really thought that much about it, just a fact of life in small tents.

A plastic film (cuben) will not allow this to happen. It is not a permiable membrane for the purpose of this explanation.

Anyway, I think Derek has plenty to think about. We are getting a bit far a'field.

You can always contact me at jdm27@cornell.edu for additional discussion.

Hi James

> But, it will allow some to pass. So, it will cool.
Being very technical … I am not sure about this.

If water vapour enters the membrane on one side and pops out on the other side, I am not sure that there would be any cooling effect.

If you are talking about the water condensing on one side, migrating through the membrane and evaporating off the other side, then there could be a cooling effect due to the evaporation process. But at the rate of diffusion expected for water through silnylon, the effect is going to be extremely small and might be almost unmeasurable.

I think a tent fly gets cold at night by radiation to the night sky. On a clear night the night sky is at about -70 C. I dare say that any wind might also cool the fabric down by evaporation of condensed water from the inside – or from the outside if the fabric is cold enough for that to happen.

Cheers

This topic is already getting a thorough treatment, it seems, but if another voice is welcome, I think the permeability hypothesis suggested by James sounds a bit fanciful.

It's my understanding that ePTFE, microporous urethane, and microporous polypropylene membranes leak at high hydrostatic pressures because liquid water is forced through tiny, perpetually open (unless obstructed) pores in the film. Silnylon leaks at high hydrostatic pressures because the liquid water creates pores where there weren't any. Silicone is very stretchy, and very impermeable, but it also has extremely low tensile strength (there are urethane condoms, but you'll never see a silicone one). Microscopic tears occur readily, particularly once the coating has been abraded. These tears are normally sealed shut, but pressure from liquid water can push them open. My point is that, unlike microporous membranes, a silicone coating has no perpetually open pores. Water vapor can't pass through silnylon unless the tiny imperfections in the coating are forced open.

I, too, have a bit of trouble seeing a significant difference in the permeability and the thermal properties of silnylon and cuben (which is spelled like "cube," not "Cuba," for the record).

Would not want to let this thread go without one of Jack Stephenson's arguments for the sealed cavity (coated inner and outer canopy) tent; that is, that the tent material does not absorb water, and so is much easier to dry out, or in the alternative, shake out and pack without carrying the water as dead weight.