By the Numbers: How does Tent Design and Materials Influence Cold-Weather Camping Comfort?

[Stephen Seeber]

Companion forum thread to: By the Numbers: How does Tent Design and Materials Influence Cold-Weather Camping Comfort?

An overview of the complex heat exchange mechanisms between the tent occupant and the outside environment – and how to test tent construction’s impact on comfort.

[Jerry Adams]

Another great article, thanks, a bit if an expansion from previous articles measuring in a lab setting

I like the thin mint equivalence, because it made me laugh uncontrollably (my wife asked me if I was okay) and because that is a unit that makes more sense than watts per meter or whatever.

I am surprised that dyneema has so much more transmisitivity. Yet another reason not to use it.

Yet your final recommendation is to use dyneena?

Yes, the internal air temperature will be about the same, but, because of the increased transmisivity what you feel will be colder.

I have noticed that if my outfit isn’t quite warm enough for the expected temp, then sleeping under the polyester tent will feel warmer

I’m looking forward to the condensation article.  That makes sense that the only solution is to make it big enough so you don’t touch the surface. But it helps to put tent under tree so it isn’t cooled by radiant heat loss.

I’m a little surprised that a double wall tent isn’t warmer.  If you have an inner liner, maybe you’ll feel that and stop, avoiding touching the outer wall with condensation on it

[Thom]

That was a great read. Thanks so much. Your work is much appreciated

thom

[Eric Kammerer]

Warmlite used to make their tents with a fairly unique design. Their current website isn’t very clear on what they do, but the wayback machine provides an earlier description:

DRY, WARM: Double walls, silicone coated fabric and sealed seams provide full rain protection, best warmth, and least condensation. The inner wall is kept warmer by an insulating air gap between walls, aided by radiant heat blocking aluminized exterior. The tent stays dry inside; warmth of inner wall eliminates the miserable condensation so common in other tents, and aids differential height chimney venting. Warm humid lighter air rises up and out the top vents, while fresh, dry air is drawn in through the lower vents.

I have no idea whether or not the claimed benefits are actually measurable. Mine seemed better than what else was available at the time — but I haven’t used that tent in years.

[Matt Dirksen]

“I’m a little surprised that a double wall tent isn’t warmer.  If you have an inner liner, maybe you’ll feel that and stop, avoiding touching the outer wall with condensation on it“

+1 to that.

While I very much love and appreciate the in-depth analysis, I’m concerned that the impact of convective heat loss isn’t being fully contemplated, which, in my opinion, may play a massive role in one’s experience that a double walled tent feels warmer.

I know in the building industry we’d typically tell clients that “a one square inch of air gap is equivalent to eight square feet of no insulation”, when discussing the importance of air sealing.

I speculate that a big advantage of a properly guyed, double walled tent is that it helps reduce convective heat loss by dampening the lateral force of wind, so the inner membrane remains more intact and doesn’t deform as much. Just like any slight movement in a sleeping back can quickly exchange warm air for cold, the same phenomenon happens to our tent with any slight movement of air. Even with a door slightly ajar, I suspect the “total ach” (air changes per hour) inside a double walled can be fine tuned.

Having used several of my “old school” four season tents out on the same winter trips in the past all at the same time, it was very obvious to the myself and the campers with me that the three double walled four season tents felt significantly warmer than the two single walled four season tents, which I suspected had to do with better convective heat loss management.
But designing a controlled experiment to test that theory might be a challenge.

[Bill Budney]

double walled four season tents felt significantly warmer

I see that Stephen only tested UL tents, not the traditional (heavier) winter tents that I think of when considering warmth (Hilleberg, Nortent, maybe SlingFin).

Did he test a Warmlite at some point? If so, then perhaps he will tell us more about how that compared to his radiant shield.

PS to Stephen: It’s another very interesting paper. Thanks for all of the hard work.

[Roger Caffin]

I would like to add a wee bit of physics to this.

A clear night sky has an effective temperature of about -70 C for infra-red radiation. Actually, I think this is not the temperature of ‘deep space’ but of the stratosphere/troposphere. Never mind, just remember -70 C and infra-red radiation. True, we don’t often think about IR heat loss.

Putting an opaque barrier between two things at different temperatures blocks the direct IR radiation between them. Roughly speaking, the barrier will be at the mid-point in temperatures. This is why having a tent of any sort is so effective. Assume ground at 0 C and sky at -70 C, then tent fly may be at -35 C, very roughly. Cold, but not as cold.

Having an opaque inner tent can result in that inner tent being at half the temperature difference between the ground and the fly. This could mean the inner tent might be around -18 C. – assuming no other influences. (There may be plenty of those.)

However, please note this is for an opaque barrier. Dyneema fabric is effectively IR transparent, so it doesn’t work. Light polyester fabric is not as bad, but it is not really ‘opaque’. Dark nylon fabric is much better.

Having a sewn-in groundsheet does not change the effective ground temperature very much, but it makes a huge difference to condensation. With bare ground (OK, grass), lots of evaporation is possible, and this will condense under the fly. This does not happen of course with a sewn-in groundsheet. It makes a huge difference when camping in the rain.

Sealing up the tent is a very bad idea if you are inside it. YOU are giving off water vapour as well. On the other hand, having a LIGHT breeze blowing through the tent all night may lower the temperature slightly (it does), but it sweeps the water vapour away. I have woken up to a dry inner tent in the morning in the snow despite it being very cold overnight. Trust me, a dry inner is miles better than a frosty inner tent: the latter showers ice crystals down the back of my neck!

Cheers

[Jerry Adams]

I think that the tent material, if it’s opaque to IR and has high emissivity, will be about 10F colder than the air temp if the sky is clear. That’s what I measure with my thermometer.

The surface of my sleeping bag is about 10F warmer than air temp because of warmth from my body.  Thus, there will be a difference of about 20F between my sleeping bag and the tent surface.

If there was no tent the effective temperature of the atmosphere is -70C = -90F using Roger’s number.  If the air temp was 20F the surface of my sleeping bag would be about 30F, so there would be 120F difference in temperature between sleeping bag and atmosphere – much larger radiant heat loss.

 

[Stephen Seeber]

Hi All.  Sorry for the delay in responding, I just returned from several days on the trail.

I will try to respond to each of the comments:

I tested a pretty wide range of tents, both single-wall and double-wall, in a fairly broad range of weather conditions. As I said in the article, there may be tent constructions that perform differently than those available to me and provide better solutions. My results and conclusions are based on weeks of outdoor tests under a variety of environmental conditions. Tent performance was measured using an array of instruments as well as my subjective impressions.

I expected my conclusions to be controversial, particularly my finding that double-well tents offer no comfort advantage over single-wall tents. However, the measured data strongly support this conclusion.

We all know that air is a great insulator.  However, in order for air to insulate, it cannot be allowed to move.  If it can move,  convection will occur and heat will readily transfer from air molecule to molecule.  Your down insulation or synthetic batt insulation works because it traps air in tiny pockets and limits air movement. Convection is prevented. This is not the case in the space between the walls of most double-wall tents.  In these tents, the air is free to circulate in all directions. It will circulate both in response to temperature gradients due to internal heat stratification or deformation of the tent walls due to puffs of wind or occupant movement. Convective heat transfer from the outer layer to the inner happens quickly and efficiently.   You can see convective heat transfer with your own eyes by viewing the video attached to figure 11, for a single-wall Dyneema tent.  This video did not work when the article went online, but it does now.  I now wish I had made a similar video for a two-wall tent, but that will have to wait.

This IR image shows a double-wall tent interior along the ridgeline.  A flap in the inner is cut open to allow us to image the surface temperature of the fly and the inner wall in the same area.  We see that the surface temperatures are nearly identical. We see that left of the tent ridge, the interior surface is actually colder than the exposed fly temperature.  This is likely because the sky temperature “seen” at the left side of the ridge is a little colder than what is “seen” to the right of the exposed fly.  The air temperature just below the inner is 36.1F, as measured on the surface of the temperature/humidity sensor hanging from a string by the open flap.

I placed temperature sensors at the ridge, between the tent walls and 6″ above the tent floor.  The inner wall is always slightly warmer than the air space between the inner wall and the fly, but not much.  My test results show, on average, the double-wall interior air temperature is only 2F warmer than that of the single wall tent.

While personal comfort experiences can be informative, they do not provide conclusive evidence due to unknown variables such as tent construction, pitching, sky view, and environmental conditions.  In my tests, these variables are systematically measured, providing a basis for my conclusions.

There may be tent designs or materials that more effectively reduce heat loss from occupants inside the tent or provide better humidity control than the ones I tested.  Certainly, that is the claim of Warmlite. Reviews and discussions of Warmlite tents often provide conflicting opinions. Unfortunately, their website has changed and is no longer as descriptive as it once was about materials and construction methods.  As was pointed out by Bill Budney, I took some data on a group outing from a Warmlite and some other tents.  Unfortunately, for that test, tree cover varied for each tent that was monitored, so I never felt the resulting data was dependable.  For what it is worth, the Warmlite was not the warmest tent.  However, if someone has a Warmlite double-wall tent and wants to send it to me once winter returns, I would be very happy to instrument it and see how it does.

On the subject of convection.  I presume by “convection,” Matt is referring to wind blowing through a tent or forced air exchange due to wind deformation. It is unclear to me that double-wall tents reduce air-flow volume in a tent.  I am assuming that a single-wall tent used in winter is pitched to the ground or wall/floor interfaces are sealed.  Forced air exchange can occur in single-wall or double-wall tents as a result of wind deformation forcing air exchange through ventilation openings. Single-wall tents like the Duplex are terrible in cold, windy weather because the fly is far from the ground and there is abundant netting between the tub floor and the fly.  My Durston Pro+ pitches to the ground but has net vents at each end, which will allow wind and spindrift to enter the tent.  For winter use, I have fashioned Dyneema panels to cover these vents.  They are secured with Velcro.

Roger brought up radiant heat loss through tent walls, which the article discusses in great detail. Included are actual measurements of both night sky temperatures and tent wall IR transmissivity of various fabrics. One of the key findings in my testing was that as heat loss due to IR transmission drops tent wall temperature below outside air temperature, convection from outside air warms the surfaces back up. You can watch this occur in the figure 11 video.  Of course, as described in the article, high IR transmissivity has a disproportionate impact on comfort when skin is exposed directly to a tent fly or a tent fly in a double-wall tent where the inner wall also has high transmissivity.  In my single-wall Dyneema tent, I wear a neoprene face mask to ensure this is not a problem in the winter.

Tent heat transfer balance has many moving parts. My comments above touch superficially on only some of the important issues. The article provides a lengthy, detailed discussion of the issues, complete with test results to quantify what is happening. My hope is that before readers of the thread append their comments based on only their reading latest comments, they read the article first and then point out the shortcomings of my analytical approach.  Then, I may have some new issues to study next winter.

 

 

[Terran Terran]

I only expect a tent to protect me from wind, precipitation, and bears.

[George H]

Terran, the tents I use provide a temp boost from 5-8 degrees, maybe up to ten sometimes . Together w sleep layers, site, etc it has great impact on what degree bag/quilt I need.

[Jerry Adams]

I’ve measured a similar temperature increase inside my tent, even when the door is wide open

I think it’s because of protection from radiant heat loss to the clear sky

If it’s cloudy or I’m under trees, then I don’t see any temperature increase

I really like Stephen’s articles because they add a little understanding to all of this

[Terran Terran]

I suspect the inside tent temperature drops when you crawl into a warm mummy bag or quilt. I think the increase you’re measuring may come from body temperature and will quickly decrease.

[George H]

I monitor inside and outside temps parallel with two Bluetooth Govee devices that records every couple of seconds continuously all night. Then I study the graphs on my phone later. Result? Well designed tents add considerable warmth to a sleep system, even after you close your eyes

 

 

[Stephen Seeber]

Terran Terran is correct. As illustrated below in two graphs.

This is a graph of interior vs exterior temperature for a single night in a double-wall tent.  I enter the tent at around 11:20 PM.  I zip the door and get the contents of the tent organized.  The interior-exterior temperature difference shoots up to a 16-degree difference between inside and outside, as seen on the bottom brown line.  At that point, I get into my sleeping bag. The interior temperature slowly cools and settles at about 4 degrees warmer than the exterior air temperature. At about 3:30 AM, I get out of my bag and leave the tent to relieve myself.  I then get back into the sleeping bag. The interior temperature shoots up to 14 degrees above the exterior temperature and then drops down to around 2F above the exterior temperature.  At around 5:45 AM, I wake up and leave the tent.  Data collection stops at about 6:15 AM.

Here is another graph:

This is an Xmid 2 tent at colder temperatures, with the inner removed.  Thus, it is a one-wall tent pitched to the ground. You see a similar pattern.  When I enter the tent, the interior is 4F colder than the exterior due to radiant losses. The interior jumps by 16F when I enter the tent. You can see I am in and out of my bag 3 times.  Then, the interior cools and is about 1F colder than the exterior.  Between 4 AM until I left the bag, the interior is about 1F warmer than the exterior.  This is because the outside temperature dropped, and the interior temperature lagged behind.

You are the heating system for your tent.  If you add insulation to yourself in the form of a sleeping bag, or extra clothes, the interior temperature will drop to around the exterior temperature.  There is no prize for a tent interior that is warmer than outside air since you are expending your energy to keep it warm.

[Bill Budney]

George, which tents have you experienced as warm?

[Thom]

I don’t believe my tent is warming much with only my face exposed.  Shaped tarp ,warmer bag user. From my time in the USMC I know what non ultralight is like.  Ha ha

thom

[Todd T]

There is no prize for a tent interior that is warmer than outside air since you are expending your energy to keep it warm.

I get the point, but must pedantically disagree.  By that logic, there’s no prize to the interior of a sleeping bag being warmer than the outdoor air.  :-)

[Jerry Adams]

George H, I will now have to google govee devices

What tent?  How many degrees warmer?

[Jerry Adams]

amazon – govee thermometer/hygrometer – one for $15, three for $40 – 3.2 ounces – that’s not too bad

[George H]

Locus Gear DCF Khufu, no inner. 5°F, sometimes more

Not an expert like Seeber, however my experiences over many seasons shows a bigger advantage. My thoughts, which could be just misguided guessing:

This tent pitch taut with very little panel flutter even in strong wind. Combined with a tight fitting, close to the ground perimeter there’s hardly any air movement inside to mix with the outside

The only mesh panel is associated with the peak vent, and only the size of a large phone

While exhaling warm breath into the tent is not ‘efficient’ it is however inevitable and should raise the temp

Unfrozen ground is often warmer than the night air and having no floor and using no ground cloth captures some of this

My Hilleberg Unna, which I only use in winter snow at altitude is also consistently warmer inside

I’ve used more airy, mesh focused tents and found them in line with the Seeber study

 

 

[Matt Dirksen]

“My Hilleberg Unna, which I only use in winter snow at altitude is also consistently warmer inside I’ve used more airy, mesh heavy tents and found them more in line with the Seeber study”

+1 to this.

All my “double-walled” winter camping experiences have been with tents that have full “solid” inners. Also, all the mesh screening on the tents was backed by solid panels. While they are definitely heavier, they were much warmer and sturdier than the mesh-flavored “double-walled” counterparts.

And that’s what’s so fascinating about Seeber’s study: that it clearly illustrates the inside mesh fabric that defines the “second wall” of the tent really doesn’t do much toward keeping the inside warmer.

Therefore, I’d personally love to see Stephen test a tent with a genuine “solid” interior with the same rigor as the other tents. I suspect it would show measurably different results than the half-mesh “double-walled” tents in the study. (I’m wondering if one of Roger Caffin’s famous four season tents can be donated for the cause?)

This whole analysis reminds me of the lively BPL discussion many years ago regarding how one chooses to define a single-walled tent vs. a double-walled tent.

If I recall, there was no consensus.

 

[Roger Caffin]

Our experience with my blue single-skin tunnel tent is that it could be, depending on the ventilation etc, a couple of degrees C warmer than the ambient outside.
Our experience with my red double-wall tunnel tent with a fabric (not mesh) inner was that even in the snow it was at least 5 degrees C warmer inside.
Those figures came from a miniature data logger I used to use on every trip.

I cannot imaging that a mesh inner tent would give any advantages at all – except maybe to keep the mosquitoes off. Mind you, that could certainly be good!

Cheers

[Stephen Seeber]

Hi Matt and George:

I think this is great.  We have some real data to work with and tent designs I did not test.

I really don’t care who is right.  I started this project two years ago to improve my comfort in the winter.  If your tent outperforms the ones I have tested and we can understand why, that is a win for all of us.

We may learn something from this.

First, the inners in the tents I tested were solid, not mesh.  In the Durston Xmid, there are large mesh panels on each side of the solid inner.  I tested with this tent as designed, and I also tested with the inner mesh openings covered. I also tested with added high and low controllable ventilation openings in the liner.

The liner tests are relevant to George’s numbers because his is a single-wall tent.  It is not very different than the single-wall Xmid where I left the solid inner out.  It is also not very different than my Durston Pro or Pro+, especially in still wind.  One difference is your fabric weight: .8 0sy, relative to my fabric weight at .51.  I expect this may impact IR transmissivity, but I have not measured that.  However, I just received four Dyneema samples for another project, ranging from .51 osy to 1.41 osy  I am in the process of measuring these now to get a better handle on the impact of changes in the Dyneema fabric.

One critical issue is how the interior and exterior temperature data is obtained.

George- can you describe how and where you mount your interior and exterior sensors?

It is important to know the sky view and sky conditions present at the time of the test. It is very important to know if you were pitched beneath an open sky, next to large rocks that block part of the sky, or underneath tree foliage. As we know, all of those things will impact the interior temperature. We also need the same information for the exterior temperature sensor.  The temperature sensor should be placed with the same sky view or shading as the tent.  So, if your tent is pitched under a tree, so should your exterior sensor.

Where I test tents at home, I have a clear sky view for my tent and exterior sensor. In the woods, it can be more difficult to have matching sky views, which will significantly impact the results.

When I calculate indoor and outdoor averages, I review the data and, by inspection, remove data obtained when I am out of my sleeping bag. As you saw in the graphs above, time out of the bag will significantly impact the average temperature difference and, in my opinion, skew the data away from what occurs when you are trying to sleep.

We need to make the same changes to your data.  Is this something you can undertake?  If not, if you can provide your CSV or xlsx files, I can process them.

George, if this interests you, send me a PM and we can work out how to move forward.

 

 

 

 

 

[George H]

Stephen, I’m an amateur just wanting to know what to expect for myself. The Govee’s were cheap and I trust them, so easy to do.

Once I saw the advantage of a good tent I stopped the data collection and just continued enjoying the benefits. I have no Excels

I rarely camp below tree line, so probably only low vegetation but maybe boulders. I always hang the outside sensor suspended from the end of a trekking pole maybe a foot above ground. The inner was near the tent floor propped on something or another

Based on what I will take away from this thread I might resume dual monitoring again this fall to see if I was wrong

I always Govee the outside on all my trips. On 10 days in Sierra ending this week I had temps between 35 and 45, but was borderline too warm in a bag with only 7 ounces of down inside a Yama Swiftline (probably a design Durston got inspo from) wearing an Alpha top and no leggings.  Average camp altitude was 11500′, and some nights it really blew hard.

Edit: the one night we cowboy camped I had pull on my VBL bag at 4am. That was with a low of 40

These are more valuable data points to me than graphs, haha

 

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