Tent Warmth Survey

I think you’re simplifying the analysis a bit…surely my breath warms up the tent some? Or does that 75 watts include my breath?

I haven’t read all of this report, but look here (link opens a PDF): An Initial Investigation of the Heating Properties of Conical Tents

On page 11, there’s a section called “Effect of Lining a Tent”. While they’re dealing with heated, 10-man arctic tents, they say “the addition of a [tent] liner causes an increase in the value of Ti’ of 60% to 100% [variance due to location of temperature measurement inside tent]. This means that a tent with a single layer liner will have a relative temperature which can be as much as twice as large as the same tent without a liner.”

Ti’ is the relative temperature rise per watt of heat input: Ti’ = (T – Ta)/Q where T is the temperature inside the tent, Ta is the ambient air temperature, and Q is the heat input by a heater.

Correct me if I’m wrong, but it sounds like they’re saying that if a tent is 5 degrees warmer than the outside with just the fly, then it will be up to 10 degrees warmer than the outside with the fly and an inner.

All this said, I chose my tent (BA Fly Creek UL2) because it has a mostly solid inner to block drafts.

"I wasn't far off when I said 3 people is 15 degrees warmer… 11.1=15 whats a few degrees amongst fellow backpackers ;)"

I wasn't trying to disagree with anyone, just trying to figure it out. 3 people with a liner maybe warms up 15 F. My calculation was overly simple : )

Interesting link John, I have to look at that some more. They were using heaters, but you can still get some useful info out of it. And they were more interested in colder temperaturers. I stay above 20 F or -10 C. That is a potential problem in info about insulation, sometimes it's more oriented towards long polar expeditions.

"Correct me if I'm wrong, but it sounds like they're saying that if a tent is 5 degrees warmer than the outside with just the fly, then it will be up to 10 degrees warmer than the outside with the fly and an inner"

I think that may be a useful conclusion.

I was just doing conduction and ignoring convection, evaporation, and radiation. I wonder what effect breathing has? It takes heat to evaporate water, but when it condenses on the inside of the tent walls it releases the heat? Maybe it's too complicated to do theoretical calculations.

I might read through it more as well. I think they were using heaters as a way to provide a quantitative heat input on demand – you could probably replace them with x number of people to get new results.

And I mentioned breathing because I'd guess that's a major source of heat input from people.

1300 W heaters – 3 of them for one test

That would be the same as 17 or 51 people : )

And "10 people" tents

Still, it's an interesting article

Jerry,

I have some past and current real world readings to add to your calculations. I've been paying attention to inside and outside temps for the last ten years or so and have spent the last 3 nights sleeping in my yard with a thermometer that records inside and outside temps and min and max for each.

I typically hang the inside thermometer about 1 foot below the peak of my tent. For all of these observations I'm using a 1 or 2 person tent with 1 or 2 persons (wife and I) in the tent. The inner tents are all fabric with an unvented rainfly that comes to within 1 foot of the ground on all sides. The inner tents are small volume, 45"-48" high with the high point at the center of the tent. These observations are for relatively windless (i.e. still) nights in the Pacific Northwest region of the US of A. Outside night time temps for the last 3 nights were in the 45-50F range. Outside night time temps for the last 10 years of random observations were in the 20-50F range.

Observations

(1) While sleeping, the inside temp stays about 10 degrees F higher than the outside temp.

(2) If I move around within the tent (e.g. getting up on my knees and changing clothes, doing sit ups, etc.) the inside temps will quickly rise to 10-25 degrees above the outside temps.

There is rarely any condensation on the inner tent, even though the conditions would favor condensation.

For example during the last 3 nights, sleeping in my yard, I've had one night of continuious rain, and two nights of 85%+ humidity. I'm camped in a low spot where cold air gathers and I'm camped on soaking wet grass. (Reminder, the fly has no vents built into it.)

The inside of the fly gets plenty of condensation but the inner tent stays dry. I think the reason for this gets back to the warmth issue. I think (but don't know) that the inner tent might be staying warm enough so water vapor within the inner tent (which feels humid) stays in the gas state until it gets outside of the inner tent and hits the cooler fly.

John,

That Canadian tent article was a great find! Thanks for bringing it to the table.

I can't follow the math but, if I'm understanding the findings correctly, I think the following might be concluded: For those of us looking for warmth, a tent with a liner (fly + inner tent?) would be indicated. For those of us looking for tent coolness a single layer tent would be indicated.

I also noted the "stag zone"* in one of the drawings. I understand this to be the area of the tent, near the apex, where air does not circulate much (stagnates) and, I would assume, heat concentrates. When it's cold and I'm changing clothes in my tent this is the exact area where I like to stick my bald head so I can absorb as much heat as possible.

Daryl

*Another possible interpretation of "stag zone" would simply be where old stags stick their heads to get warm.

Hi Daryl

> The inside of the fly gets plenty of condensation but the inner tent stays dry. I think
> the reason for this gets back to the warmth issue.
Yup. The fly can get quite cold, so water preferentially condenses on it rather than on the slightly warmer inner tent. Even just a few degrees difference can do this.

Cheers

When you're sleeping in your tent and it's 10 degree F warmer, does that have a fly?

Jerry,

Yes, the tents I'm using all have flys that come within a foot of the ground all around the inner all nylon fabric tent (no or very little netting on inner tent).

Daryl

Dan,

"With that said, tent inners could be made from light fabrics that are no heavier than the widely used no-see-um netting. A fabric like EightD (or is it TenD now?) weighs about the same as netting (0.7oz) and presumably would be warmer. It's affordable too, so it may be a good choice for a solid inner fabric"

The myog tent I've used in the yard for the last few nights is made of the fabric (or something close to it) you mention.

So since I am only looking for warmth and not worried about the tent being too hot it is a zero-weight-added path (for me) to keeping warmer at night.

The 10 degree added tent warmth while sleeping allows me to carry a slightly lighter sleeping bag. Looking at sleeping bag chrts it looks like I might be saving about 4 ounces. A typical 20 degree bag, for example, might be 4 ounces heavier than a 30 degree rated bag.

The 10-25 degree added tent warmth while moving around allows me to carry less upper body clothing for wearing in the tent. So I might be at least saving the weight of a pullover, say 4 ounces.

My current one person tent with fly weighs about 20 ounces. If I'm saving 8 ounces as described above I'm down to a net of 12 ounces for a 4 season shelter.

Daryl

So, maybe an unlined tent is 5 degree F warmer than ambient, lined tent 10 degrees

You get an additional 5 degree F with a liner. If it was 6 square yards, and made with 0.67 oz/yd2 fabric, it would weigh 4 ounces plus any tie-outs if you had any.

To get a 5 degree warmer quilt, you'de have to use maybe 2.9 oz/yd2 Apex rather than 2.5 oz/yd2 (ignoring fact they don't make such a thing). If you had a 3 square yard quilt, it would weigh an extra 1.2 ounces.

Maybe this is just a calculation for people that want to over-analyze : )

Another thing is, for us in the PNW where it rains too much, if it's raining, I would rather the tent was as little as possible. Leave it set up until last, then unpitch and put the wet mess in a waterproof bag. I'de rather not have a liner in that also.

Tents feel warm largely b/c they eliminate convective heat loss [they act as a wind break]. I like ventilation in summer with mesh panels, but mesh panels aren't required in the winter time. Additionally, mesh panels should be higher on summer tents. This became immediately apparent a few summers back while camping on a windy ridge at the edge of the CA/NV border. Sand was blowing under the fly and straight into my tent [what a mess come morning]. People who require summer tents with good ventilation, but also rain protection are better served by tents that have two doors [with awnings] than a tent with tons of mesh. My Bibler Fitzroy with two doors is better in summer [ventilation wise] than my lightweight, mesh everywhere, single door Mtn Hardwear summer tent [with fly on]. So no, the inherent warmth of a tent is not a concern [and they don't have much thermal resistance anyway], ventilation/breathability is the key. Being able to vent and achieve air flow when you're hot yet being able to close the tent up in the cold without and condensation issues.

For me tent features are all about rain, wind stability, bugs and ventilation. Warmth is a good thing, but if I'm dry and the thing stays up, my sleeping bag and clothing will handle the warmth issues.

My shelter is often as wet from condensation as from rain and that can be both inside and out.

As to warmth, when using a hammock I've noticed that the insect net adds some warmth. I imagine it is slowing the warm air transfer and diffusing light breezes. I was surprised at the difference and not noticing it as much with tents. Then again, the net on my hammock unzips full length and swings back where a tent has only a small door.

We have a bit of experience with this, making large conical shelters, and heating them , and also adding liners or nests. I have not done any testing with a static BTU source but here are some non scientific findings.

When running a stove a liner adds a fair amount of warmth while the stove is running. Yes the apex can be much warmer than ground level. Sometimes in 20F the ground level of the tent can be 65 -70 while the cone is well above 100, probably closer to 120 – 130F.

A small nest with DWR on 3 sides, seems to add close to 10F in a small micro climate

Yes, condensation always forms on the outside, unless the layers touch. The same for frost

In one case, a few inches of snow fell overnight, the tent was pitched on soil, but the rest of the surrounding area was snow. The temperature was about 15F outside in the morning, but inside the tent it was above 40. I attribute this to a couple factors, mainly the inside of the tent the day before had been probably 80 F due to the sun hitting it, even though outside it was likely never above 32. The snow then provided insulation about 2.5 – 3 ft up the outside wall. The liner provided a small but second level of insulation. So, you had ground that was warm and giving off heat, and then an insulation layer of snow added around about 50% of the tent fabric. I wouldn't count on this to keep me warm and take a lighter bag, however I would be pleasantly surprised to wake up and find my tent well above freezing after a night of snow.

When it gets cold, we plan to experiment and test essentially reflective roofs near the apex, to see if they minimize heat loss where most of it is loss.

Regarding summer use, effective ventilation is the best answer. You can still use the same DWR's etc, as long as you have good venting. This is more than just apex venting. Controlled venting via doors or the base goes a long ways.

Several of you have mentioned other factors that would trump the warmth factor when selecting a tent. Caustrophobia might be another one.

The small enclosed inner tents that I like might not even be tolerable for some of you. The inner tent I've been sleeping in this week, for example, is 6 feet long, 48" high, 18" wide at the ends and about 30" wide at the midpoint. Here are some photos:(blue foam is only there to keep tent off muddy ground)

Jerry,

"So, maybe an unlined tent is 5 degree F warmer than ambient"

I measured the inside/outside temp difference last night with the fly only (no inner tent). You were right…..it was about 5F degrees (about 55F inside and 50F outside).

Daryl