additional warmth provided by rain fly in double wall tent

I've actually carried a two thermometer meter to measure the differences between the inside and outside of my Fly Creek 2. I have consistently seen 5-8 F warmer temperatures inside the tent than outside. There have been a few occasions when the difference was even greater (these were generally times when the outside temperature was below 15F. I also feel more comfortable in the cold with the rain fly on because it helps to block wind, and also reduces my perceived radiation cooling.
Jim

i have carried a double reading thermometer a few years back, and seen temps of around 6 degrees between inside the netting vs outside the tent measured about 6" off the deck, in even a good evening breeze. this in a conventional type 2 hoop tent that was all but 100% netting.

With small volume inner tents that (1) are solid nylon and (2) have flys that come to within 6" of the ground all around I usually get about 10 degrees difference between inside and outside temps while I'm inactive.

With almost any activity (e.g. changing clothes) I can raise the inside temps up to 25 degrees above the outside temps for short periods of time. It only takes a few minutes to do so. When I'm cold and wet it feels wonderful to get in the tent and change into dry clothes within this +25 degree environment.

The inner tent is also wind free in most cases so wind chill is not a factor.

Seems like an obvious question here is whether you need to have a double walled (versus single wall) tent to get this effect. I don't see why you would. So maybe the question is in reference to enclosed shelters more broadly. Clearly, tent design and the amount of ventilation will be a factor for both double and single walled.

Learned this at the upper end of Lyell Canyon, summer 2011. 8 x 10 flat tarp. Cold wind from earlier in the evening had died down. Clear, chilly, high elevation, we decided to sleep under the beautiful stars. I lasted about an hour, too cold to get to sleep. Got up and erected the tarp over us. The effect was dramatic. I think it was enough for the tarp to more or less trap our rising body heat. Slept quite well after that.

Mina

Radiant cooling accounts for something like 7-10% of your heat loss. Tarps/flys will generally stop most of this. Assuming radiation occurs in all directions, you will save about 5% by using a tarp over you, ignoring the ground.

Convection depends on the air circulation under a tarp/fly. This is HIGHLY variable bit I figure about another 10%. Sometimes, with no wind or breeze, this is close to none. Sometimes, the wind is soo heavy you can easly add 10F or more.

A single walled tent/tarp supplys no insulation. But a double walled tent will add about an R1 to a tent, assuming a 1" air space all around. Not that for some meshes, non-noseeumm meshes, this can be a bit less, but it helps. Usually I figure 2-3 degrees per R digit, but this is by guess and by gosh. It is usually more, but this does not take into account open edges on a tent.

Exhaled heat trapped (another 2-3%,) moisture/condensation (another 1%,) and general air insulation of smaller tents (~32sqft, ~40" high) accounts for another 4-5%.

The overall heat loss to the ground is highly variable, too. Rocks, hard packed earth, and ice will be colder than snow, forest duff, or lawn, for example.

Overall, I figure about 10-15F increase due to the tent/tarp. It could be more on a still night. Best case is about 25-30F, assuming ideal conditions. But, you cannot allow for best case.

> 5-8 F warmer temperatures inside the tent than outside
Easily. Sometimes as much as 5 C (9 F).
Of course, the wind factor can be huge as well.

Cheers