That's essentially how 4 season tents work, the tent is full nylon (no mesh) and the fly is full and basically acts as a second tent, creating that air gap.
some 4 season tents even have little foamies on them that help keep the gap, preventing snow from squishing the fly against the tent and eliminating that insulating volume: Putting a tent inside another tent would work, but it would work better if the tents were closer in size, then you wouldn't have a large of a volume of air to try and warm.
If you really want to insulate your tent, try putting a blanket or old sleeping bag in between the tent and the fly.
In a pinch you can use dry grass or leaves, that's what Siberians do, they make a shelter inside of a shelter and fill the gap in-between with grass.
outdoors.stackexchange.com/users/4148/ShemSeger
I don't know what your situation is exactly, but my advice would be to instead invest in a good down sleeping-bag .
Or bring someone to cuddle with.
A good sleeping bag will provide that pocket of still air you are looking for and is going to be much more compact and lightweight.
For about a year and a half
while at college I lived in a three-season two-person tent which was inside a cloth-teepee.
I only had a 20 degree (Fahrenheit) sleeping bag
and it got pretty cold
(I slept in my boots some winter nights).
I perceived the main benefit to be peace of mind that my stuff was protected a little more in case the elements misbehaved while I was gone.
It worked for me in my situation, and, without having collected any data to quantify, felt more comfortable than just the two-person tent, but I am not sure you would want to do this where you have to carry both tents.
outdoors.stackexchange.com/users/1957/DudeOnRock
Tent in a tent will not really work.
Firstly the chance of getting two tents that are designed to work this way is essentially nil.
You will end up having to find two tents that are compatible - one larger than the other.
The smaller tent must still be usable, and the larger one so large that it will not touch the smaller one anywhere.
Secondly you're going to have issues with condensation.
If you are looking to create an air-pocket, you will have to zip or close up all the ventilation holes.
This will create an environment in which you will have a lot of condensation on the inside tent.
Like others suggested: Use a decent sleeping bag (and liner) to get the warmth you need, and a good-enough tent to keep you dry and out of the wind and free of bugs.
outdoors.stackexchange.com/users/8248/Johan
A tents job is not insulation.
Your tents job is to provide shelter from wind, etc.
Heat will (and should) pass though tent membranes very eaily.
Therefore adding a tent inside a tent will add practically no extra insulation and therfore no benefit.
The outer tent will protect you from wind/rain and provide shelter.
The inner tent is doing nothing.
outdoors.stackexchange.com/users/None/
Among the problems with this idea is that pole ends from the inner tent can damage the floor of the outer tent.
If you need to be warmer, augment your sleeping gear with an inexpensive blanket or two.
Check around at used clothing stores or buy new.
outdoors.stackexchange.com/users/11340/Pooneil
Makes a huge difference.
I did it this weekend.
The inner tent was very comfortable wearing long underwear, even outside the sleeping bag.
The outer tent was cold and drafty.
Outside was 30 degrees.
outdoors.stackexchange.com/users/15059/Brian
Yes.
I recently spent over a year living off-grid and used a smaller tent inside a larger one to conserve heat in the smaller one.
The inner tent was my sleeping area while the extra space inside the outer tent was used for storing non-essential gear and items which needed to be protected from exposure to the elements.
For short camping trips, using 2 tents is not very practical, and you'd probably do better to just put a blanket or other insulating cover over a single tent.
You will need to use some sort of barrier between the tent poles and the floor of the larger tent, such as bits of cloth or even styrofoam pressed over the pole tips.
I used a tarp with an emergency blanket laid over it and set up on top of the blanket.
The outer tent does not provide a great deal of insulation, but it does reduce air-flow between the outside world and the smaller tent.
This can greatly reduce heat loss, especially on a wet or windy day.
Since your body puts off heat naturally and the inner tent retains heat more efficiently, it can become toasty inside the inner tent.
If you have a camp heater, it can be used in the outer tent to keep temperatures up without making the inner tent stuffy or over-warm.
Another trick for conserving heat is to put a blanket on the floor of the tent (even if you are only using a single tent) to act as a barrier between the ground and your sleeping space.
outdoors.stackexchange.com/users/16603/Roger Golden
I know that a pocket of still air is an excellent insulator.   Would placing a tent inside of another tent provide any substantive benefits to warmth? For a concrete example, consider placing a small two person dome tent inside of a much larger 6 or 8 person family tent.  I'm interested in comparing tent alone / tent with fly / tent in tent.
Tent inside of a tent - any improvement in warmth?
outdoors.stackexchange.com
<tents><insulation>
outdoors.stackexchange.com/questions/11214
outdoors.stackexchange.com/users/6282/J Kimball
A tent in a tent will not really work as a tent's job is  not  insulation, but to provide shelter from wind, bugs, etc. If you need to be warmer, augment your sleeping gear with an inexpensive blanket or two,  invest in a good sleeping-bag and liner and consider putting a blanket on the floor of the tent to act as a barrier between the ground and your sleeping space. The benefit and practicality of using two tents is disputed. Blankets, decent sleeping bags and liners help with keeping warm.
Some people report that this works well, others disagree. Blankets, decent sleeping bags and liners help with keeping warm.