Since I live in a rather cold part of the world (South Chile) I often get bombarded by marketing to make my house/apartment better insulated. There are all kinds of cladding and coatings to improve house insulation. There are also "technologies" like passive house (german: passivhaus) design, which say that even if you generate a small amount of heat, if you have a well insulated house, you can enjoy that heat for hours. Same goes for double pane windows- they are recommended as better insulation as well. Instead of spending money on generating heat (gas, electricity), they recommend you spend money on insulating the house better.
Most of these recommendations are to reduce the conduction loss of heat (measured by R-value of materials used) in the house. However, my experience and experimentation tell me that it is the convection losses which determine the average room temperature (more specifically, the average air temperature in the room, which is what we care about, in feeling hot or cold-we as humans come mostly in contact with the air in the room), and conduction losses are a minor part of the heat loss from a heated room. Spending money on better insulation to reduce conduction losses of the house is a wastage of money. You are better off investing that money in generating more energy inside the house. Double pane windows are also a waste of money.
To reduce convection loss (the loss due to direct air draft or air flow from the outside), sealing (or more technically, weatherstripping) the crevices and gaps in the doors and windows, a cheap solution, is enough of a barrier, and is good enough to reduce significantly the overall heat loss from a heated room or house. Beyond that, you can't do much in terms of insulation; and the only way to make the room hotter is to generate more heat.
Observations to support these conclusions:
1. Consider a room in a house or an apartment in the winter.
[temperatures are approximate]
Outside air temperature=10C
Room has just one electric heater of 2000W.
If I divide this room into two identical parts using even a relatively thin (3 cm wide) wooden wall or partition (as is the case in many houses and apartments), I have found from personal experimentation that the part of the room where the heater is located is much warmer than the other part which does not have the heater.
Before partition: Steady state average temperature in the room = 11C when heater is off
Before partition: Steady state average temperature in the room = 20C when heater is on
After partition: Let "A" be the part of the room with the heater; and B the part without the heater. Experimentation shows that A reaches an average temperature of 22C, while "B" has average temperature of 11C.
Because the average temperature in part "B" is much lower than in part "A", and the temperature in part "A" is about the same as it was before putting the partition, and that of part "B" the same as the outside temperature, it is clear that the thin wooden wall served as wonderful insulation for maintaining the temperature in part "A". If a substantial part of heat was going through the wooden partition wall from part "A" to part "B", the temperature in "B" would be significantly closer to "A", and not so close to the outside temperature.
Furthermore, since the wooden wall is quite thin, it does not block conduction loss-it only blocks convection loss from part "A" to "B". It follows that part "A" must be losing most of the heat due to convection to part "B" before the wall was erected.
Looking at it another way:
The entire room has a standard brick (or concrete) wall around it, which is much thicker than the wooden partition inserted to divide the room in two parts. If the steady state temperature in the two parts "A" and "B" was primarily determined by the outer brick wall's insulation, it would not cause such a dramatic difference between their temperatures in these parts. It seems that the outer brick or concrete wall acts more as a block against convection loss, rather than conduction loss. If the brick (or concrete) wall's thickness were reduced, or if it were replaced by a wooden wall, I surmise that it would cause no difference in the average temperature in parts "A" and "B", or the temperature of the undivided room.
If I changed the heater settings (1000W instead of 2000W), or used another heater (4000W), the general result doesn't change.
I get additional confirmation from this from the fact that people in cold places often heat just one room. When they do this, the other rooms of the house or apartment are quite cold-which shows that the thin room-room walls, normally of wood, serve well to keep the heat completely inside the heated room.
Igloos also work for the same reason-they block the convection loss from the inside of the igloo. Igloo walls are made of snow-and improving their insulation by putting more insulation cladding of funky materials is clearly ridiculous. Improving the insulation of a brick or concrete wall, which are quite thick anyway, by the same funky materials is equally silly.
If conduction losses were more important than convection losses-opening a window in a well insulated house with thick brick walls and loads of insulation claddings would not lead to heat loss from the room. But experience tells us that that is silly-that the air draft from the open window will be felt right away and cause a substantial drop on the value of the average room temperature; and this is another way to show that convection losses dominate the loss of heat from the house.
The average temperature inside a room depends only on the size of the cavity (total air volume) and the heater power, if most of the convection losses are blocked. It does not depend on the wall type, it's thickness, it's insulation properties, or the insulation cladding on it.
The comments section on this page on greenbuildingadvisor.com has good data from people in the home building/home maintenance industry, and pretty much agrees with the conclusions I presented here.
2. If insulation worked so well to keep the heat in the house, from symmetry, it should keep the heat from getting into a similar house in the summer. But we never hear of people spending loads of money in warm countries on insulation to keep the "air-conditioning" more efficient. No one thought of putting thicker walls or massive cladding on the walls in warm countries to increase the efficiency of their air-conditioners. No one recommended double pane windows either. There are no passive house designs for hot countries. Insulation is insulation-and from symmetry, if it works well in cold weather, it should work equally well in warm weather. From practical experience in warm weather, people have realized that they just need to increase the air-conditioning power when it gets hotter outside. They support spending money on generation, and not better insulation in hot weather.
In summary-the average room temperature is determined by convection losses. Losses due to conduction are nor a major factor is deciding the average temperature inside the room.
This can be extended to the uselessness of double pane windows. Double pane windows are an unnecessary expense. The important thing is to prevent convection loss; and a single pane window does as good a job as a double pane window to avoid convection loss. To minimize convection loss, you do not leave any air holes or small openings in the windows, doors, etc. That should be sufficient to get rid of all "air flow" or "draft" loss or convection loss of heat.
Conduction losses, which is what all these technologies like insulation cladding, passive house etc. are trying to prevent, are not very important. A wall which is even a few centimeters thick to prevent direct contact with the outside air, to reduce convection loss, is all you need.
Technically, conduction losses and materials are specified by their R-value. From this article, it should be clear that R-values don't matter much; they are worrying about a minor cause of heat loss from a house or apartment when talking about R-values and choosing materials based on that. Fixing air leaks in the house or a apartment is the only thing you need to do to reduce your heating (or air-conditioning) costs.
The recent Grenfell Tower fire in London shows that insulation cladding can cause other problems-it appears that the insulating materials used outside the concrete were highly flammable, causing an increase in the intensity of the fire. Many organic or petroleum based fillings and layers are touted as insulation for houses (including styrofoam), and putting a material which is not useful in insulation, as I have shown in this article, which in addition causes other problems like increase in the risk of fire, or of emission of toxic fumes sounds like a very bad idea to me.