The points in favor of a internal thermal insulation are that: the intervention is economic; the other condominiums do not need to agree; brings some benefit in terms of lower consumption for heating. The points against, however, are many more: the thermal bridges remain as they are; if there is mold on the walls, it stays where it is (indeed it tends to get worse) and just avoid seeing it; the risk of condensation increases; the walkable surface decreases; that saving on gas does not necessarily come back in terms of comfort.
Therefore? So, and let's get to the point, theinternal thermal insulation it is an intervention to be considered only if there is no alternative, that is, only if it is not possible to increase the insulation of the walls with an intervention from the outside - the classic thermal coat - or with the insugfflaggio of insulating material in the cavities. The latter solution, which in turn has numerous critical issues and is not decisive, when possible may be preferable to upholstering the house with insulating panels.
Having said what must be said, to bet on a internal thermal insulation one must have solid and unassailable reasons. But sometimes they exist. One may be that the building you live in is subject to architectural constraints and cannot be isolated from the outside with the external insulation system. Another is that there is no agreement between condominiums and rather than zero insulation, an internal coating is better. Other reasons frankly do not come to mind.
Given at this point that you have passed the decision-making phase, consider the intervention of internal thermal insulation like a major surgery: you are acting on the skin of your home, from the inside too, and everything you do will somehow modify - for better or worse - the internal atmosphere in terms of thermohygrometric balance and emissions, and with it living comfort.
Choose natural, non-emissive, hygroscopic and breathable insulators. Better a wool and cotton tank top or a synthetic one? The reasoning to be done is the same and considering that usually a internal thermal insulation it is not a huge expense, it is not on materials that economy must be made. Natural blond cork, sheep's wool, vegetable fibers etc. they can also cost a little more per square meter but it's worth it.
An important feature in an insulating material forinternal thermal insulation is its hygroscopicity, that is, the ability to absorb the water molecules present in the surroundings without compromising the insulating properties. Another is that it is breathable, so as not to block the exchange of air, and a third that can be applied without the need for a vapor barrier. If the characteristics of the environment make the vapor barrier essential, make sure it is intact and not pierce it with the shelf plugs.
The technique. Get out of your head that accomplish a good one internal thermal insulation is an ordinary do-it-yourself operation. This does not mean that you cannot do it alone, but that if you decide to do it you must be good and have some technical knowledge. Insulating from the inside does not mean gluing insulating panels to the walls, an operation which, in addition to being of little use, would worsen the living comfort. If anything, the effective system is that of the internal counterwall, which also exploits the insulating properties of the air and allows a good amount of hygroscopic and transpiring insulation to be inserted into the cavity.