Now, there are all kinds of solutions and they all have caveats. As someone said here, old houses are build so that walls are semi-permeble, meaning that heat constantly wanders to the outside. That also means that the dew point is pretty far towards the outside in winter - water can get out of the house. If you insulate the inner wall, the dew point will move more towards the inside, meaning your walls will get wet and water will take longer to move outside. If you insulate the outside, water cannot get out at all - you must find another solution to get the moisture out of the house. It gets more complicated the more you dive into this topic.
Hence, I still do not understand the logic why isolating only a subset of walls instead of all of them would be a problem.
As I said, I had an energy efficiency specialist at my home which said that isolating 3 walls would be 75% as efficient as isolating 4 and totally recommended. So far, I have not seen any solid logic supporting the fact that isolating only 3 walls instead of 4 would *increase* the risk of mold compared to either, 4-walls all insulated, or, zero walls insulated.
As the house cools at the end of the day the uninsulated wall will be colder than the other walls and if it's below the dew point condensation will form on it. If the temperature differential is high enough in comparison to difference in insulation between the walls then enough water will leave the air to keep the dew point below the temperature of the insulated walls and so that one wall will collect the majority of moisture (at 30C a meter cubed of saturated air carries about 30mls of water so yes condensation on a surface can heavily impact humidity). Once a wall becomes moist it will still acumulate water if it's below the dew point.
You likely will have seen real life examples of this when you look at single pane windows in otherwise insulated houses and seen them fog up/ have condensate form on them while the walls around them remain dry (if you haven't seen this but have been to houses with single pane glass in your area then condensation won't be an issue where you live).
As to your 75% efficiency point it will in fact have a lower efficiency as heat transfer increases with temperature differential. Since the room has a higher temperature differential with the outside the less insulated wall will be faster at transfering heat.
That is not to say that insulating 3 walls is a bad idea, it will almost certainly improve a rooms U value, but the worse the insulation on the remaining original wall, ceiling, and floor the less difference the 3 walls will make to the rooms U value and thus how quickly the room transfers heat.
Warm/er air can hold more moisture than cold/cooler air. If part of the house is warm, and you happen to cook and/or shower in it, it will have a high moisture content.
But water vapour diffuses through-out an enclosure. So that water in a gaseous phase will equally spread everywhere (eventually).
When it reaches the uninsulated part/s of the structure, the air is colder and so can't hold as much moisture leading to condensation. Get lots of condensation, and you have an environment that is suitable for mold growth (which can then release spores in the air and mess up indoor air quality (IQA)). Not to mention that water is a universal solvent, and so will turn your structure (stone, masonry, concrete, wood) to mush over time.
The place of condensation will most likely be a surface that is a large temperature change: so in the winter your inside is warm, and your uninsulated outside walls are cold.
Today, it's generally solved by having an airtight sheet (plastic) between the insulation and the inner wall. Sometimes placed inside the insulation. This ensures a temperature gradient that will not cause condensation inside the house.
You'll generally put a sheet that's porous but not airtight outside the insulation, to reduce heat loss due to convection and also provide an extra layer of defense against moisture damage due to wind-blown rain.
This technique necessitates good ventilation (often powered) if it's used throughout the entire house. This is to avoid saturating the inside air with humidity due to breathing and other activity, itself leading to the condensation you're trying to avoid. It can sometimes be just on e.g. floors and ceilings. Then, the rest of the structure might still be leaky enough that mostly passive ventilation will do the trick.
Older houses handle the problem by just being so leaky that condensation won't be a problem.
That said, insulation is still the right answer, but it ain't cheap. OP should probably tear down that beautiful old house for something more modern.
Hotter air has more energy and thus will naturally "flow" towards cooler areas with less energy until equilibrium is reached.
In this example, the higher temperature air holds more moisture. That moisture will be carried along and dispersed towards the cooler wall, where it could condensate.
But a wall will stop condensating water when reaching a certain degree of humidity. So I’m not entirely convinced one can simply divide the amount of humidity in the air by the amount of uninsulated walls (or sqm).
The air will not have more moisture, but rather all the moisture problems will be towards/on the uninsulated wall.
> But a wall will stop condensating water when reaching a certain degree of humidity. So I’m not entirely convinced one can simply divide the amount of humidity in the air by the amount of uninsulated walls (or sqm).
Condensation will never stop on a cold wall until all the moisture in the air is removed. If you boil a pot of water the air will fill with moisture, and the entire house will have a high humidity. But because one particular wall is cold because of a lack of insulation the condensation (water going from vapour to liquid phase) will be concentrated on that particular wall.
The condensation will continue to occur as long as you continue to add moisture to the air via cooking and bathing (hot showers, baths).
That's the bit I am missing here, it just does not add up, condensation is not something that simply keeps accumulating water on a wall if it's cold enough, there's a saturation at some point. The question is, how fast is this saturation reached? If it's fast, then isolating one out of 3 walls may not create additional moisture on the non-insulated wall.
Sorry, I hope I don't come over as being stubborn, I'm genuinely interested in insulating (I have to do it at some point), I just like to understand things and so far I do not understand why isolating 3 walls out of 4 would generate additional moisture on the remaining wall.
' I do not understand why isolating 3 walls out of 4 would generate additional moisture on the remaining wall.'
you gey problems with mold way before you are anywhere near 'the physical limir of condensarion'. You dont want to be near it.
The question is not whether the amount of consensation will double or quadruple. It is weather you will have mould problems. If your wal starts getting wet, you will have mould peoblems.