Comment by Helmut10001 | Hacker News Reader

Comment by Helmut10001 | Hacker News Reader

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I do not have one at hand, but you should find this in almost any guide. It is not complicated: If you insulate 3 walls and leave one wall untouched - this wall will always be the coldest. That means the water contained in your air will condensate always at this wall (because it is the local minimum), resulting in mould and mildew on this wall over time.

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.

But your logic seems to indicate that only one wall will have condensation instead of 4. It’s still an improvement no?

You're concentrating 4 wall's worth of condensation onto just one wall. Instead of being spread out, all the moisture gets on the one wall.

No, that is not how condensation works, otherwise you would have automatically more condensation on the wall in a larger room. The amount of condensation on the walls is not dependent on the size of the room or the fact that it condensates on the other walls. It depends on air humidity, air temperature, wall temperature and finally, very important, wall humidity. Water will not keep on accumulating on the wall, it will reach an equilibrium at some point, when the wall itself has become moist. The fact there is condensation or not on the other walls has negligible impact on total air humidity, most of all in larger rooms. And that's the only factor out of the 4 I could imagine being impacted by condensation (or not) on the other walls.

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.

The point is it won't reach an equilibrium. The outside of the house changes temperature with the day night cycle. The one uninsulated wall will be closer to the outside temperature than the other walls.

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.

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