What is Ubakus really doing? And what is that 2/3 - 1/3 rule?

[Garald]

I've read Ubakus really uses a very simple model to predict condensation. So, what is it, and where can I read about it? What do more sophisticated models (such as the one used by WUFI, which I can't use, as I don't have Windows - just failed to run it using WINE) really do?

 

TL;DR: I would like to install outside insulation on a wall that is already insulated on the inside. I want to actually understand (the physics behind) how to make choices so as to prevent condensation - I can't just follow rules of thumb such as the "1/3-2/3 rule" and "at least 5 times as much resistance to water vapor on the inside as on the other" as they can't really be simultaneously obeyed in this sort of situation.

[SteamyTea]

The physics is quite simple.

Air is a mixture of gases. Nitrogen, oxygen, water vapour, argon and the other traces gases, carbon dioxide, methane, nitrous oxide.

 

Nitrogen and oxygen are fairly thermally stable in the earth atmospheric temperature range.

Water vapour (about 0.25%) is anything but stable. It can be a solid, a liquid or a gas. Sometimes all three at the same time (triple point).

Temperature is not the only thing that makes it change state, air pressure can as well.

This is why when you look at some more sophisticated RH and AH models they ask for air pressure.

 

Water is also strange molecule. It has different thermal properties and different temperatures, and to make matters worse, expands, rather than contracts, when between 277K and 273K, with the latter temperature being a phase change temperature.

 

Then there is the way that it likes a nucleus point to change phase on. Mineral wool gives it billions of these, so like to condense and freeze in it.

 

As water changes state, it releases a lot of energy, which can warm surrounding gases and solids. This prolongs the phase change time. 

 

It really is a messy business.

 

But there is another way to model it.

Statistically.

Build an array of phase change data points that correspond to the local RH, temperature and maybe air pressure.

Then look at the probabilities.

You will generally find that the high risk times i.e. condensation does not happen very often, or for very long.

 

It is also worth remembering that a wall has surface temperatures slightly lower than internal air temperature on the inside, and often much higher than outside air temperature on the outside where it is heated by the sun.

 

Our old mate @Ed Davies wrote and interesting bit about humidity on his website.

https://edavies.me.uk/2017/03/vapour/

[Alan Ambrose]

From memory doesn’t ubakus allow you to choose from a bunch of different moisture models? I’m guessing that a typical moisture analysis usually isn’t so marginal that the choice of moisture model means much anyway.

[MikeSharp01]

5 hours ago, SteamyTea said:

 

Our old mate @Ed Davies wrote and interesting bit about humidity on his website.

Anybody know what has become of Ed? Not seen here for some time and his last blog post is also some time back. Hope all is well.

[SteamyTea]

On 23/06/2024 at 22:04, MikeSharp01 said:

Anybody know what has become of Ed

https://edavies.me.uk/2023/10/abandoned/

 

Seems he has not posted at the other place since October 2022.

Edited Wednesday at 15:56 by SteamyTea

[Alan Ambrose]

That's sad, it looks like it was going well.

 

Re Ubakus: yes, here are the moisture models. The OP seems to have vanished, but I would encourage him to try the model options and see what difference it makes.

 

 

[Garald]

51 minutes ago, Alan Ambrose said:

That's sad, it looks like it was going well.

 

Re Ubakus: yes, here are the moisture models. The OP seems to have vanished, but I would encourage him to try the model options and see what difference it makes.

 

 

I haven't vanished!

 

At any rate, I take the DIN this-and-that models are very simple 1-D models one can do with pencil and paper?