Cottage 1830 diffusion open

[oliven1]

If you have a dew point in the wall with that definition, then it is seriously misconstructed.

What?
You almost always have a dew point somewhere in the wall, that's why you have a plastic in the wall.
Why do you think otherwise you have plastic in the wall?

 

[Daniel 109]

To avoid condensation in the wall. With that definition, you no longer have a dew point in the wall.

 

[oliven1]

In order to avoid condensation in the wall. With that definition, you no longer have a dew point in the wall.

I'm afraid you haven't quite grasped this. You always have a dew point somewhere in the wall (with a few very rare exceptions) but a normally insulated house always has a dew point in the middle of the wall.

Then, of course, one should have the dew point on the right side of the vapor barrier. The dew point doesn't disappear just because you have a vapor barrier.

 

[Daniel 109]

You mean just as I wrote in post 13. A definition that was not accepted then.

 

[Plupp 405730]

I would say that the dew point disappears with the plastic because if it didn't, the wall would become damp and eventually rot, the entire function of the plastic is to prevent the humid indoor air from migrating into the wall. People often talk about a zero point inside an exterior wall where the cold and warm parts meet, preferably somewhere in the middle of the construction, where you naturally don't want condensation.

 

[oliven1]

You mean just like I wrote in post 13. A definition that was not accepted then.

you wrote in post 13 the following:
"When talking about the dew point in a wall, it usually refers to the place where the temperature corresponds to the indoor air's dew point."

which is an incomprehensible text and not at all what I said in post 18, how do you mean it's the same?

 

[oliven1]

I would say that the dew point disappears with the plastic because if it didn't, the wall would become damp and eventually rot away. The whole function of the plastic is to stop the moist indoor air from migrating into the wall.
One usually talks about a zero point inside an exterior wall where the cold and warm parts meet, preferably somewhere in the middle of the construction; you don't want condensation there for natural reasons.

No, you will always have a dew point in the wall, it's pure physics.
Then it's our task to design a wall where moist air never reaches the dew point.
Normally, one utilizes the vapor barrier for this.

 

[Plupp 405730]

no, you will always have a dew point in the wall, that is pure physics.
then it is our task to design a wall where moist air never reaches the dew point.
Normally, you use the vapor barrier for this.

Have you read what I wrote?

 

[[old rusty]]

you wrote the following in post 13:
"When talking about the dew point in a wall, it's usually referring to the location where the temperature corresponds to the indoor air's dew point."

which is an incomprehensible text and not at all what I said in post 18, how do you mean it is the same?

I would say that Daniel's post describes well what is usually referred to as the dew point.

Then perhaps more words would have been used if one wants to educate someone who has no idea what the dew point is.

 

[oliven1]

I would say that Daniel's post describes well what is commonly referred to as the dew point.

Then perhaps more words would have been used if one wants to educate someone who has no idea what dew point is.

but no, you can't say that because it can just as well be the outside air.

The dew point occurs at the temperature when the air simply cannot hold the moisture as water vapor and instead releases it as free water (condensation).
It happens when warm air is cooled down and then cannot hold as much moisture.

 

[oliven1]

Have you read what I wrote?

understood that you wrote that the dew point disappears just because you install a vapor barrier

 

[Plupp 405730]

The plastic in the wall prevents the moist warm indoor air from reaching the cold part of the construction. The term dew point is not used inside the wall; instead, one can talk about the zero point where warm and cold meet. Simply put, the dew point is the temperature air must be cooled down to for the water in the air to turn from vapor to its liquid form: water! When the cooling occurs on a surface, water droplets are formed in the form of dew. This is prevented with the construction plastic, and thus there is no dew point left inside the plastic.

https://byggipedia.se/byggteknik/stomkomplettering/yttervaggar-och-funktionskrav/

 

[[old rusty]]

but no, you can't say that because it could just as well be the outside air.

the dew point occurs at the temperature where the air simply cannot hold the moisture in the form of water vapor and instead releases it as free water (condensation).
happens when warm air is cooled and it then cannot hold as much moisture.

Absolutely.
It can be warm and humid outside, which then causes condensation in the wall, physics doesn't care which.

But isn't that how we build in Sweden? More in hotter latitudes with frequent AC use where you put plastic on the outside of the wall.

 

[oliven1]

Absolutely.
It can be warm and humid outside which then causes condensation in the wall, the physics doesn't care.

But isn't that usually not how we build in Sweden? Rather, it's in warmer latitudes with frequent AC use that plastic is placed on the outside of the wall.

no, reverse moisture migration is something considered in every design.
many foundations have problems with this.

 

[oliven1]

The plastic in the wall prevents the moist warm indoor air from reaching the cold part of the construction.
One doesn't use the word dew point inside the wall; instead, one can talk about the null point where warm and cold meet.
Simply put, the dew point is the temperature to which air must be cooled for the water in the air to transform from vapor to its liquid form: water! When the cooling occurs on a surface, water droplets in the form of dew appear. This is prevented with construction plastic, and thereby there is no longer a dew point inside the plastic.

[link]

One always talks about the dew point when it comes to walls, why wouldn't one do that?
Just because you install a vapor barrier doesn't mean the dew point clearly disappears.
All that happens is that you prevent moisture from reaching the dew point, but claiming it would disappear is completely incorrect.