[offthehook]

I once had some licensed building inspector telling me that If I were to have over pressure in my house, mold would apear inside the walls.

Now when I look inside our heated barn with steady under pressure & high humidity levels, Ice builds up inbetween seams leading the nearby wooden areas to attract mold.

Can anyone please explain me the right principles that are over or under pressure related in hopes to avoid getting mold in my home?

i thought it was almost impossible to fully insulate a houose so that overpressure is tough.

[offthehook]

Thanks for reply BG.

Meanwhile I found the answer myself tho. It's all equally bad, under pressure, over pressure, neutral, all the same.
Where it seems to come down to is the amount of relative humidity present in the house that's causing the mold and in disregard of pressure.

[GMT]

no idea how true, but I have read that salt water kills/prevents mold. Not sure how you'd use that info, but given as the water evaporates, it will leave the salt behind, it would seem a fairly long term solution so long as there isn't enough moisture to have water collecting sufficiently to start running.

[hotrod69]

It's more about proper air movement. Make sure your attic vents aren't blocked and that there are enough of them to allow for adequate air flow.

I know this is kind of an old topic but since I am a engineer I was thinking of making some facts clear i this thread.

How to start..
Well first of you have your temperature inside your house, that temperature decides how much water your air can hold.
Example: 5*C can hold about 0,005kg H2O / kg of air and 20*C can hold about 0,015 kg H2O / kg of air. (i.e the humidity here is 100%)

Then you have the outside air which is colder and can hold even less water.
Example: -15*C the air can only hold 0,001kg H20 / kg of air.

So why care how much water the air can hold? Because if you pull warm air to a cold environment the humidity will rise, and when it reaches 100% the air will begin shed water in the form of condensation on (cold) surfaces nearby. And where you have liquid water in a construction you have a problem.

Even a humidity of 75% in constructions with aaaany form of organic material, for example wood is the absolute last threshold for mold. Never ever have more than 75% humidity in a construction.

So basically, if you overpressure a house when you have a higher temperature inside (and in most cases a higher amount of water in the air [kg/kg] than outside), you will force the excess air trough the construction rather then trough the ventilation system.
This will in almost every case (with any form of organic material) cause mold.
And the higher the overpressure, temp/humidity difference and so on the worse it will get

If you want to calculate how much water you can push trough the construction without the humidity reaching over 75% inside the walls you can use a diagram by Mollier. Google it, it will take 10minutes to learn but after that it's child play. You just follow the lines with you finger.

As an licensed building engineer, I would never ever overpressure a normal house.
Overpressure can be used for "firecells" in building and such, so not all bad, but for where people live, no way.

This is written from a tempered climate perspective, like most parts of Europe, these rules change when you have a tropical climate. As you will notice if you use the diagram by Mollier.

Hopes this can clear thing up! And if you have any questions just ask away, but do it on the boards so everyone can learn!

Originally Posted by BasementGrower

i thought it was almost impossible to fully insulate a houose so that overpressure is tough.

It is not the insulation that "tries" to keep a house airtight, it is the plastic foil that should be mounted on the inside of the insulation that should do that.

And modern houses in Europe are pretty air tight. A total leakage of 0,3liter/sq m of outside wall is normally a target now a days (at a overpressure inside the house of 50 Pa).

But this has nothing to do with the mold issue, air tightness does not matter more than if you get the construction wet, it will take longer to dry out. And that in it self is pretty bad because organic material+water+time+heat=guara nteed mold.

Hope somebody find this helpful!

[offthehook]

So basically, if you overpressure a house when you have a higher temperature inside (and in most cases a higher amount of water in the air [kg/kg] than outside), you will force the excess air trough the construction rather then trough the ventilation system.
This will in almost every case (with any form of organic material) cause mold.
And the higher the overpressure, temp/humidity difference and so on the worse it will get

So why care how much water the air can hold? Because if you pull warm air to a cold environment the humidity will rise, and when it reaches 100% the air will begin shed water in the form of condensation on (cold) surfaces nearby. And where you have liquid water in a construction you have a problem.


I noticed that the same happened when I had underpressure.

Thanks for explaining TJ.

But what do you have to say about 'underpressure' or 'neutral conditions'.

From your engineers stand point this might be interesting as the guy explaining me this once before, never could explain exactly why overpressure would be bad when I see the same happening with underpressure.

You got to know that I live in an extreem cold country.
Once up in lapland with minus 50C we had ice chunks building up under our beds made up of our own sweat.
Even though at sleeping temperature we had the regular + 18.
That kind of cold seems to make drafts of it's own that attach to humid surfaces like a claw of some gost if ya ask me.

Lapland? God morgon!

Well if I understand your situation right, you have a barn with low or no insulation and you have ice building up on the inside of the outer-walls of the barn?

This does not have a straight link to the pressure in the barn (but it is a factor)

This has more to do with that the wall is colder than the inside airs dew point. This leads to condensation on the cold wall which then freezes to ice.

The plus point in your case is the equation, organic material+water+time+heat=mold, if you have ice you have no heat, and no mold.

Constructions with low/no insulation are less perceptible to mold due to the fact that the construction can quickly dry out when the temperatures outside starts to rise. And since there is no heat inside the wall when its icy = no mold

Hope this helps! If something isnt clear, just ask! =)

Originally Posted by offthehook

But what do you have to say about 'underpressure' or 'neutral conditions'.

Underpressure is always the way to go in constructions where you care about mold or some kind of rotting.

Neutral conditions (in a whole building) should never be the aim, because it's hard to calculate the exact pressure drop in the whole of the ventilation system, the drop is what what gives you the real world extraction and intake rates.

Originally Posted by offthehook

From your engineers stand point this might be interesting as the guy explaining me this once before, never could explain exactly why overpressure would be bad when I see the same happening with underpressure.

Just to really clarify, when you overpressure a house you send WARM air trough the construction and when you underpressure the house you send COLD air trough the construction. And cold to warm is good when we are talking mold prevention . Not the other way around

As said before, this is not the way to think in tropical climates with high temp and high humidity.