negative pressure vs vapor pressure

[vato]

I'm designing a wall and roof envelope for high interior/low exterior humidity, high interior/low exterior temperatures.
Mech Eng's plan to create .05"wc neg pressure inside.
I was attempting to use fiberglass insulation with vapor barrier on interior to save money.
I'm now considering sprayed in closed cell insulation which would create a vapor barrier on both sides of the wall and
cover the dew point and either direction of vapor pressure.
I did ask the ME for wall insulation/vapor barrier recommendations, but he defers to the architect and this seems like an ME question.
What magnitude of negative pressure reverses my vapor pressure?
thanks for any thoughts on this

 

[Compositepro]

What you describe does not seem unusual. A good vapor barrier on the inside should be all you need. However, be careful to insure there is no unintendedvapor barrier on the out side.

 

[vato]

It's more like 70% humidity on the interior 0% exterior
70deg int/0deg exterior.

 

[willard3]

The blind leading the blind

 

[vato]

Thanks willard3. I guess if you're not blind, then you can answer the question.
Please lead us now.

 

[willard3]

First, find out the difference between vapor pressure and air pressure.

 

[LittleInch]

I'm afraid I'm not following the question. Negative pressure reverses my vapour pressure??

If you mean at what pressure less than atmospheric will the vapour pressure of water reduce, it will be a lot lot more than 0.05"wc, Can you explain the question a bit better and put some numbers in it please.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[dbill74]

Vapor pressure vs Negative pressure: although similar and somewhat related, are quite different. In simple terms, vapor pressure is a partial pressure or the amount of pressure exerted by the vapor in the air. Negative pressure is the total pressure (vapor + dry air) and is less than atmospheric (a vacuum).

One thing of note I find curious is that the building is being designed with an overall negative pressurization.

 

[vato]

Thanks dbill,
The building is a MJ grow op so odor control is crucial. This way all exhaust is filtered all the time. If a door is opened, air goes in hopefully.

I think I better understand the relationship between the two pressures now, thanks. Maybe negative pressure moves the location and/or temp of the dew point, a very very tiny amount? Seems like the neg pressure could, if large enough, affect the temp that condensation would occur, but it would
make the door impossible to open.

 

[dbill74]

In this context 'negative pressure' is a relative term and needs to further defined to identify negative to something else. In this case the inside of the building is negative to the outdoors. Reading my previous post I did not explain that properly/clearly. It is dealing with 'total pressures' still.

What this means practically is that outdoor air is going to try to get into the building through cracks and seams and any other way it can (this is known as infiltration). Having a negative building will not directly impact the temperature in the building although infiltration air when it mixes with the indoor air can. How this interaction works is something you should leave to the project ME, it is his job to take that into consideration.

When a building is not properly balanced and the pressure differential between the indoors and outdoors is sufficiently high, I have seen doors kept open (often accompanied with a whistle) or are hard to close. As for your door, with a pressure differential of 0.05" w.c. you will not have any problems opening/closing doors. This again is something that your ME is being paid to be responsible for.

You talked about vapor barrier location, this is something the architect details, along with framing and insulation. I'll have to get back to you regarding having the vapor barrier on the inside or outside, but one thing is certain, you only want one vapor barrier. If you have multiple vapor barriers as you mention in your OP, you will get moisture trapped between the barriers and you will have mold problems.

 

[Compositepro]

The vapor barrier always has to be on the "hot" side to avoid condensation. Having the differential pressure causing infiltration of "dry" rather than "humid" air is a good thing.

This has been posted before and is very relevant (adventures in building science):

https://www.youtube.com/watch?v=rkfAcWpOYAA