Understanding Vapor Diffusion
paul_schertz
| Posted in General Questions on
Can someone help me understand one aspect of moisture diffusion?
Moisture diffuses from an area of high concentration (high vapor pressure) to an area of low concentration. This is typically from inside the home to outside. What occurs when the outside relative humidity is at 100% even though its vapor pressure is lower than that inside the home? Does the diffusion stop or does it continue and just condense on the outside surface? My intuition is failing me on this one.
Thanks
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"This is typically from inside the home to outside"
Those are bold words! The water vapor content of air (humidity), inside and outside of a house varies greatly by location. They way it moves, is as you've stated, from higher concentration to lower concentration. The resistance to that movement, is essentially the perm rating listed on all the building products.
It stops when equilibrium is reached. That doesn't mean it has to condense on either surface.
The situation you are describing may occur when it's colder outside than inside. Water vapor migrates from higher vapor pressure to lower vapor pressure, even where there is an RH gradient in the opposite direction. At some point in the assembly condensation will occur; then water transport is governed by sorption processes if it is an organic material such as wood. Fortunately, a well-placed vapor retarder will largely prevent this. An interior vapor retarder is recommended for northern climates.
If you ran a humidifier (or say, boiled a pot of water) in a 'room at 100% RH' would the humidifier stop pushing out vapor? Would the boiling water not convert to gas?
Or, would instead the net rate of condensation in the system increase proportionally while evaporation at the source continues in accordance with local temperature and vapor pressure?
Then consider that the house in your scenario is essentially a humidifier.
This scenario could not exist without a temperature differential between the two environments.
[said slightly differently, the system is on a balance, and although on net it may be in equilibrium at the exterior face of the sheathing (saturated), there can still be flows (drives) from areas of evaporation to areas of condensation (warmer to colder). In this case they're happening from inside the house to outside.]
If the temperature on one side of a wall is below the dew point on the other side of the wall, there is a risk of condensation occurring within the wall.
Generally the flow of moisture in a wall is from wetter to drier, and from warmer to cooler. In most situations the wetter side is also the warmer side. You want to construct the wall so that the warm, wet side has a vapor barrier and the cold, dry side is vapor open so that any moisture that gets into it will be driven out. In a heating-dominated climate the barrier goes on the inside and in a cooling-dominated climate it goes on the outside.
The question is interesting because I don't think it IS generally intuitive.
Part of the puzzle might be that there's often a notion held that condensation is an absolute event. It's easy to think that dew-point creates a sort of 'cap' against which no further vapor movement will occur—as if any object below dew-point is a black hole towards which all water will move towards. But we must remember that vapor condensing on a surface doesn't happen instantaneously (it is a rate) nor does it completely stop all water molecules from escaping (maybe at absolute zero?).
Would a 40F object placed in a room with air at an average dew-point of 41F collect condensation at the same RATE as a -40F object? Of course not, yet they both have net accumulation (flow from a gas in the air to a liquid on the surface). But the rates will be very different and hints that vapor may even migrate from one towards the other. Molecules are always coming and going and the phenomenon of condensation is one of net flows. The rate of H2O molecules leaving a colder object will be lower than that of a warmer object, even if they both are below dew-point.
We can use this concept to see that drives will still occur even with condensation events within a wall.
Let's say half-way through a wall, the materials are at dew-point of the adjacent air. More H2O molecules will be staying then going, but still some will leave. Which way do they exit? Well of course they go every which way, but the net flow will be towards the lower vapor pressure (which is created by the colder temperatures further out in the wall).
If condensation occurs at a fast enough rate that the adjacent air loses enough vapor (without being replenished) to then have a lower VP then even the outside air, then the drive could be towards that object from both directions. But this wouldn't be common within a wall (if it did happen it would be transitory, like after a cold night with a suddenly warm and humid day).
If you placed an AC coil inside your wall, then it may be common.
Your two replies are very helpful. You seemed to understand my intuition block. Rate is key.
I'm going to mull that over some more - sometime it takes a while for my intuition to adapt.
Thanks
Probably any discussion of moisture diffusion should include the fact that air movement is far (perhaps 100X) more significant. Including air from the interior, into the cavity and then returning to the interior (interior side air barriers are important). Even with fantastic exterior side air sealing, you may have wind buffeting causing 10X more moisture movement into the wall than diffusion.