You have probably heard a lot of advices about vapor barriers and vapor retarders. And maybe you have walked away, even more confused than before. The problem, I think, is that you've been told what to do "Put it on the warm-in-winter side," or "Never use one" but you haven't had the physics of what happens explained to you, and that’s why I think you didn’t understand it.
In this article, I'm not going to get into the details of vapor barriers, I'm simply going to explain what happens in a wall cavity with and without a plastic vapor barrier installed. Here is said, if you have used plastic paint inside or outside on the walls it is to be seen as vapor barriers or vapor retarders as these paints will prevent the vapor to move or even prevent the condensation inside a wall to evaporate.
Plastic on the inside
1. Hot Humid Weather
I'm writing this article because I see a lot of houses having problems with humidity on the inside or their outside walls, or even both, and you know that can't be a good thing.
Think of a day in June, just a few months ago, I saw condensation on the outside of a window...at 1 o'clock in the afternoon on a sunny day. The dew point of the outdoor air was 25.5° C. The window had a single pane of glass. They had the air conditioner running, so the indoor temperature was probably 23° or below. Humid air hits the cool surface of the window and condensation occurs.
Now, imagine that the pane of glass is actually the paint on your inside wall (a sheet of plastic). As told in one of my earlier articles, the direction of heat transfer is always from a higher temperature to a lower temperature.
If it's a typical wall, chances are good that water vapor in the outdoor air will find its way into the wall cavity, eventually finding the vapor barrier (the paint on the inside wall). If that wall allows outdoor air to infiltrate and the paint is below the dew point, condensation is the likely result. If those conditions last long enough, the condensed water will accumulate inside the wall.
The truth, though, is that the water vapor in the outdoor air is rarely the source of moisture that will make the paint and plaster to fall off the wall. More likely is that moisture from a wet foundation makes its way up into the wall by capillary action, or bulk water from leaks around openings gets into the wall cavity. The presence of an interior vapor barrier makes drying out the cavity harder to do, though. Therefore it is important to use breathable paint so the wall can dry out.
Without the vapor barrier (plastic paint), water vapor hits the wall and diffuses through to the drier (in summer) indoor air. By plastic paint, you cut off the natural drying mechanism and water that finds its way into walls can stay there longer and do a lot of damage.
2. Cold Weather
In cold weather, a vapor barrier on the interior side of a wall probably won't cause any problems. The humid air is indoors, and the dry air is outdoors. The vapor barrier still cuts off drying to the indoors, but it keeps the water vapor in the humid indoor air away from the cold surfaces inside the wall if the wall is insulated. If not the humid air inside will condense on the cold surface of the paint and cause mold.
Plastic on the outside
3. Cold Weather
Plastic paint on the outer surface of a wall in cold weather could cause problems. The humid air is indoors. Assuming there is no insulation. If water vapor diffuses or infiltrates into the wall cavity and finds the cool surface, moisture problems can occur.
Of course, you can have moisture problems here even without the exterior vapor barrier because of “the rule of material wetting”. Warm materials dry more quickly than cold materials. Therefore it is important to use KefaTherm as exterior paint, as it has insulation capacities, as it also breaks the surface tension of the water and lets it evaporate faster to the air. KefaTherm gives you dry walls.
4. Hot Humid Weather
The problem occurs with a vapor barrier when it prevents drying to the drier space. In a building with air conditioning during hot humid weather, the drier space is indoors. The humid air is outdoors. The wrong place to put a vapor barrier is on the inside because any humid air that gets into the wall cavity is blocked from drying to the inside.
If the vapor barrier is on the outside, it prevents the humid air from diffusing into the wall cavity and finding the cold surface on the other side of the cavity. So, putting a vapor barrier on the inner surface in cold weather, and one on the outer surface in hot weather would be a solution, but difficult to practice.
It's not just a climate issue
We can summarize the vapor barrier issue like this:
- A vapor barrier's job is to keep water vapor in humid air from diffusing through one side of a wall and finding a cool surface inside the wall.
- When a vapor barrier is on the side of a wall where the dry air is (i.e., outside in winter or inside in summer), moisture problems can occur.
- A vapor barrier reduces the movement of water vapor by diffusion. Holes in the vapor barrier that allow humid air through may allow a lot more water vapor into a construction than the vapor barrier is allowing trapped water to evaporate.
Many houses actually have vapor barriers both on the inside and on the outside of their walls as they are painted with plastic paint. People are painting their houses to protect against wind and weather, but as you probably can understand, the water vapor is coming in everywhere, and with vapor barriers on both sides of the wall, the water can impossible evaporate to the outside again as it is trapped inside the wall. Of cause, as mentioned above, it is more likely that the moisture is from a wet foundation making its way up into the wall by capillary action, or bulk water from leaks in roof or terraces, or leaking water or drain pipes, that lets water into the wall cavity.
Enhancing drying vs. preventing wetness
Understanding moisture is one of the most important aspects of making buildings do their jobs properly and not fail prematurely. We know now that mid-twentieth century building science incorrectly ascribed magical properties to vapor barriers. Water vapor from indoor air wasn't the source of most moisture problems. Water ingress from damages and defects on, or even missing climate shield is mostly the cause of moisture problems.
Our understanding now is that it's generally more important for walls to be able to dry than it is to block water vapor with materials like polyethylene and paint that is impermeable.
The fact that many houses have moisture problems, the argument for enhanced drying potential becomes much stronger. The products from Kefa System dries out the walls by capillary effect and keep the walls warm with the best insulation capabilities.