In cold climate areas it is normal to put a vapor barrier on the warm side of the wall so that the humid interior air cannot get into the wall and condense during the winter. But you may not be aware that this vapor flow can also occur in the opposite direction, namely humid summer outdoor air may allow vapor to move toward the interior, causing moisture build-up in the wall.
There’s a well-known chain of courtyard style hotels that often uses EIFS as the cladding. These are often two- or three-story wood frame buildings using DensGlass as the sheathing. Recently, hotel owners in a southern city were having complaints about mildew smells and staining of the wall covering on exterior walls. The vinyl wall covering was also coming loose in places. The owners of this hotel and I took apart the wall from the inside and found the fiberglass cavity insulation had mildew, and the sheathing and vinyl had mildew too. There were water stains on the cavity side of the drywall and evidence of water accumulation at the bottom of the stud cavity. There was enough moisture that mushrooms were growing in the carpet.
Initially, we thought the windows were leaking but after water pressure testing we concluded the water wasn’t due to leaky windows, caulking, flashing or the air conditioners. Where was the moisture coming from?
The owner suggested that the EIFS coatings themselves might be leaking. Having the EIFS coatings leak is almost unheard of but we tested the coatings anyway, and they were just fine.
Water Vapor 101
I suggested that the moisture might be condensation coming from outdoors. I did some engineering calculations, and lo and behold it was moisture from outdoors. The vapor was flowing inward but got stopped by the vinyl wall covering, which was acting like a vapor barrier. The hot and humid outdoor summer air moisture was trying to flow towards the cool, dry air-conditioned building interior.
Water vapor is a gas and can work its way through wood, paper, paint, insulation and many other building materials. The process is slow but over time a good bit of water can be transferred. If the amount of moisture in the air is greater than its capacity at any given temperature, the dew point has been reached and condensation occurs-the vapor reverts to liquid.
EIFS coatings and insulation are partially permeable to water vapor. This is a necessary feature of the coating design so that moisture flowing in the winter does not condense on the inside of the EIFS lamina. This ruins the lamina and can cause it to debond from the foam. The partial permeability of the foam and coatings also allows the wall to slowly dry out if there are water leaks.
How much condensation there is, and where it is, and how long it’s been there, will determine whether or not the wall will be damaged. If the wall warms up again, the liquid water will become a gas again and the wall can start to dry out. Condensation also impacts insulation in the sense that reducing insulation properties can affect the cost of heating and cooling.
How to Fix this
We talked about putting a coating on the outside of the EIFS finish. The idea was to block water vapor from getting into the wall in the first place. This would work during the summer but in the winter it might cause the problem mentioned above with water backing up inside the EIFS near the lamina.
The vinyl was the culprit but the owner was insistent on keeping it for appearance purposes and ease of cleaning. We talked to the vinyl people to see if there was a type of vinyl that was more permeable, and the answer was no. The vinyl people suggested putting thousands of tiny holes in the vinyl to let moisture pass through. We made a sample and it yielded much higher permeability and the holes were essentially invisible. The problem with this approach is that the special pin that holed the vinyl was not a commercial product.
The solution ended up being to ventilate the drywall by putting a slot along the top of the inside of the wall and simply letting the vapor flow through unimpeded. This solution was simple and easy to retrofit. Since the mildew-ridden walls needed to be rebuilt anyway, we had the built-in opportunity to make this simple change.
This kind of “reverse vapor flow” can be detected using engineering calculations. Thus if you are working on a project that has prolonged periods of hot humid summer weather, work out a solution before the problem surfaces. EIFS manufacturers offer this analysis service for free as part of their technical support. There are also sophisticated real-time software simulation programs that take into account a huge number of wall design factors and determine such things as the ability to dry out if it does get damp.
This condensation problem is not limited to stud walls. It can also occur on block and concrete walls that have high vapor resistive paints on them, such as you might find in a laundry or indoor swimming pool.
The idea of coating the EIFS finish to block vapor flow has other implications, such as in cold climate areas. It is common for owners to spruce up older EIFS projects by painting. The trick is not to use a paint that acts like a vapor barrier. The EIFS manufacturers make paints that are compatible with the finishes and not particularly vapor resistive.
This problem of vapor condensing in the wall applies more to traditional barrier EIFS than to the newer EIFS with drainage. The drainage cavity tends to ventilate the wall, making it easier for the wall to dry out. However, a source of the problem is not the EIFS itself but indoor wall coverings. Thus, this condensation problem can occur in many types of wall claddings, not just EIFS.
The idea of having vinyl wall covering with lots of tiny holes was intriguing enough that the vinyl people looked into offering it as special product. It turned out the market was not large enough, so they did not commercialize it as a product. But one of the engineers at the vinyl company figured out how to do it differently. He uses what looks like a rolling pin that has hundreds of little needles sticking out of it. Once the vinyl is on the wall, you roll this perforation tool over the vinyl. Voila! It’s pretty slick. W&C