Waterproofing and Vapor Proofing
March 7, 2011
The concept of waterproofing is easily understood; something either leaks or it doesn’t. Water leaks are easy to find; they always leave a trail such as a drop, trickle, stream, or occasionally, a small river. It comes as a surprise to many that because something is waterproof does not mean it is vapor proof. Water is a liquid molecule that is easily seen. Vapor on the other hand is a gas. We know when it is raining out because we clearly get wet. Humidity on the other hand (moisture vapor) can be very elusive to detect unless unusually high. When it comes to water and vapor proofing structures, the vapor part is very hard to detect and impossible without specific instruments. Both water and moisture vapor can be a very destructive force on many materials used in modern building construction.
With tighter building envelopes there is a growing need to think in terms of moisture management which includes both liquid water and water vapor. Current industry standards only address the water aspect of moisture management. Older and less energy efficient methods of building construction used in the past would allow for drying vast amounts of moisture or moisture vapor in a structure. That is no longer possible with the house wraps, insulation and other energy code requirements in place today. I am not sure how accurate or the basis of the statement but one respected building science professional stated older homes were capable of dispersing hundreds of gallons of water in vapor form, but with newer homes, even 10 can be a problem with tight building envelopes. Surprisingly, despite millions of dollars in tile-related water damages to structures every year, very little study has been done on effective moisture management for tile installation systems. Ceramic tile, given its limited use in the big picture of the building envelope, has not come under the moisture management microscope thus far. But, when you stop and consider that the average shower sees more moisture than a rain forest in a year, sooner or later we will get their attention.
The overwhelming majority of tile and related moisture management is applicable to interior structures. The need for effective waterproofing and moisture management today is much greater than in the past for reasons already mentioned. As the level of moisture management needed and methods of construction vary widely there are very few products out there that work universally in every application. Manufacturers offer many different products so that those needs can be competitively met. The current tile industry test for waterproofing products, ANSI A118.10, uses a column of water over a piece of membrane that is visually inspected after 48 hours for evidence of moisture penetration. If no trace of water is observed (after 48 hours) the product is deemed waterproof. There is no requirement under waterproofing for vapor permeability. What is permeability? It is the ability of vapor to pass through a given material. All materials, even steel are permeable. There is no such thing as an impermeable material; all building materials and variations thereof have different “perm” ratings. Under building code, a material with a perm rating of less than 1 is considered impermeable. How they arrive at that number is where things can get very technical so I will give you the short version. The test used to determine permeability is ASTM E-96. The test used for building code is ASTM E-96A. However, there are 5 versions of that test, 96A-96E, and all providing different values. To make things a little more complicated, different manufacturers test their products using the test that best suits them. For the purposes of this article, we will use the values under ASTM E96A.
Waterproofing products we typically use in the tile industry can vary widely in permeability from as little as .02 to as much as 4 and are still considered waterproof. That means they leave no visible water on the underside of the product being tested after being exposed to a column of water for several days. Vapor may pass, there is no restriction, but actual water may not. There may well be instances where we would not want any vapor (meaning minimal) penetration such as a steam shower that was in use 24/7 as vapor passage can have a cumulative effect on the wall cavity. Then again, there are instances where we want a waterproof surface but it needs to breathe, such as a tile installation over a concrete slab on grade in an area with a high water table. The variations of installation requirements can be numerous and should always be given through consideration prior to product selection. Let’s take a look at some examples.
There is no tub/shower area where the wall cavity of a structure would not benefit from no water penetration and reduced moisture vapor. Regardless of what geographical area you reside in, it is always a bad idea to drive any type of moisture into the wall cavity whether composed of masonry or wood. Bad things happen when moisture is trapped in a confined space with little to no ventilation. While backer boards are generally unaffected by water some allow for relatively large quantities of moisture retention by drawing water from the cement grout or in some cases, absorbent wall tile. Not too many weeks go by without the “I used backerboard; why is my tub and/or shower leaking” claim, falsely believing that all backer boards are waterproof.
Ideally, tile in wet areas should be installed directly over surfaces that are both waterproof and have low permeability. This can be done in several ways. Some backer boards themselves have these properties and can be waterproofed by treating the seams. It can also be accomplished through liquid and sheet membranes, even a few certain types of thinset mortars. An added benefit of stopping water at the surface is the overall humidity in the surrounding area will be reduced by not having to provide the air to dry out a larger wet mass of wall due to the thinner moisture retaining profile of the tile and membrane. Very few homes or commercial structures have means of adequate ventilation to deal with the higher levels of moisture typical in the tight energy efficient building envelopes used today. Any reduction of moisture vapor is a welcome plus to healthy air.
All showers should be properly waterproofed. Showers which have a much greater surface area than tub/showers and are typically more confining, which also means less airflow. With all the products on the market at just as many price points there is simply no reason to not have an effective moisture management system in showers. Think of a shower as a rain forest contained in a tile structure with only one means of escape, the drain. The conventional full mortar shower was a tried and true method that has worked for hundreds of years when properly constructed. Today backer board and mortar shower bases are the common products used in shower applications. Most cement backer boards recommend a vapor retarder such as 15# roofing felt or 4 mill plastic over the studs prior to application of the board. In some geographical locations this is not allowed due to exterior vapor drive in the wall assembly. This makes a good case for recommending a full moisture management system in every shower.
Steam showers have gone from a blip on the radar 10 years ago to an increasingly common installation. Steam is a vapor that will penetrate the smallest pores without a moisture management system in-place but which one to use? This is a very gray area in both waterproofing products and code. Given their relatively new popularity, there is not a long history to draw on so we must apply some basic common sense principles. All areas of a steam shower must have an appropriate moisture management system. Water vapor in a wall cavity is bad and without completely covering all surfaces with the appropriate membrane and sealing all penetrations troubles are likely. If the steam shower is somewhat of a novelty item, seeing only occasional use in a residential application, many waterproofing products with perm ratings higher than 1 may be up to the task. While there is no clear guidance yet from code or the tile industry documents, common sense says the greater the use, the less permeable the system should be to avoid damage to the surrounding structure. This limits the amount of liquid or trowel applied systems up to the task. Many do not recommend their products for commercial application which anticipates daily use for long periods of time. Actual anticipated use should be carefully and realistically evaluated. Make your choices wisely when working on steam showers in any setting.
Throughout this article I have replaced the terms waterproofing with moisture management. When using this term we are correctly indicating that moisture in both liquid and vapor form should be managed. Once you can create the thought process in your customers that homes should be protected from not only leaks but excessive moisture vapor, you will find many will embrace or at a minimum be more receptive to the associated additional cost of moisture management. If we continually refer to moisture management as waterproofing you will most assuredly meet continued resistance in trying to explain the benefits of a drier tile installation in a tight building envelope. In the current market, a little extra income while providing your client with a healthier building is a win-win opportunity.