Water Proof verses Vapor Proof

Discussion in 'Tutorials' started by jadnashua, Jan 30, 2015.

  1. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Joined:
    Sep 2, 2004
    Occupation:
    Retired Systems engineer for defense industry.
    Location:
    New England
    Water and a shower can bring happiness and financial disaster…it must be managed properly to avoid the disaster part. Water is somewhat unique in the environment we live in: it can and does exist in three forms - solid, liquid, and gas (ignoring plasma state for those that may care). Hopefully, you’ll not see solid in your shower! But, it is still a possibility in the walls if vapor is not managed properly and I’ll try to explain.

    First, you must realize that there is a difference between waterproof and vapor proof. Think about a jacket with Gore-Tex™ in it…it does not let LIQUID water through it (assuming it is a quality product, assembled well – hopefully, like your shower!), but it is specifically designed to let water VAPOR through it. It does that by using one of the characteristics of a water molecule – liquid water has covalent bonds that tend to cause them to stick together – think beads of water on your freshly waxed car; they ‘look’ like a bigger molecule than they actually are because lots of them are bound together. Water vapor, on the other hand, has gained enough energy to break the liquid water molecules apart from their ‘clump’, and is a single molecule, not stuck together with other water molecules. It is smaller, and can pass through holes in the Gore-Tex™.
    The walls and pan of your shower have the same issues. Gravity allows you to get by with water resistant walls made of materials that are not degraded by moisture, a shower pan requires some means of waterproofing. The hassle is that since moisture can get into walls when constructed in this manner, and the more common means of construction does not have walls that are vapor proof, that moisture can get into the walls, potentially condense, and create problems. This is the reason why when using say cbu on the walls with tile on top, you want a vapor barrier behind it to keep that moisture from penetrating and accumulating inside of the walls and causing damage.

    Now, let’s consider the use of a surface waterproofing – they come in two types: liquid and fabric sheet goods. Lots of examples of these two types. Companies that make liquid include: Custom Building Products, Laticrete, Mapei. Companies that make sheet goods include: Laticrete, Schluter, Noble. Those listings are not complete. As a general class, the liquid membranes are considered waterproof, but not vapor proof. As to sheet membranes, those generally are both waterproof AND vapor proof. Now, there are limits. In the building industry, the ability of a material to pass water vapor is measured in perms, and anything less than one is considered vapor proof (but can still pass vapor). When it comes to showers, you have two general classes: conventional showers and steam showers. For a conventional shower, vapor management isn’t as critical (but is still important) as with a steam shower. A steam shower will have a much higher vapor pressure, and like anything, pressure tends to drive the material further into and through things. Because a commercial steam shower tends to be used sometimes continuously or at least a lot more frequently and for longer periods of time verses a residential steam shower, the industry has set different minimum requirements on managing water vapor. A residential steam shower must be built with materials with a perm rating of 0.75 perms or less; a commercial steam shower drops that to 0.50 perms or less. Few, if any liquid water proofing materials can qualify for use in a steam shower, but work just fine in a conventional one. When it comes to sheet membranes, all of the listed companies make at least one sheet membrane that meets the commercial steam shower rating, but not all sheet membranes from all manufacturers are recommended for use in a commercial steam shower. It is important to understand what, and how the shower is planned to be used to then select the required material for successful long-term reliability. To gain higher perm ratings, the primary means is to make the sheet membrane thicker. This is both good and bad, and that discussion is left for another time. Making a liquid membrane thicker, one might think, would help, but it has functional problems. The stuff needs to dry after being applied, and if the surface was totally vapor proof, the lower layers could not then dry out. The stuff tends to be somewhat pliable, and when applied thicker than designed, it isn’t as good a base for tile on top. It is extremely important with a liquid membrane to get the proper thickness of the wet coats – there is a specific min/max for each brand that must be adhered to if you want a reliable install that meets its individual specifications.

    So, it is important to consider both liquid and vapor water in a shower, and even more critical in a steam shower. Knowing this and the industry requirements can help you choose the product best suited for your installation. Everything in life has some cost/benefit relationship – the industry guidelines, when followed, will provide you with a reliable result IF you read and understand and follow the installation instructions of the materials you choose, AND pick one suitable for your application. Do that, and any one of them out there will produce a quality result.
     
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  2. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Joined:
    Sep 2, 2004
    Occupation:
    Retired Systems engineer for defense industry.
    Location:
    New England
    Attached is a table I compiled with some (not all, but the major ones) of the waterproofing materials available in the USA, both liquid and sheet types. If you have some additions or can fill in the blanks on some of those already in the document, please post it, and I'll update the document as an ongoing chart for reference.
     

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