Does Cement Board Need A Vapor Barrier?

Cement boards are a common backing material used due to their immense durability and strength that it provides. Vapor barriers help control how moisture interacts with your materials by preventing moisture from entering critical places, such as inside your walls. The question now is if it’s necessary to have both to increase the longevity of your home.

You should only install a vapor barrier on your cement boards with the guidance of an architect or engineer. As much as vapor barriers are good for your home, incorrectly placing them and putting them in unplanned locations can prevent moisture from evaporating and can also cause moisture to be redirected to other areas within your walls.

Read on as we’ll be going more into cement boards, vapor barriers, how vapor barriers are typically installed on cement boards, and the benefits of having a vapor barrier on your cement boards.

Before anything else, we just want to let you know that dealing with moisture is a very situational and specific problem for each home. There are many causes, solutions, designs, and processes needed to regulate a home’s moisture properly. What may be beneficial for some homes may be detrimental for others, and it’s important to let you know that there is no one-size-fits-all solution for this type of design.

Cement Boards: A brief overview

Cement boards, also known as fiber-cement sidings, are a mixture of cement and fibers such as glass made sheets. They can be used for multiple purposes, such as adding support for walls(exterior and interior), floors, counters, etc.

The main benefit and strength of cement boards are their durability. They’re capable of reinforcing different parts of a structure, can handle impact forces well (useful for floors), are general moisture resistant by themselves( can even be waterproof if made with portland cement), and have a good fire-resistance rating.

The main disadvantage of cement boards is how hard they are to work with. Remember that a big part of the costs is paying the laborers( cement boards are difficult to cut into the right shape) in construction. The mobilizations of materials( cement boards are heavy) and that making the material itself is quite expensive to other boards such as gypsum.

Cement boards are often used in the industry as an underlying feature. You won’t see them, and they serve as a base for other finishes( such as tiles, other boards, etc.). They’re a good surface to work on because they can handle all sorts of stress put on them while retaining their shape.

The different types of cement boards are classified the same that cement is also classified, which means that they’re categorized based on the different materials that make up the cement board, the ratio of those materials, and the main purpose of that cement board(Ex. Water-proofing, Fire-retardant, etc.)

Vapor Barriers: A brief overview

Vapor barriers are used to prevent moisture(water vapor) from transferring through materials.

For the most part, the placement of vapor barriers highly depends on your house’s relationship with moisture. Moisture is good for keeping heat indoors, which is good for places that experience winter but bad for tropical countries. In balanced climates that don’t experience extreme temperatures, vapor barriers may not be needed at all.

It’s also important to note that vapor barriers are not recommended for buildings with an internal temperature that vastly differs from outdoor temperatures( with a few exceptions). This is because the temperature difference will cause water vapor in the air to condense(when warm air collides with a cold surface).

There are many ways that one can have a vapor barrier put up as long as it serves the function of reducing the amount of water vapor that passes through the material that it’s on. Vapor barriers are classified based on their permeability, which is the rate at which water/vapor passes through a material. Here are the most common types of vapor barriers used in the industry:

  1. Paint

The performance of paint as a vapor barrier will highly depend on what type of paint is being used. The best paint for vapor retarding purposes would be oil-based since oil never mixes with water under normal circumstances.

  1. Glass

Glass is capable of completely blocking off water. Glass is often installed as a vapor barrier in fiber-form or spun into blanket-form( glass wool).

  1. Foil

Foil is great for homes that need to keep thermal heat inside while keeping the moisture out. Sauna foil reflects heat back at the source while at the same time has a low permeability rating.

  1. Plastics

Plastics such as PVC can also be used as a vapor barrier.  Plastic as a material is one of the best for waterproofing and water resistance, which is why it’s used for raincoats, umbrellas, lifeboats, pipes, containers, etc.

  1. Wood

Wood, by itself, is not a great vapor barrier. However, considering how easy it is to install and with a few waterproofing finishes, it can be. Wooden boards covered in resin can help reduce the amount of vapor seeping into the building.

The usage of these materials and how they’re installed will depend on how you want to control the moisture within your house. Each material and type will have its pros and cons and may not be compatible with the rest of your house.

Just because a material is highly impermeable, meaning no water vapor can pass through it, doesn’t mean that it’s the best. Sometimes, we want to let water vapor pass through materials so that it doesn’t build up. Either way, vapor barriers are a must-have for improving air quality, energy efficiency, and the longevity of your house as long as they’re used properly.

Note: The lower the permeability of a material, the better it is at keeping moisture out.

We’ll now be going into how a vapor barrier is installed on a cement board with all these said.

Installing vapor barriers on cement board

As mentioned earlier, it’s essential to note that you’ll need to install your vapor barrier in a certain way, highly dependent on how your house is built and the conditions that it experiences. 

Vapor barriers are usually installed on cement boards through their back, meaning between the board and the rest of the interior wall. Since cement boards can handle heavy loads, almost any form of a vapor barrier can be used and limited by space inside the wall cavity. 

As much as vapor barriers sound like they have no downside, they can cause more problems than benefits for your walls if misplaced. A clear example of this is preventing the moisture inside your walls from evaporating or misdirecting your moisture towards vulnerable parts of your house when your cement boards are already doing just fine.

Unfortunately, there is no step-by-step guide on how to go about this properly. Every building is designed differently, and your engineer/architect should have already planned out how the moisture control system inside your walls should work.

What you can do instead

If you looked up this article, we’re guessing that you have some moisture problems, and you currently have cement boards installed. Given that cement boards are difficult to work with for a DIY job, you’ll need to contact a handyman to assess the situation.

If you want a quick DIY solution for moisture problems within your walls, then we have got a solution for you.

The safest and easiest way to attain moisture protection is to seal up any gaps in your walls. You can simply achieve this by applying sealing tape and caulk for any of your walls. Take note that you need to first identify where the moisture is coming from before sealing appropriately. You’ll also need to determine if the moisture in that location is a good or a bad thing for your home, as water vapor can also play a role in maintaining ideal temperatures.

A general tip is that water and moisture will always find their way inside a home, but the good thing is we can control how it enters and exits through the planned usage of barriers and ventilation. The majority of water vapors enter a building through small cracks and gaps instead of permeability since things always take the path of least resistance. 

Final Thoughts

You should only install a vapor barrier on cement boards with the advice of an architect or an engineer. Incorrectly placed vapor barriers can cause moisture to build up in unwanted areas, which can cause materials to deteriorate. Vapor barriers come in all shapes and sizes, each with its strengths and weaknesses.

The simplest solution to keeping out moisture is to cover up any gaps and holes in your walls. If you plan to address moisture problems, always remember that it should be appropriately redirected to help your house( such as temperature control). 

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