Condensation & Vapor Diffusion in Metal Building Cavities
When we talk about moisture in non-conditioned metal buildings, most people picture visible condensation on cold metal walls or on the bottom of the roofing dripping down inside the building. What’s less obvious — but far more important — is what’s happening microscopically inside the wall cavities of a building, especially if the walls and ceiling are closed up.
Do you know how moisture moves, where it accumulates, and why controlling that movement is essential for long-term building durability? If you have a semi-conditioned building (one that may occasionally use heating or cooling) or one that stores moisture-rich items like plants, dirt, feed, etc. and you have a vapor barrier on the interior, read on to learn more about controlling condensation and troubleshooting moisture problems.
Understanding moisture dynamics — condensation, capillary action, and vapor diffusion — helps you design walls that can successfully keep moisture out.
The Three Main Mechanisms That Bring Moisture Inside Your Building
Inside a wall (or roof) cavity of your building, moisture travels in three primary ways:
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Condensation – When Vapor Turns to Liquid
Condensation occurs when warm, moisture-laden air encounters a surface that is cool enough for water vapor to change phase into liquid. In metal buildings, this often happens when warm interior air contacts the cold steel skin — especially at night after a sunny day. This is exactly what causes “sweating” on walls or ceilings. As building-science experts note, the real problem isn’t just water forming on a surface — it’s when that moisture stays wet long enough to initiate corrosion, mold, or decay.
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Capillary Action – Liquid Wicking Through Materials
Capillary action is the tendency of liquid water to be drawn into a material’s tiny pores, like a sponge. A good everyday example is a paper towel. When you dip one corner into a spill, the water doesn’t just stay at the bottom — it climbs upward through the towel. That happens because the paper has thousands of microscopic pores that pull water along.
Building materials behave the same way. Concrete, wood, insulation fibers, and even dirt all have tiny pathways that can draw liquid water in and spread it out. Once water enters one of these materials, it doesn’t need pressure or airflow to move — it simply wicks along on its own.
Without careful detailing — like drainage spaces — moisture can be wicked into insulation or building materials and remain there, leading to long-term problems. Items like dirt or wood piles, feed storage, or even a bunch of adults gathering can all put moisture into the air.
The key to controlling capillary action is interrupting the path water uses to travel. This is done by adding capillary breaks (materials that water cannot wick through), keeping absorbent materials from directly touching wet surfaces, and using vapor barriers and insulation layers that don’t absorb liquid water.
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Vapor Diffusion – Vapor Driven by Pressure
Even when there’s no liquid present, moisture still moves as vapor. Vapor diffusion is the migration of water molecules from areas of high vapor pressure to areas of low vapor pressure — such as from warm, humid interiors toward cooler exterior surfaces or vice versa.
Not all materials allow vapor to pass through equally. Some act as vapor barriers, others as vapor retarders, and some are vapor open — letting moisture pass freely. Understanding the permeability of each layer in a wall assembly is key to predicting how and where moisture will move and preventing it from getting where it doesn't belong.
Drying vs. Wetting: The Balancing Act
The goal of any moisture-control strategy is to try our best to make the conditions unfavorable for moisture to form and to create an impermeable layer between the interior of the building and its metal skin (where condensation is most likely to form).
In a non-conditioned space, you can tackle condensation and vapor by either sealing the building up so it's airtight inside or by fully venting air through the building (bringing in outside air and flushing it through the building and out the exhaust). More info about venting a space for moisture control can be found on our website here: Using BlueTex in a Carport to Prevent Condensation
For the purpose of this article, we're discussing buildings that are sealed up inside using a vapor barrier. One note on this: a common mistake in wall design is to install vapor barriers on both sides of a cavity. This effectively “sandwiches” moisture in place and eliminates drying potential — a recipe for trapped moisture and eventual damage. In non-conditioned or semi-conditioned buildings, you only need 1 vapor barrier layer and adding more than that could lead to problems trapping moisture between the two. If you have a fully-conditioned metal building, there are specific instances where two vapor barriers can be (carefully) utilized for maximum moisture control.
Design Principles to Keep Moisture Under Control in Non-Conditioned Buildings
Here are key principles for managing moisture in wall cavities:
✔ Control Air Leakage
Moisture moves much more readily with air than by diffusion alone. Sealing gaps, penetrations, and transitions helps minimize the transport of moist air into cooler wall cavities, where it could condense. This is why overlapping your insulation and vapor barriers, along with using a vapor barrier seam tape, are critical when installing metal building insulation for moisture control.
✔ Strategically Place Vapor Barriers and Retarders
At minimum, a vapor barrier should be placed on the warm side of your insulation (so closest to the interior of the building) in colder climates to limit inward vapor drive. In mixed climates, you typically find the vapor barrier closest to the exterior. When using a single layer of BlueTex™ alone in your building, this is easy to install since there's no other insulation being used.
✔ Provide Drying Paths
Allowing moisture to escape — toward the interior or exterior — ensures that moisture doesn’t accumulate. Back-venting your insulation (the area between the metal skin and your insulation) is another path to keep things dry. You can passively or actively push air between the metal skin and the foil surface of your BlueTex™ to help it stay dry back there. Or you can seal up the bottom of the walls and close any ceiling venting and make a dead air space behind the insulation. The advantage to this, if you do it properly, is that dead air can also give you some R-value!
For more in-depth moisture control recommendations, see our guide on causes of moisture in metal buildings causes and the companion page on moisture prevention techniques in metal buildings.
BlueTex Vapor Barriers: Part of Your Moisture Control Strategy
Both BlueTex 2mm PRO and BlueTex 6mm Supreme act as vapor barriers that help manage moisture movement in wall cavities — but they’re suited to different applications:
BlueTex 2mm PRO — A reliable vapor barrier and radiant barrier solution that reduces moisture migration and heat transfer in many non-conditioned metal building applications. This is a great option in warm climates or climates that are particularly dry.
BlueTex 6mm Supreme — A thicker vapor barrier and radiant barrier that delivers superior moisture control, especially in wet, cold climates where buildings are regularly heated but not 24/7. The 6mm Supreme’s enhanced thickness and barrier properties provide improved resistance to vapor diffusion and moisture accumulation in climates prone to condensation cycles. The radiant barrier layer also helps keep heat inside the building in cold seasons, so it stays away from the metal exterior.
Placing these products correctly within your wall assembly helps shift the first condensation surface away from the cold metal to a warmer, controlled surface (the interior BlueTex™). This limits the potential for liquid forming inside the cavity and supports drying when conditions change. We have some more tips on controlling moisture inside a building on this blog post: Moisture Control Tips for Metal Buildings
Understanding what’s happening inside a wall cavity — how moisture condenses, how water moves through capillary action, and how vapor diffuses through materials — is fundamental to designing moisture-resilient metal buildings.
By thinking of walls as moisture systems, not just insulation sandwiches, and by using products like BlueTex 2mm PRO and BlueTex 6mm Supreme in assemblies with good air sealing and/or back ventilation, you can dramatically reduce moisture risks and keep interiors dry and effective at controlling temperature inside your building year-round.
