The Interior Finishes Effect on Moisture Retention

The Role of Interior Finishes

Think of your walls and ceilings as a barrier between the inside and outside. This barrier, which is part of the building envelope, controls heat and moisture. During the winter, the interior of your home is often warm and relatively humid due to activities like cooking, showering, and even breathing. The outside air, however, is cold and dry. This difference in temperature and humidity creates a force that pushes moisture from the warm, humid interior toward the cold, dry exterior.

The interior finish on your walls and ceilings, such as paint or wallpaper, acts as the final surface the moisture encounters. If this finish is a vapor retarder—meaning it slows down or stops the movement of water vapor—it can have a big impact on what happens to that moisture.

Flat vs. Glossy Paint

• Flat (Matte) Paint: This type of paint is porous, which means it has tiny microscopic holes. This allows water vapor to pass through it relatively easily. In the winter, moisture from inside the home can travel through the flat paint and into the wall covering. This allows moisture that's been driven into the painted material from the interior side of the wall, to evaporate back into the rooms air.

• Glossy (Semi-Gloss or Gloss) Paint: This paint contains more resins and is less porous, making it much more of a vapor retarder. It acts like a slight vapor barrier, trapping moisture vapor behind it. This trapped moisture can condense into liquid water when it hits the colder back side of the drywall. If this water can't dry out, it creates the perfect environment for mold growth between the paint and the drywall. For this reason, it is generally recommended to use flat paint on exterior walls in colder climates to allow the walls to "breathe."

The Problem with Wallpaper

Wallpaper presents a similar, and often more severe, problem. It is a very effective vapor retarder, essentially sealing the wall surface. During the winter, moisture is attracted to the cold exterior wall, but the wallpaper blocks it from passing through. The moisture then condenses on the back of the wallpaper and against the drywall. This trapped moisture provides an ideal food source and environment for mold to grow, often without any visible signs until the problem is severe. The mold can then release spores and mycotoxins into your home's air, impacting your health. This is why in many colder climates, applying wallpaper to an exterior wall is a major risk to a building's health.

It's a reasonable assumption that if mold isn't visible on an inspection, but is showing up on air samples, and wall paper is on exterior wall, odds are that there will be mold growth hidden behind it.

We want to avoid wall finishes that act as a vapor barrier or retarder on exterior walls.

It's critical to understand how a wall assembly functions as a system. The materials chosen for each layer—from the exterior cladding to the interior paint—all influence how moisture moves and is managed within the wall cavity. While we know that air transport is the primary driver of moisture movement, the finishes on both the inside and outside of the wall play a critical role in controlling moisture retention.

Understanding Permeance

The key property to consider is permeance, which is a material's ability to allow water vapor to pass through it. It's measured in "perms." Materials are generally categorized into three groups based on their perm rating:

• Vapor-Impermeable: <0.1 perms (e.g., sheet metal, polyethylene film)

• Vapor-Semi-Impermeable: 0.1 to 1.0 perms (e.g., some paints, foil-faced insulation)

• Vapor-Semi-Permeable: 1.0 to 10.0 perms (e.g., latex paints, most building wraps)

• Vapor-Permeable: >10.0 perms (e.g., fiberglass batts, house wraps, gypsum board)

Winter Months: The Interior Finish

In the winter, the vapor pressure differential drives moisture from the warm, humid interior to the cold, dry exterior. The interior finishes directly impact this process.

• Vapor-Impermeable Finishes: Materials like oil-based paints, vinyl wallpaper, or interior-side vapor barriers (polyethylene sheeting) have very low permeance. They act as a vapor retarder, slowing the movement of water vapor into the wall cavity. While this can be beneficial in cold climates, it can also trap any moisture that does get into the wall, preventing it from drying back to the inside. If warm, moist air leaks past these barriers through small gaps, it can condense on the back of the cold drywall or on framing members, leading to significant moisture problems.

• Vapor-Permeable Finishes: Finishes like latex paint and most gypsum board have high permeance. They allow water vapor to pass through them relatively easily, so they offer little resistance. In this case, the wall's ability to manage moisture depends heavily on the materials further in the assembly.

Summer Months: The Exterior Finish

In hot, humid climates during the summer, the vapor pressure differential is reversed, driving moisture from the humid exterior to the air-conditioned interior. Here, the exterior finishes are critical.

• Vapor-Impermeable Finishes: Materials like vinyl siding, stucco, or brick with low-perm paint or parging can trap moisture that gets into the wall cavity (from rain or air leaks). This is especially problematic if the sheathing or insulation behind these finishes can’t dry to the exterior. Moisture can then move inward and condense on the cool surface of an interior vapor barrier.

• Vapor-Permeable Finishes: Highly permeable exterior finishes, such as most house wraps and permeable siding, allow the wall assembly to dry to the outside if it becomes wet. This is often the preferred strategy in hot, humid climates, as it provides a path for moisture to escape.

A fundamental principle of building science is that a wall assembly should be designed to manage and dry out any moisture it encounters. A common strategy is to make the interior side of the wall "tighter" than the exterior side. This is often described by the phrase, "tighter on the inside, more permeable on the outside." This design ensures that any moisture that finds its way into the wall can easily escape and dry out, preventing long-term damage.

Ultimately, moisture retention in a wall assembly is a complex interaction between bulk water management, air sealing, and the permeance of every material from the finish to the framing.

Examples of Permeance in Common Wall Coverings

Permeance is a measure of how easily water vapor can pass through a material, rated in "perms" (grains per hour per square foot per inch of mercury vapor pressure difference). Higher perm ratings mean the material is more permeable. The following examples are based on typical products and applications, but actual ratings can vary by product, thickness, and number of coats.

Paints

The permeance of paint is heavily influenced by its type and thickness. A single coat of paint has a much higher perm rating than multiple coats.

• Latex (Acrylic) Paint: Generally considered vapor-permeable to semi-permeable. It's often classified as a Class III vapor retarder (1.0 to 10 perms), allowing a moderate amount of moisture to pass through.

  • Flat Latex Paint: Typically has a high perm rating, often in the range of 5 to 27 perms.

  • Semi-Gloss Latex Paint: Has a lower perm rating than flat paint, usually around 5 perms.

• Oil-Based (Alkyd) Paint: These paints are less permeable than latex paints.

  • Flat Oil-Based Paint: Can have a high perm rating, similar to latex.

  • Semi-Gloss Alkyd Paint: Has a very low perm rating, often less than 1.0 perm, which can make it a Class II vapor retarder. Multiple coats can significantly reduce its permeability.

Wallpapers

The permeance of wallpaper depends on its material and any special features, such as perforations.

• Vinyl Wallpaper: This is often the most problematic wall covering for moisture. The vinyl acts as a vapor barrier on the interior of the wall.

  • Vapor-Impermeable: Many vinyl wallpapers are considered vapor-impermeable, with a perm rating of less than 0.1 perms. This can trap moisture behind the wallpaper, especially if it gets in from an air leak, leading to mold and mildew.

  • Micro Vented/Perforated Vinyl: Some vinyl wallpapers are manufactured with tiny perforations to increase their permeance and allow trapped moisture to escape. These products can have a perm rating of 8 to 15 perms.

• Non-Vinyl/Paper-Based Wallpaper: Generally more permeable than vinyl, allowing walls to "breathe" better. Their perm ratings can range from semi-permeable to highly permeable.

Important Note 

When evaluating perm ratings, remember that air leaks are far more significant than vapor diffusion. A small hole in a wall assembly can move more moisture than the entire surface area of the wall by vapor diffusion. Therefore, focusing on air-sealing (eliminating drafts and leaks) is often the most effective strategy for moisture control, regardless of the permeance of the wall coverings.