Benefits of Vapor Semi-Permeable Insulation on Masonry Walls in Hot-Humid Climates

Home / News / Benefits of Vapor Semi-Permeable Insulation on Masonry Walls in Hot-Humid Climates


An area’s average climate determines, in part, the types of materials used in residential and commercial construction. Hot and humid climates include parts of North and South Carolina, Georgia, Florida, Alabama, Mississippi, Louisiana, Texas, and Mexico according to Herbertson’s Thermal Regions (a modified version of the Koppen Classification scale).

Herbertson defines a hot and humid climate as a place that receives more than 20 inches of rain annually and exhibits one or both of these conditions:

  • A wet bulb temperature (thermometer covered in a water-soaked cloth) higher than 67 degrees Fahrenheit for 3,000 or more hours during the warmest 6 months of the year.

  • A wet bulb temperature higher than 73 degrees Fahrenheit for 1,500 or more hours during the warmest 6 months of the year.



Hot air and water vapor must be managed through the use of building materials and techniques adapted to the climate. For instance, heat always moves toward cold – which means that, in sunny, humid areas, hot air will gradually replace air-conditioned indoor air unless builders take steps to keep moisture out of the building.

In high precipitation areas, moisture can threaten the integrity of the structure and its materials. To keep standing pools of water, wind-driven rain and leaks from weakening the building over time, contractors need to incorporate construction technology that keeps moisture out and allows it to dry if it finds a way in. Among some of the most effective materials used are weather resistant barriers and vapor-transmitting insulation.


Understanding Air Flow, Moisture, and Heat


According to the Second Law of Thermodynamics, heat transfers from high temperatures to lower temperatures. This effect can be slowed or halted through selected building materials, but heat never flows away from cold of its own accord.

Building science illustrates how heat can move through walls and affect indoor temperatures, putting stress on heating, ventilation and air conditioning (HVAC) systems. Hot air tends to rise in open areas while cold air sinks. Heat transfers across elements of the building envelope via 3 distinct physical modalities: convection, conduction and radiation. The Low-Emissive (Low-e) surface(s) of some insulation products can reduce one or more of these modes of heat transfer to help improve the energy efficiency and comfort in buildings.

Water intrusion can wreak havoc on a building and reduce its ability to regulate indoor temperatures and maintain its stability and reliability. In addition to rainwater and flooding, moisture can also get into a building through water vapor transmission.

Rainwater can infiltrate into walls with its own momentum. The surface tension qualities of water can even cause water to curve around an object and through openings into parts of a building not intended to be exposed to bulk water. Flashing, gutters, grading and other methods of directing bulk water away from the building are used as a first-line of defense to protect buildings from bulk moisture.


Choosing the Best Materials for the Climate


The importance and our understanding of building science has evolved over the past half century or so. Thermal insulation is now common. Our living environments are designed to control air changes and HVAC technologies make living in hot-humid regions not only possible, but comfortable.

Climate and precipitation are major factors in a builder’s specification decisions. In hot-humid climates, materials must be able to withstand the elements and contribute to the building’s energy efficiency. As a result, today’s homes and commercial buildings built in zones 1, 2 and 3 enjoy a living environment that’s designed to limit air infiltration to limit moisture accumulation across the envelope. Insulation technology can also significantly increase R-value, the measure of insulation effectiveness. And, using semi-permeable insulation is wise in hot-humid climates because it allows moisture to escape when it finds a way in.


The 3 Classes of Building Materials


Vapor retarders are used by designers and builders to prevent or slow vapor transmission. Examples include:



  • Polyethylene sheeting

  • Aluminum/organic-faced rolls of fiberglass insulation

  • Asphalt-coated kraft paper

  • Vapor repellant paints and gypsum

  • Flexible sheet reflective air barriers


Vapor control products come in three varieties: vapor impermeable, semi-vapor permeable and vapor permeable. Their permeability is measured in units called perms, which represent the rate of moisture transfer through a given material:



  • Vapor impermeable – Less than or equal to 1 perm and able to keep out almost all moisture. Examples include asphalt kraft paper, oil-based paints, vinyl wall panels and polyethylene sheeting and flexible sheet reflective air barriers.

  • Semi-vapor permeable – Between 1 and 10 perms, which allows a small amount of moisture infiltration. Examples include some latex paints, expanded polystyrene, fiber-covered isocyanurate and perforated reflective insulation commonly used on masonry walls in hot-humid climates.

  • Vapor permeable – Anything greater than 10 perms, which permits moisture to pass relatively easily. Examples include unfaced fiberglass insulations, unpainted plaster, unpainted stucco and cellulose insulation.


A material’s ability to slow vapor transmission also depends on the area’s climate. In the hot and humid South, concrete block is a common building material because it can withstand high winds and hurricanes better than other materials and building methods.

And, while it performs well in terms of preventing moisture from entering the interior, prolonged exposure to humid conditions can lead to problems ranging from mold and mildew to gradual degradation of structural materials and other moisture-susceptible building components such as insulation.

Choosing a semi-permeable insulation that also reduces radiant heat transfer can be a smart choice for architects and builders concerned with durability and sustainability. Semi-permeable insulation allows the envelope to dry if moisture finds a way in and also improves thermal performance and comfort for those living or working in the building.



Fi-Foil and Masonry Semi-Permeable Insulation


Fi-Foil Company’s VR Plus Shield™ Reflective Insulation for Masonry Walls is constructed of three layers of reflective insulation designed to reduce radiant heat transfer through walls and increase a home’s overall R-value. VR Plus Shield Perforated version is recommended for masonry wall applications in hot-humid climates, adds R-7.0 and can be used alone or as part of a system. Perforations along the premium aluminum sheets allow moisture to pass through the insulation layers.

For more information on Fi-Foil’s VR Plus Shield™ or any of Fi-Foil’s other products and services, such as Silver Shield™, HY-Fi®, Retroshield® and accessories, visit the website or call 888-513-4972.

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