Published December 10, 2024
Controlling moisture is one of the most challenging aspects of any building project. It gets complicated for a few reasons, including:Layered solutions: Success in managing moisture involves a variety of approaches that must work together as a unified system.
Multiple disciplines: It requires experience, skillful application and mutual understanding at every stage of a project by designers, engineers, builders and tradesmen.
Geography: Different climates and locations demand their own combinations of solutions. A warm, humid climate requires techniques that prevent outside air from moving into the building. A cold climate needs an emphasis on preventing condensation in wall cavities as warm air moves from inside to outside. A sea-side building site presents different challenges than a site just a few miles inland.
Technology: There is a lot of science built into the various approaches to manage moisture, and understanding the sometimes cryptic terminology is helpful for specifying and sourcing the appropriate materials.
What follows are simplified explanations of relevant technical terms and specifications, and an overview of the roles of different materials.
Essential Definitions
Permeability (Perms): The measure of a material’s ability to allow vapor – that is, gaseous water – to pass through it. A higher “perm rating” indicates easier passage of moisture.
- 0.1 perm or less: Vapor impermeable
- >0.1-1.0 perm: Vapor semi-impermeable
- >1.0-10 perm: Vapor semi-permeable
- >10: Vapor permeable
Water Vapor Transmission Rate (WVTR): The rate at which water vapor passes through a material. It’s generally expressed as grams per 100 square inches per 24 hours. It may also be referred to as the Moisture Vapor Transmission Rate (MVTR).
Absorption Rate: How quickly a material absorbs water when exposed to moisture—expressed as a percentage of weight increase.
Water Penetration Resistance: A material’s ability to resist water penetration based on the amount of pressure applied. It’s relevant in foundations, where porous materials like cinder block may be in continuous contact with water-laden soil; and on flat roofs and other surfaces where water can pool. It’s measured in inches of water column, expressed as pounds per square foot (psf)—or pascals (Pa) in the metric system.
Condensation: The process by which water vapor in the air turns to liquid by coming into contact with a cooler surface.
Vapor diffusion: The ability of gaseous water to move through certain materials like masonry and wood. It’s relevant for enclosed spaces like wall cavities, where there is a temperature gradient to encourage condensation.
Capillary action: The ability of liquid water to move through porous materials like masonry and concrete.
Capillarity or capillary suction: The tendency of a porous material to absorb and transport liquid water through capillary action. There are several ways to measure it; the international standard (ISO 15148) places a dry sample of material in contact with water and measures the mass of water absorbed over time. It’s expressed as a Water Absorption Coefficient (W); the lower the number, the less water a material will take up through capillary action.
Drying potential: How quickly a material or assembly can dry out when exposed to air. It’s expressed as a percentage per hour.
Hygroscopicity: A material’s ability to absorb moisture from the air. It’s expressed as a percentage of weight increase at a given humidity—with a higher number meaning a material will absorb moisture more readily. It’s relevant to indoor humidity, condensation concerns and indoor air quality.
Hydrostatic pressure: The force exerted by liquid water against a structure, such as a foundation wall or basement floor, due to water accumulation in the soil. It can cause water infiltration in below-grade spaces.
Materials for Moisture Control
The materials used to manage moisture serve four main purposes:
- Preventing water vapor or liquid from getting in.
- Helping water vapor or liquid to escape.
- Mitigating water’s effect on the building’s materials, interior environment and occupants.
- Managing water’s state to facilitate any of the above purposes.
Air barrier: A system of insulation, sealants and other materials that create an uninterrupted layer to block air flow – and the associated migration of moisture – though the building envelope.
Cladding and siding materials: Fiber cement siding, vinyl siding, metal panels and other systems that resist water absorption.
Drainage plane: A layer within the wall assembly that directs condensation inside the exterior cladding away from inner wall components to prevent mold and structural damage.
Drainage systems and materials: Includes perforated drainpipes around foundations and drainage boards installed against foundation walls to prevent hydrostatic pressure build up.
Insulation: Critical for reducing temperature gradients that cause condensation inside wall cavities. Moisture-resistant insulation types include closed-cell foam (spray and sheets), fiberglass and mineral wool. [See related blog post: Sorting Out Insulation Types and Their Properties.]
Moisture-resistant flooring: Porcelain tile, luxury vinyl tile (LVT) and rubber flooring for wet areas like bathrooms, kitchens, laundries, etc.
Mold-resistant drywall: Panels that include a non-absorbent surface and mold inhibitors in the gypsum—designed for high-moisture areas. Examples include Greenboard and fiberglass-faced drywall.
Moisture-resistant windows and doors: Featuring frames that resist water absorption and are sealed to prevent leaks; and windows with double- and triple-glazing, sealed gas-filled glazing and low-emissive coated glass to minimize condensation.
Rain screens: Systems of materials to create an air gap between the cladding and the wall sheathing, allowing drainage and evaporation of liquid water that penetrates the cladding.
Vapor barriers and retarders: Plastic sheeting, commonly HDPE (high-density polyethylene), placed on the warm side of insulation to prevent moisture vapor from passing through walls, floors and ceilings. Foil-faced insulation can serve as a vapor retarder.
Waterproof coatings: Acrylic, epoxy or bitumen-based coatings applied in thin layers on walls and roofs. They cure into a hardened layer that repels and resists water. Less durable than waterproof membranes, they’re most often used as a protective topcoat rather than a primary waterproofing solution.
Waterproof membranes: Applied in liquid or sheet form to create a thick, uninterrupted flexible layer that prevents penetration of liquid water. Designed to withstand higher hydrostatic pressure than waterproof coatings, they’re used on foundations, below-grade walls and roofs. Examples include EPDM (Ethylene propylene diene monomer) rubber, PVC and bituminous membranes, and liquid coatings of polyurethane or acrylic.
Let Best Supply’s expert team identify and deliver the materials you need to manage the complex issues around moisture control in your next project. Request a quote here.