Installing windows for vapor control

The vapor control layer of a home is defined as the part of the building enclosure designed to control the rate at which water vapor moves through the material or assembly by way of diffusion. The goal is simple: to avoid or reduce condensation inside the assembly during cold or humid seasons.

Of the four control layers, vapor is the least understood, which may explain why its risk is sometimes overstated. In reality, air leaks move far more moisture than vapor diffusion. Still, a vapor control strategy is needed for most buildings. 

Where to place the vapor control layer? How vapor-open or vapor-tight it should be? The answer to both depends heavily on the climate.

• In cooling-dominated climates, building assemblies often need to limit vapor diffusion from the exterior inward. That can mean using more vapor-resistant materials toward the exterior side of the wall.
• In heating-dominated climates, vapor control is typically placed on the interior, or warm-in-winter side of the assembly, to slow the movement of interior moisture toward cold surfaces.
• In mixed climates — where both heating and cooling are required — the ideal vapor control layer would change seasonally. While true “smart” assemblies have yet to be developed, variable-permeance membranes can partially fill that role.

Typical guidance:

• Hot and humid climates: Avoid strong vapor retarders at the interior; assemblies must be able to dry inward.
• Cold climates: Place the vapor retarder toward the interior (warm-in-winter) side to allow for drying outward.
• Mixed climates: Ideally, the vapor retarder has drying potential on both the interior and exterior sides.

Hybrid building assemblies, such as those with exterior insulated walls or unvented roofs, need more careful design so the structure stays warm enough to avoid interior condensation. 

I have seen window condensation in both hot and cold climates. The phenomenon is caused by water condensing on a cool — and sometimes cold — surface. This surface is typically the glass or glazing portion of the window but can also be on the window sash or frame.

Exterior window condensation 

In hot, humid weather, air-conditioned homes can cool the exterior surfaces of windows and doors. When those surfaces drop below the outdoor dew point, condensation forms on the outside of the glass. This is normal and not a problem. Windows are designed to handle exterior moisture, like rain, snow, and humidity.

Interior window condensation 

In cold weather, the opposite can happen. Indoor air often contains more moisture than outdoor air. When that warm, humid air contacts cold window surfaces, condensation can form on the interior side. This is the scenario we want to avoid.

Repeated wetting of interior window components can lead to the following problems:

• The frame’s finish could be stained or peel. 
• Wood components could swell or deteriorate.
• Mold growth may occur in severe cases.
• The window may require more maintenance and have a reduced service life overall. 

There are two basic strategies for reducing window condensation on the inside of a home: warming the window’s surfaces or lowering the interior humidity levels. Let’s start with warming the window. There are two ways this can be accomplished.

1. Choose a window with a higher U-Factor or condensation resistance value.
2. Direct warm air to blow across the window’s surface, typically by using a forced air heating system.

Reducing interior humidity loads is the simplest solution. Often this is accomplished using air conditioning or a dehumidifier. Another option is to use a ventilation system, such as balanced mechanical ventilation for exchanging indoor with outdoor air during cooler weather.

There are limitations to these strategies. Some homes require higher humidity. Maybe an occupant has a medical condition where dry air is harmful, or maybe there is high quality interior wood finishes or valuable artwork that requires higher humidity.   

In those situations, the better approach is usually to invest in higher-performance windows and thoughtful air distribution rather than trying to dry the house. 

If windows are properly air sealed, vapor diffusion at the window-to-wall interface is rarely a durability concern. Air movement — not vapor diffusion — is responsible for the vast majority of moisture transport.

Condensation on window surfaces is governed by basic physics: temperature, humidity, and surface conditions. Exterior condensation is harmless. Interior condensation is manageable.

It can be controlled by:

• Choosing windows with appropriate thermal performance for the climate.
• Directing warm air across the window’s glass.
• Managing indoor humidity levels.