What Exterior Walls are Made of

Many people are curious about what exterior walls of houses are made of; their components and how they work together. There is a wide variety of materials that can make up walls.

Exterior walls of houses consist of a support system, typically a framing system or masonry; insulation; sheathing consisting of panels that provide a continuous enclosure; a weather barrier usually consisting of flexible sheet products; and the exterior finish material such as brick, wood, or numerous manufactured products.

Let’s take a deeper look into these walls and how the different components work together

Wall construction components

The primary purpose of the exterior wall is to provide a sealed enclosure for your house providing protection from the weather. The support system of a wall will be either structural or non-structural, determined by whether or not it carries the weight of upper floors and the roof.

Most houses tend to be built of structural walls built of light-weight framing systems or masonry such as brick or concrete blocks. However, if the house is designed around a structural system consisting of columns and beams made from heavy steel members, concrete, or heavy timber, the exterior walls will typically be non-structural as they are not used to support any floor or roof loads.

Framing systems are what you typically see when houses are built in the U.S. and Canada. This system, commonly called stick framing, consists of small but long rectangular pieces of lumber or light-weight steel from which wall panels are created. Wall panels are made by placing these pieces (the sticks) evenly between bottom and top plates made of the same material. This system creates a light-weight but strong and efficient support system using minimal amounts of material.

Masonry consists of uniformly sized rectangular units stacked together to form walls. The materials typically used are clay brick or concrete block. Bricks were most commonly used for support walls in older homes before stick framing and larger-sized concrete blocks were created. Concrete blocks are typically used where lumber is not readily available.

Insulation is used to provide a thermal barrier allowing the inside of a house to remain comfortable in temperature extremes. Insulation for stick framing systems is typically batt insulation. This insulation is usually made of loose fiberglass sheets that are narrow and long (the batts) placed between the framing members. Other insulation options include rigid wall panels that are placed on the outside of framing or masonry systems, or foam insulation that can be sprayed between framing members.

Sheathing provides a continuous enclosure on stick framing systems. In the U.S. sheathing for houses typically consists of plywood sheets of a standard size of 4 feet by 8 feet spanning across the wood framing. However, newer technologies such as rigid insulation wall panels consisting of extruded foam sheets or various fibrous material can act as the enclosure for a house.

A moisture barrier is placed over sheathing in stick-framed systems to keep water out of the wall system. This barrier can consist of tar paper, which is a flexible sheet that is impregnated with tar; plastic sheeting that is wrapped around the house; or a sprayed-on flexible membrane.

The exterior finish is what you see on the outside of the house. There are numerous materials available, but can be grouped into the following categories:

• Masonry, which can consist of brick, natural stone, similar man-made materials, or glass blocks.
• Siding, which can be created from a wide variety of materials, sizes, and patterns.
• Stucco, which creates a uniform flat finish material that can be made of cement-based mixtures or as a man-made finish over rigid insulation panels.
• Glass, consisting of large panels of glass set into a framing system or without a framing system as structural glass.
• Decorative, such as mosaics and decorative tiles.

How wall components work together

Keeping moisture and outside air from getting into the wall and the house along with keeping out extreme temperatures are the main goals of wall construction.

Early exterior masonry walls were made from two or more interwoven rows of brick stacked vertically. The interior face of this wall might have been plastered as a finish, while the outside remained exposed brick or also plastered. If exterior brick was exposed, rain would wet the somewhat porous materials, but then dry out after the rain stopped. Conversely, any water vapor created inside from cooking or bathing would be controlled at the inside of the wall by its being able to dry out. This ability for the wall to dry out is how destruction from freezing water in the wall was minimized.

Once framed wall systems were developed the method of controlling moisture and temperature changed. Water and interior water vapor is damaging to wood causing swelling, warping, and rotting of the material. The wood frame had to be covered to create enclosure and a means to keep moisture away from the wood had to be developed. This was accomplished by applying an initial layer of boards over the framing and then covering that with felt sheets impregnated with tar. The exterior of the house would then be covered with finished wood boards that were painted.

Keeping interior water vapor from damaging the wood was not as difficult. Early interior walls had plaster applied over an interior sheathing of wood boards. This plaster might take on some of the interior moisture but could dry out before its getting to the wood.

Early houses typically had little to no insulation, with the interior spaces being heated in the winter to a level that would counteract any cold temperatures that could transfer through the walls. Air conditioning was not available so summer temperature was controlled by opening windows to let in breezes.

Today we have a wider variety of material that can be used and mixed-and-matched together. The advent of air conditioning with heating now controls interior temperatures and water vapor better. However, once the push for increased insulation and energy efficiency became common new conditions have been created that affect walls.

The biggest challenge today is keeping moisture and water vapor from getting trapped inside the walls. Our need for better energy performance has brought about more tightly sealed houses. For framed systems we now have exterior building wraps that both control moisture but also covers up any gaps that could allow material to dry out.

Interior walls now typically use gypsum board as an interior surface on which finish material is placed. To keep interior vapor from getting inside the walls a vapor barrier is applied over the studs and insulation prior to installation of the gypsum board.

This barrier is best created by an appropriate plastic sheet over the framing before the gypsum board is applied. Batt insulation with a vapor-resistant backing material attached to the studs requires more effort to make sure any gaps are sealed at the studs, and so is not as effective. Special primers and paints can also act as vapor barriers but are not readily available.

The use of masonry over framing systems created an interesting challenge. Brick on framed systems typically consists of a single row of bricks stacked vertically. If any moisture got through the brick it needed a way to get out.

This face brick is typically placed about an inch away from the sheathing, tied to the wall with anchors. This gap between the wall keeps moisture away from the framing and dries out the backside of the masonry through the use of wicks through the masonry mortar joints that pulls moisture out from behind the brick.

Material Options for Your Exterior Walls

We have generally shown the means by which we can control moisture, vapor, and temperature through our various wall systems. New technologies have developed some interesting alternative systems such as insulated blocks and structural insulated panels that can be used as the support system while also insulating the house.

We also have a variety of exterior finish materials that can be used. Below is a summary table showing some of the material options available from the categories discussed above.

In addition to the above, structural concrete can be exposed as long as consideration is given to how it’s formed and finished. Precast concrete panels, though not often used for houses, can have a variety of finish options.

Each material above could have an article to itself. However, please use the guide as a general overview of materials you can consider for your house.