The Anatomy of a Wall


A wall is a barrier between the inside of your house and the outdoors, but the function of a wall extends far beyond containing your stuff and protecting you from the elements.  Walls consist of a complex system of materials working together to regulate temperature and moisture within your home. To do this, a wall must consist of several layers.  Each layer has a specific function and its relative placement depends on many factors, such as climate, siding material, and sheathing material.  Ultimately, the wall system is meant to protect the structure, so each layer must either be protected or able to withstand the elements to which it is exposed.

Fig. 1: Barrier Wall -               -                -             -              Fig. 2: Cavity Wall

Barrier wall metal siding  cavity wall siding

(Images via Building Design Envelope Guide: www.wbdg.org/design/env)

A wall is considered a 'barrier wall' if the outermost layer (cladding) is completely air-tight. An example of a barrier wall is a poured concrete panel with seamless metal cladding . Most residential walls, however, are 'cavity walls.' 

HighR wall 12 web rev resized 600A cavity wall includes air space within the wall assembly and a drainage plane.  Elements of a cavity wall include: exterior cladding, a drainage cavity, a drainage plane, an air barrier system, a vapor retarder, an insulating element, a sheathing system, a frame, an interior wall board, and some type of interior finish.  Some of these layers can serve multiple purposes. Rigid insulation inside the exterior wall cavity can serve as the air barrier and drainage plane.  Each component comes in a variety of materials, and must be compatible with the other components, the configuration of the wall, and the climate.  Even the interior finish can affect the performance of a wall!

Elements of A Cavity Wall: 

Exterior Cladding is the outermost layer on the exterior side of a wall system.  It is the home's first line of defense against the elements.  Cladding is divided into two general categories: Masonry (stone, bricks, clay, concrete, stucco) and Siding (wood, wood fiber, aluminum, vinyl, and fiber cement) More information about siding and cladding.

Drainage Cavity or Air Gap collects rainwater that passes through the exterior cladding and directs it back outside.   It allows air to mix and water vapor to condense on the drainage plane, or escape.

Drainage Plane: is a water repellent material that is designed to shed rainwater and condensation down and out of the wall system.  Its function is similar to that of the exterior cladding, but it is located inside the wall assembly.  It can be applied to the surface of rigid insulation or on the sheathing material.  Examples of materials that are used as drainage planes are building paper, 'housewraps," such as Tyvek, Typar, and Raindrop, and foam insulation.  Drainage planes are also vapor barriers: A vapor barrier is meant to prevent migration of water vapor.  Each layer of the wall has a different permeability.  Permeability is a measure of how well a material allows water vapor to pass through it.  A vapor barrier is less permeable than other materials in the wall system.

Air barrier: As the name suggests, an air barrier is meant to block air from entering the wall system.  The air barrier must be continuous in order to prevent air exchange between the inside and outside of the building.  It must also be strong enough to resist air movement without deforming. 

The air barrier also sometimes serves as the vapor barrier, in which case it should be impermeable to liquid water, but should also allow water vapor to escape from the inside of the wall system to the outside.  The air barrier can lie on the inside or the outside of the exterior insulation (see diagrams below, via Meditch Murphey Architects Blog http://meditchmurphey.com/blog/2009/05/ask_mike.html)

cross section wall insulationcross section wall air barrier
 

Insulation controls heat exchange. Different climates require different levels of insulation.  The measure of a material's ability to resist heat exchange is known as its R value (the higher the better).  Below are some approximate R values expressed per inch of thickness:

Expanded Polystyrene (EPS): R value of 4

Extruded Polystyrene (XPS): R value of 5

Polyisocyanurate (PIC): R value of 6

If your insulation will be exposed to water vapor, for instance, you should choose a material that does not lose its insulating properties when affected by moisture. 

Cavity Insulation fills the cavity (open space) in a wall system. Cavity insulation can be made of polyurethane, fiberglass/glass wool, rock wool, or cellulose. Insulation should always be placed somewhere on the exterior of the wall system.  If the structure is not insulated it can deform through expansion or contraction due to temperature and moisture fluctuations.  Cavity insulation may or may not be exposed to moisture depending on where it lies relative to the vapor barrier.

Exterior Insulation:  Exterior Insulation prevents water vapor from condensing (from water vapor to liquid water) inside the wall, which would cause mold. 

Rigid Exterior Insulation can also act as a drainage plane.

Examples: extruded polystyrene.  Siding with built-in exterior insulation is an option offered by CertainTeed that Fiderio & Sons can install.

Structural elements distribute live and dead loads to the frame.  The frame is 'the skeleton' of the wall and its configuration indicates the type of wall. 

Sheathing is a structural component of the wall system.  It can be made of plywood, OSB, or insulation panels.  It attaches to the frame of the wall, keeps the structure straight, and serves as an anchor for other layers in the system. 

Sheathing Materials include OSB, plywood, fiberboard, and exterior gypsum

In Connecticut, wall sheathing should have a minimum R value of 5.

Interior walls are the layer that you see from the inside of your home. Gypsum wall board, also referred to as drywall or sheetrock, is the most common material. This is the chalky stuff that falls apart when you try to hang your pictures.

Finish:

Wallpaper, when in style, can serve as more than just decoration; it can also insulate!  

Paint: Again, consider the permeability to avoid mold growth inside the wall.  

Textile wall coverings allow your walls to breathe and absorb unwanted sound, but can pose a fire hazard, so check with the experts first to make sure you are not violating local fire code. 

Thirty-plus years of experience counts for a lot, and it has taught us that every house is unique. We know construction, but you know your home. It is always good practice to be an educated customer. If you learn the basics about the structure of your home, you can more easily identify problems and address them before they lead to costly repairs. Increasing your knowledge will faciliate effective communication between you and your contractor. The more you can tell us the better we can prevent potential problems during your remodel.  

As a thanks for reading and learning, we are offering a free consultation to discuss any concerns you have or any projects you'd like to us get started on.