Exterior walls should provide more that just protection from the elements, or structural strength to support the ceiling and roof. Walls should also be designed to provide thermal comfort, energy conservation and water/moisture control.
The roof, the attic or the windows are usually seen as more important for comfort and to reduce energy bills than walls. But that's not necessarily so.
Walls Are Critical for High Comfort and Low Heating and Cooling Bills
External walls need a thick layer of insulation material. Without it, heat will flow to the outside to the inside of the house, according to the season. Wood, steel, concrete and other construction materials do not stop heat flow.
Consider high levels of insulation for your external walls.
The ideal levels of wall insulation vary greatly, according to the climate zone.
They go from 2-3 inches/5-8 cm of a rigid insulation in hot climates, to 10 inches/24 cm of mineral wool or rigid foam in very cold climates (or, according to Passive House Standards, to R-50/Metric U-value 0.113, that is, around 12 inches/30 cm of mineral wool or rigid foam).
If you are building a new home, do it right that first time. Retrofit wall insulation is expensive. You need more than just high levels of attic insulation, or high-performance windows.
Wood- and Steel-Frame Walls
Filling 2x4 walls (or even 2x6) stud cavities with insulation is not enough.
Wood and steel framing members are a source of heat transfer. They can degrade the insulation of the walls by 20%-30% (wood framing) or 50% (steel).
To reduce thermal bridging to a minimum, you need to apply a continuous and thick enough layer of foam or dense mineral wool to the exterior (or interior) of the walls.
The strength, the durability or the moisture-resistance of masonry walls depends on the materials they are made up of, and their properties.
Similarly, masonry doesn't provide significant thermal insulation.
Thermal Bridging Through Walls
Steel, wood, concrete and other construction materials aren't insulation materials; without proper insulation, heat will pass through them. Only insulation materials – in significant amounts, properly installed - can reduce thermal bridging to very low levels.
Insulated sheathing, that is, a continuous layer of a rigid insulation all over the exterior (or the interior) of the walls is the only way of minimizing thermal bridging.
Exterior Walls & Moisture damage Guide
Exterior walls should be kept... dry. That’s obvious, but hard to achieve.
In part, the difficulty comes from the many mechanisms through which moisture can penetrate the walls.
And also because walls aren't designed properly, or with water-resistant materials, or haven't good first lines of defense.
Are New Types of Walls - ICFs, SIPs, AAC - Advantageous?
Modern walls can be built with ICF (Insulated Concrete Forms), AAC (Autoclaved Aerated Concrete) or Structural Insulated Panels (SIPs).
But are these new construction materials worthwhile?
Protect Your Walls: first lines of defense
Walls are designed to protect people and their property. But to be effective at this role, they should also be "protected" as much as possible.
Siding or special paints, are obvious first lines of defense. But there are other first lines of defense: roof overhangs and well-designed ground and surface drainage systems, in rainy climates. Or shading in hot climates...
Low and High Thermal Mass Walls (and Floors)
Concrete and other masonry materials can store heat up to 10 hours or more, before beginning to release the heat - an ability that can be used in space heating (and cooling) strategies, in some climates.
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House-Energy Video on Walls