Structural Integrity Starts With the Frame, But Relies on “Control Layers”

When we think of a home’s structure, we first think of lumber. Wood frame construction remains an economical and sustainable choice for residential construction for several reasons.  Properly managed forestry practices make wood a renewable resource.  Structural lumber reaching the lumber yards today is more “on grade” than in years past thanks to improved inspection processes which require lumber producers to rate every piece of lumber as it is produced, and “graders” are closely monitored by regional inspection groups.

Engineered lumber products make use of short pieces of wood that would otherwise end up in a landfill, making them a good ecological choice; furthermore, glulam products, created by gluing the thin pieces of solid wood together, are larger, straighter, more stable and easier to align.  Advances in technology with engineered wood now make wood a viable substitute for steel, so we’re now seeing more use of wood even in commercial construction.

Light structural lumber milled from softwood trees (spruce, fir and pine) that are sawn and machine-planed to standard dimensions (2×4″, 2×6″, 2×8″, etc.) remains the material mainly used for construction of single family homes. Engineered posts and beams are commonly used for residential roof framing but are more and more being used for structural framing particularly where wide spans are needed.  The added costs incurred by using engineered wood members can often be recouped in labor cost savings; framers spend less time installing them and because the engineering beams are made with openings left for wires and pipes to pass through them, there are savings in time and labor costs later from electricians and plumbers.

Controlling moisture is fundamental to the proper functioning of any structure, and it starts with the framing.  In general, framing lumber with a moisture content up to 14% is considered acceptable.  As long as the material is not carelessly handled at a yard or jobsite, it will dry out fine in a normal building cycle.  The Journal of Light Construction warns that fast construction schedules can contribute to callback problems when the house framing isn’t given adequate time to dry on its own before wall and ceiling finishes are installed.  To avoid callbacks such as nail pops, drywall cracks, and open trim miters caused by moisture issues in the framing lumber,  the Forest Products Lab recommends checking the moisture content in about 10% of the framing as good practice before interior finish and trim is installed.

But there is more to a wall, of course,  than just the frame. The Building Science Corporation (BSC) calls the perfect wall an “environmental separator.”  To keep the outside out and the inside in, the wall assembly has to control water, air, vapor and heat.  If the insulation is placed on the inside of the structure, it fails to protect the structure itself from heat and cold. Thus, the control layers should be located on the outside of the structure in order to protect the structure, says BSC.   The “Perfect Wall” concept entails keeping the structure of a building within the conditioned envelope.

In theory, there should be four control layers—the water control layer, the air control layer, the vapor control layer, and the thermal control layer—so that they wrap the building outside of the structural components. For example, one way of controlling air and vapor would be applying a fluid-applied membrane system to the wall sheathing. Thermal control is achieved by adding insulation to the wall exterior. Claddings are not only an aesthetic choice.  They also function as the water control layer and can act as an ultra-violet screen.

But building a perfect wall involves more than just sealing everything up as tight as possible.  Moisture vapor movement through walls occurs naturally, so it is important to design wall systems that can manage moisture vapor.   The goal is for the assembly to dry both inwards from the control layers—and also to dry outwards from the control layers.   Vapor-impermeable exterior wall sheathing, (insulating sheathing products) can slow the process even trapping moisture in the wall cavity and should never be used on the inside of the assembly.

There’s also the control layers on the roof structure to consider.  BSC’s “Perfect Roof is sometimes referred to as an “inverted roof” since the rainwater control layer is under the insulation and roof cladding. For example, underlayment is attached to the sheathing, then topped with insulation, and then clad with the material of choice, creating a continuous layer that controls water, air, vapor and heat over the whole roof.  Then, the control layer for rain on the roof should be connected to the control layer for rain on the wall, the control layer for air on the roof connected to the control layer for air on the wall, etc., to form one continuous protective shell over the entire structure.

Rather than risk the wall “recipe” being built just right and in a timely manner, especially when the skilled labor force is still in short supply, some builders are taking a harder look at alternatives to stick-built framing.  Rising lumber prices are another reason to explore alternatives.  Ready-to-install Structural Insulation Panel (SIPs) wall panels are made by bonding a sheet material — most commonly OSB— onto both sides of an expanded polystyrene (EPS) foam core. By themselves, these materials are not strong enough to support loads, but once they’re made into panels they can be used for structural elements like walls, roofs, and floors.  Building with Insulated Concrete Form (ICF) blocks filled with concrete to form the entire perimeter wall system is another option.  But the fact remains that wood still makes the most sense for most builders and their buyers, with over 90% of the homes built in the U.S. constructed with timber.

The key to staying current on construction practices is an appetite for continuous improvement.  Savvy builders make it their mission to stay tuned to building science and open to new ideas while carefully vetting the cost and viability of alternative products and techniques.

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