There is more to the exterior design of a home than just a pretty façade. In residential projects, system design of the building envelope seldom receives the attention it deserves. More time and resources need to be spent on what is the homes’—and occupants’—main defense: the building envelope.
Given the issues of building resilience, durability, design, and client comfort, more single-family builders are leaning on performance standards to create high-performance, high-design home exteriors. It all starts with understanding a few important principles.
Addressing Condensation Issues
Many folks attribute moisture issues with rainwater. From attic rain up north to wet batts in Phoenix, builders are becoming more aware of another kind of water intrusion: condensation. Opposing exterior and interior temperatures create condensation where cold, warmth, and humidity meet, which turns vapor into droplets. Depending on the outside temperature, temperature difference, and relative humidity, these droplets may form on the siding, on the water-resistive barrier, on back of the plywood, on the studs, and even on the drywall. (Typically they need a surface and do not accumulate in the middle of an insulation batt, though batts can get soaked from the condensation accumulating on framing surfaces.)
The location in the wall where this water will form depends on the dew point temperature. If the dew point is in the wall cavity, and the wall stays wet longer than it stays dry, all of the water-sensitive materials (wood framing, sheathing and/or drywall) can be structurally compromised. More worrisome and immediate, though, is the mold and mildew that thrives in such high-humidity conditions. Over a cold winter or hot summer, the moisture persists. Indoor air quality plummets and affects the health of the home’s occupants.
There are two ways to control condensation: 1) Limit the amount of humid air inside the building assembly. This is typically done with good air sealing; air leaks carry far more humid air into an building assembly than vapor diffusion.
2) Raise the temperature of the building assembly to keep materials above the dew point temperature. This is increasingly done using continuous insulation, which greatly improves overall thermal performance and relocates the dew point condensation away from the wall cavity.
Combining Condensation Control and Bulk Water Control
Condensation in walls can be devastating, but by far the most common failure mechanism is exterior water seeping into walls. The best way to keep rainwater from soaking a building assembly is by creating dimensional separation. This necessary disconnect can be achieved in a variety of ways.
An increasing number of both production and custom residential builders have found simple ways to implement rainscreens. Vertical batten nailers (“strapping”), nylon matrix mats, or siding that installs with rainscreen clips are all readily available and effective ways to uncouple the cladding from the rest of the wall assembly. This disconnect is commonly referred to as a “drained and back-ventilated rainscreen” (DBVR).
Adding 1 inch to 1.5 inches of foam, or using a foam-sheathing combination (sometimes referred to as “structural insulation”) is fairly easy and inexpensive to build in because most rainscreen claddings don’t require additional framing to accomplish a robust, high-performance, thermally broken building enclosure, solving for both condensation control and water control.
For the hard-core stucco builders, a new way of thinking about exterior design is in order since it’s difficult to address water leakage and condensation issues with a direct-applied stucco. To address both water and condensation control with a stucco exterior, a cavity needs to be created with a drainage assembly over foam sheathing. One pro tip? There is a hybrid solution that can bridge the cost concerns on entry-level homes: Employing a rainscreen cladding only on the south side(s) of the home where it’s needed most will make a huge difference in performance while moderating the cost average for the entire exterior.
Does Stucco Still Have a Place as a Siding?
Stucco is the South and Southwest’s vinyl siding equivalent, just as brick is in Texas and the Mid-Atlantic. In northern U.S. markets, stucco is pretty much a fringe material either because of moisture and freeze-thaw concerns
Knocking on traditional stucco is fighting talk for many old-school builders and plasterers who appreciate its benefits. It’s thick, durable, and inexpensive. It is fire- and pest-resistant. But three-coat stucco has a lurking secret not enough people talk about: thermal mass. Because stucco isn’t ventilated, it doesn’t address dew point. It can also cause homes to use more energy than necessary.
The Southern exposure of any home in the Sun Belt region gets absolutely crushed by solar gain. A day with 100-degree ambient temperature can easily result in a 150-degree wall temperature all day. Homes with concrete masonry unit block or wood-framed homes with direct-applied stucco use much more energy into the evening and early morning than a home with a ventilated rainscreen cladding. A stucco home holds the heat so the HVAC has to work longer and harder. Some builders have been using a drain wrap between the sheathing and felt paper layers to create more drainage. Even if that can be achieved, however, it’s not enough space to uncouple the stucco from the structure to allow the needed separation and ventilation that would allow the wall to cool in near-real-time to manage condensation.
Materials Matter
With modern design firmly rooted in today’s vernacular, demand for new materials is high. But if you look around, you’ll probably notice a more modern take on traditional materials, too. White or black brick and architectural concrete rank high on Pinterest boards for modern home design. So why aren’t more people using them? The scope of work, schedule, trade labor, and cost are the main reasons. For instance, people love smooth, board-formed concrete walls but not the engineering, forming, pouring, cure time, stripping, sacking, and sealing involved in the install process, let alone finding time in the construction schedule, locating a specialty contractor, and dealing with cracks and spalling.
With exteriors, you definitely have a trade-off. Often, the more environmentally friendly and healthy the material, the less durable and higher maintenance it is, therefore increasing life cycle costs and environmental impact substantially. Wood is a good example. By contrast, metals have a high embodied energy in manufacturing and even geopolitical footprint, but they are highly durable and resilient.
Cement, sand, pulp, and water make a formidably durable and affordable combination. You need robust raw materials like this to make something that is sustainable from the standpoint of performance, maintenance, and longevity. It also helps that products made with these raw materials are generally within reach for almost every construction budget. But manufacturers need to invest in decarbonizing their final product that uses as little of these raw materials as possible.
For instance, manufacturers can offset cement and sand usage by incorporating fly ash, a useful post-industrial by-product of coal power that would otherwise end up in landfills. Recycled post-consumer pulp fibers can be added for flexural strength; these act like rebar in concrete. Generally, the less water used in the manufacturing process—whether we are talking ready-mix concrete or some thin fiber cement products—the better. A dry manufacturing process typically results in a more durable product that can be made into a wider bandwidth of products.
If a manufacturer can balance responsible use of raw materials and manufacturing in a low-impact manner, they can achieve a beautiful, high-design, high-performance modern rainscreen cladding product that's affordable, workable, and long-lasting. This is a potent combination loved by home builders and homeowners alike.
