We plan to build a large deck outside a southern sliding glass door. What are the best environmentally conscious materials to use? Are there any general efficiency design tips? Stevie N.
Dear Stevie: People don’t often think about a deck’s efficiency because it is outdoors, but the proper design and materials can influence your air-conditioning costs and especially peak cooling load. As with your plans, most decks or patios are located just outside a large glass door. With a deck or patio, more reflected and conductive heat passes through the glass into your house as compared to just natural foliage out there making proper design important.
Having a comfortable deck or patio allows your family to spend more time outdoors even during the peak afternoon heat. When people spend more time in the outdoor heat, they become more accustomed to it. This allows them to set the air conditioner thermostat a few degrees higher without sacrificing comfort. When cooling your house, each degree you raise the thermostat setting can save 5% on your electric bills..
Your house is already completed so your door location is fixed. Decks are not used as often in the mornings and the morning sun is not intense so it should be comfortable. With a south-facing deck and the sun directly overhead around noon, shading the deck is simpler. By afternoon, the hotter western sun can be partially shaded by the house. In very hot climates, the north side is an excellent location.
Constructing a decorative partially open wall on the southwest side of the deck is effective for shading. This also provides privacy. The simplest design uses just standard posts covered by lattice. The openings in the lattice allow some breezes to pass through. Planting climbing vines along the lattice also enhances the cooling by natural transpiration (evaporation of moisture from the leaves).
A more complicated, but better design for a shading wall is to install 6- or 8-inch horizontal slats instead of lattice. By tilting the slats at the proper angle, the direct afternoon sun can be blocked while providing ample area for breezes. The best angle for the slats depends upon your area’s latitude and the deck’s orientation to the sun. Make some sample cardboard slats to test for the best angle.
To block the sun from a more overhead direction, build a pergola over the portion of the deck closest to the house. It requires some detailed saw cuts when building one from lumber. Complete composite pergola kits are available for the inexperienced do-it-yourselfers. Both types are attractive, and by allowing plants to grow up and over it, they can provide additional shading.
In addition to saving energy, you can make environmentally conscious choices for the decking materials. For the deck framing, engineered lumber can be used instead of standard solid two-by lumber. Engineered lumber is stronger and often made from smaller wood pieces so less prime wood is required overall. Its strength may allow for fewer posts and longer spans without creating a springy feel.
The choice of actual decking material is most important. Pressure-treated wood is abundant and the least expensive material to use. It also has the nicest appearance and feel on bare feet and it is the easiest material to work with. The only environmental drawbacks to wood are its limited life and the cleaning and sealing chemicals which must be applied every year or two to extend its life.
Composite decking is another popular option. It is typically made from 95% recycled materials, primarily from plastic bags and sawdust and you work with it exactly like real wood. It does require periodic cleaning to minimize stains and mildew. For almost no maintenance, select composite decking with a thin solid vinyl top coat. This top coat has an actual grain rolled into it to simulate real wood.
Cellular PVC decking is a third option. I used cellular PVC decking on a second-floor deck off my bedroom. It uses more virgin materials than composites do, but it is very durable, low-maintenance, lightweight, and does not stain or mildew. Both cellular and composite decking can be installed with hidden fasteners.
If your deck is exposed to the direct afternoon sun, select light-colored composite decking to reduce the heat buildup and heat radiated to your house wall and windows. I thought dark brown composite decking would look best for my first-floor deck. It actually gets too hot to walk on with barefoot in the afternoon.