by James Dulley
Engineering Consultant

Q: I want to build a deck off my family room sliding glass door. I want it to be as environmentally friendly and energy efficient as possible. What design and material suggestions can you give me?

A: Adding a deck to a house, especially if you do most of the work, increases the resale value of a house by several times its material cost. Since you are building it by an existing sliding glass door, there probably are some concrete steps or landing which makes a stable base near the house. You should also always securely attach a strong ledger board to the house wall framing or masonry.

A deck can actually be energy efficient in several ways. If having a deck allows your family to spend more time outdoors, then you should be able to set your central air conditioning thermostat higher during the times you are primarily outdoors. Setting the thermostat higher, even for just a few extra hours per day, will reduce your electric bills.

If you are outdoors more often, you will become accustomed to the heat and will be more comfortable indoors without as much air conditioning. I try to air-condition as little as possible during summer. When I visit someone’s house which is well air-conditioned, it feels too chilly for me because I am not used to it.
A properly designed deck with a pergola or some type of tall side wall facing south or southwest can also
provide shading for your house. This is particularly helpful if it can also shade the glass patio door. Even with the most efficient glass in the door, a tremendous amount of heat transfers in through the glass when it is exposed to the direct afternoon sun.

In addition to saving energy, you can make environmentally friendly choices for the decking materials. For the deck framing, engineered lumber can be used instead of standard solid 2x 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 decking materials 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 the easiest material to work with for most do-it-yourselfers. 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 option. Trex recently developed a new decking material, Transcend, which is a very environmentally friendly material. It is made from 95 percent recycled materials, primarily from plastic bags and sawdust.

This decking is different from other composites in that it has a thin polymer top cap. This reduces staining and mildew growth as compared to typical composite materials. It is more expensive than other composite materials, but its minimal maintenance makes it a reasonable investment.

Cellular PVC decking a third option. I used light-colored Azek 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 and does not stain or mildew.

If your deck is exposed to the direct afternoon sun, select light-colored composite decking to reduce the heat buildup and the heat radiated to your house wall and windows. My first-floor composite deck is dark brown and it is too hot to walk on barefoot in the afternoon.

Building a vertical 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.

Another option for the shading wall is to install horizontal slats. By tilting them 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 deck. You can build one from lumber or buy a composite pergola kit. This is attractive, and by allowing plants to grow up and over it, it can provide additional shading.


James Dulley is a nationally syndicated engineering consultant based in Cincinnati.

If you have a question about energy use or energy-efficient products, send it to: James Dulley, Electric Consumer, 6906 Royalgreen Drive, Cincinnati, OH 45244; or visit www.dulley.com.