Save Your Skin – Systems Design for Your Building’s Exterior

by Maurice Levitch, AIA

A successful exterior finish system will look good, keep the weather out, and last a long time. Choosing the right system, however, is not as simple as deciding between stucco and T-111 wood siding.

When designing a new building there are many exterior finish material choices. The options narrow when looking at the design style, and again when considering the construction budget. The most important factors, though, may be the least considered.

With a systems approach to selection, design, and construction of a successful exterior finish system, you will need to consider all of the following elements together: climate, type of construction (wood, masonry, steel, etc.), architectural elements such as overhangs, building orientation, door and window selection and detailing, roof design and material, gutters and downspouts, flashing details, building/house wrap, lifecycle cost, maintenance requirements, and last but not least, aesthetics. As with many other parts of a building, the use of any exterior finish material should be considered as a system both in itself and as part of the entire structure.

Like your skin, the skin, or envelope, of a building should allow your building to breathe while protecting you from the elements. Although skin is your body’s true siding, most of us wear clothing as an additional layer for modesty and protection. If you consider your clothing as siding, you can easily see that after getting wet you and your clothing dry out much faster and with more comfort if you can take your clothes off to dry; and no matter what type of rain gear you’re wearing you almost always get wet.

A “plastic wrap” policy – plugging every hole, caulking every joint – is ineffective. Wood shrinks and expands, caulked joints fail, and natural forces like gravity, capillary action and wind-driven rain will cause water to go where it’s not wanted. Water will enter the outer envelope – it’s one of the things it does best!

The key is in the envelope design. If there is a drainage plane behind the siding – with a gap that allows infiltrating water to run down and out – water won’t sit behind the siding and turn to vapor. Here’s the theory: let it in, get it down, and get it out. Or, more simply put, “drain the rain”.

Rain-screen siding systems allow for water to run behind the siding material and to travel out and away from the building at the bottom of the wall. By adding furring strips over the vapor barrier and behind wood siding, or using “wrinkled” house wrap behind stucco, you can increase the life of the siding and improve the performance of the exterior finish system. While a rain-screen system requires technical expertise and research on specific materials as well as careful construction, it will pay off in the end.

You may have heard of the problems with Frank O. Gehry’s Stata Center project at MIT, where the Owner has filed a lawsuit against the architect and contractor for “design and construction failures” related to leaks caused in part by problems with the exterior envelope. The construction firm claims they warned the architect of flaws in the detailing and the architect refused to make changes to improve the design.

While you can certainly make it harder on yourself with a complex building design, chances are you are not a star architect earning a $15 million commission. Even if you are, it is better to design and build a great envelope rather than rely on your professional liability insurance. Whatever skin you use, make sure yours is protected!

This article was originally published in the March 2008 issue of Builder/Architect magazine.