Much Ado About Stucco

By Michelle N. Delehanty, PE, PMP
MDC Systems^® Consulting Engineer

In recent years, the popularity of stucco siding for houses has soared.  Or, depending who you ask, it’s the notoriety that has increased so steadily over the past decade.  Good or bad, stucco is being talked about among builders, homeowners, and even in the news.

Stucco is a durable, eye-catching, and cost effective material that has been used for centuries.  This option is both attractive and versatile to consumers in that it can be applied to many different home styles.  Stucco has been known to stand the test of time; however, way it is used today – in composition and assembly – isn’t necessarily your grandfather’s stucco.

History of Stucco

In the evolution of building materials, Portland cement stucco is a relative new comer.  It was patented in England in 1824, and not perfected to its current state until about 50 years later.  Portland cement stucco is stronger, harder and quicker curing than traditional lime plasters, lending itself to be the most commonly used cement stucco of today’s construction, along with plethora of synthetic stuccos that mix Portland cement and acrylics or other hydrocarbons.

So what has changed with the subtraction of lime from the stucco mixtures?  The Portland Cement Stucco, although stronger and more convenient, is more brittle, and therefore more likely to crack under some load conditions.  The lime gave the stucco a natural ability to “heal” stress cracks that develop.  Additionally, the Lime Stucco is moisture permeable.  This statement would lead you to believe that Portland is better when it comes to moisture.  This is not really the case, because with Portland cement stucco, there is a greater chance that the moisture will become trapped in the wall, as opposed to permeating back through the Lime stucco and evaporating into the outside air.

With the development of today’s stucco technologies over the past few decades, the code has taken awhile to catch up.  Many of the homes built 10 years ago were based on codes that hadn’t been updated since the early 90s.  As a result, the construction of these houses can sometimes be perceived as unfit for today’s standards.

Typical Stucco Siding Assembly for Newer homes

Homes built in the last quarter century are typically framed by 2×4 or 2×6 studs.  On the interior of this framing is ½” gypsum wallboard, or drywall as it is commonly referred, laid out in four foot by eight foot panels with joints taped and a painted finish.  On the exterior of the drywall, a vapor retarder and insulation are required, usually a product with both elements is used – fiberglass batt insulation with foil or kraft paper lining is included and installed in the cavity spaces between the stud framing.  Both the vapor retarder and insulation have varying levels of moisture and temperature resistance, respectively, and are specified based on the code and climate when the house is designed.

Sheathing is placed on the exterior side of the studs and insulation and used as a structural element for lateral support as well as a backing to the weather resistive barrier and cladding.  The sheathing for a wood framed stucco house is either plywood or a synthetic wood particle board such as oriented strand board, or OSB.   The weather resistive barrier placed on the exterior of the sheathing can be a number of different materials, from sheathing paper to an asphalt-saturated felt.  This material would be specified by the home’s architect or dictated by code.

Finally, the outermost layer is, of course, the stucco itself.  The Portland Cement stucco is spread over an expanded metal lath in a one, two or three coat system.  The three coat system consists of a 3/8” scratch coat, a 3/8” brown coat, and a 1/8” finish coat, for a total thickness of 7/8”.

Is there a problem?

The main complaint to arise from the recent boom of stucco homes is moisture.  The penetration of water through the stucco surface and the inadequacy of the “drying out” process are leading to an array of problems, including constant moisture that could lead to mold, deterioration of the sheathing, or worse, structural damage.

Water infiltration is caused by:

• Cracks in the wall: Stress cracks are one reason that moisture can penetrate the stucco assembly.  Portland cement stucco shrinks as it dries, which normally creates small hairline cracks in the finished surface.  Larger cracks may form, however, if there is significant movement in the structure, since stucco is a nonstructural and relatively rigid coating.  The structural members of the house- the wood framing and OSB sheathing- all expand and contract at different rates than the stucco coating, causing stress cracks.  If the crack begins as a surface crack, the water penetration and freeze/ thaw of water will expand the crack, allowing more water to enter and the cycle to continue.
• If there is inadequate sealant between dissimilar materials, water will find a way into the wall.  When materials of a different nature, such as stucco and a roof eave, meet, the gap between them is vulnerable to water.  If the water enters an unsealed gap, it can run behind the stucco and potentially find its way beneath the weather resistive barrier.
• Improper flashing: Flashing is a building detail installed within exterior elements such as the roof, windows, and walls to direct water away from the building. If an element in the stucco wall is not flashed correctly, the water could actually be directed into the wall, having disastrously effects.
• High sitting plant beds: When one hears of walls getting wet, it is usually assumed to be from the rain, coming in and running down. An often unthought-of cause for water infiltration is mulch sitting right up against the house. If the ground becomes saturated, the wet mulch sitting against the house could allow for water to follow capillaries up under the stucco to the sheathing beneath.

Retaining Moisture:

• High Interior Humidity: One factor in the inability for a wall to dry is a high interior humidity. By looking at the indoor and outdoor temperatures and humidities, we are able to measure the transference of moisture through the wall. It has been found that under certain conditions, a high humidity indoors hinders the wall system from drying out completely before the next saturation takes place.
• Direction of wall face:  Unfortunately, something that cannot be changed in a house is the alignment of the walls. The typical direction of the driving rain coupled with the sun and wind exposure has a great impact on a wall’s ability to “dry out”. We have found that in our region, the Northeastern facing walls usually have the most saturation.
• High sitting plant beds (again): Besides being a cause for water intrusion, mulch against the house can also prevent water from escaping in two ways. First, not allowing air flow on the outside of the stucco will cause the moisture to stay put in the wall, essentially not allowing it to “dry out”. Second, and more significant, the mulch or topsoil could be blocking important elements included in the wall to allow water to escape such as the weep screed or weep holes. It is important that these components are always kept free from obstruction.

What is the solution?

The worst thing you can do if you suspect there may be moisture issues in your stucco home is to do nothing.  Next month, we will discuss what actions should be taken to mitigate the risks so that issues do not arise in your stucco home as well as what to do if you suspect there already is a moisture problem.