To Install Stucco Right, Include an Air Gap

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To Install Stucco Right, Include an Air Gap

How should stucco be installed to avoid the wall-rot problems plaguing thousands of U.S. houses?

Posted on Nov 12 2010 by Martin Holladay, GBA Advisor

In many areas of the country, hundreds of stucco-clad homes have suffered wall rot. Although building scientists are still researching the causes of wall rot behind stucco, it’s clear that all of these walls got wet and were unable to dry.

Among the reasons that have been proposed for the recent epidemic of stucco-clad homes with wall rot:

  • Many of the houses had OSB sheathingMaterial, usually plywood or oriented strand board (OSB), but sometimes wooden boards, installed on the exterior of wall studs, rafters, or roof trusses; siding or roofing installed on the sheathing—sometimes over strapping to create a rainscreen. , which is more susceptible to rot than plywood or board sheathing.
  • Many of the houses had only one layer of Grade D paper or asphalt felt under the stucco rather than two.
  • The homes lacked a ventilated air gap between the stucco and the sheathing.
  • In many of the homes, interior polyethylene prevented the walls from drying to the interior.
  • Many of the homes had flashing errors.
  • Some of the homes had high indoor humidity coupled with an imperfect air barrierBuilding assembly components that work as a system to restrict air flow through the building envelope. Air barriers may or may not act as a vapor barrier. The air barrier can be on the exterior, the interior of the assembly, or both..

If you’re planning to install stucco on a wood-framed wall, you can benefit from the many lessons learned from all of the failed stucco installations of the last twenty years. Before we tackle the question of how to avoid such failures, however, let’s delve into some stories from the recent wall-rot plague.


GBA Encyclopedia: Stucco and Synthetic Stucco

All About Wall Rot

How Risky Is Cold OSB Wall Sheathing?

When Sunshine Drives Moisture Into Walls

Reviving an Old Debate on Vapor Barriers

All About Water-Resistive Barriers

A failure rate of 51%

I wrote a special report on rotting stucco-clad walls in Minnesota for the May 2006 issue of Energy Design Update. The article began, “In Minnesota, home inspectors have discovered that hundreds of recently built homes — especially stucco-clad homes — have rotting walls. In spite of years of efforts by construction experts to educate builders on improved water-management details, wet walls continue to take a staggering toll on builders and insurance companies.

“According to Ron Glubka, the chief building official in Woodbury, Minnesota, the city’s wet walls represent ‘the largest construction defect problem in local history.’ … Glubka [reports that] the building department has issued permits for wall repair work for 344 out of the 670 stucco-clad homes built in Woodbury in the 1990s — a failure rate of 51%. … In most of the Minnesota cases, the necessary wall repair work has not been minor or cheap. In one case discussed in the November 19, 2005 issue of the Star Tribune, a Minneapolis newspaper, the stucco walls at the Woodbury home of Steve and Debbie Long required $174,860 of repairs; the total bill was paid by their builder’s insurance company. ‘The costs incurred boggle my mind,’ says Paul Ellringer, an engineer and building consultant in St. Paul, Minnesota. ‘The worst case I’ve seen is one where the repairs cost $700,000. Just in Minnesota, the problems have been costing hundreds of millions of dollars a year.’ ”

Builders called to fix failed stucco often flub the repair

Unfortunately, the expensive repairs made to rotting stucco houses have themselves proved faulty. The Energy Design Update article noted, “As further evidence that some Minnesota wall repairs have been based on faulty diagnosis, experts note that some rebuilt houses are beginning to fail for a second time. According to [building consultant MacGregor] Pearce, ‘In a significant number of houses, the repairs to the stucco have failed too.’

“Among those who have seen ‘failed rebuilds’ is Paul Ellringer. ‘I’ve had two clients where their house was rebuilt twice,’ said Ellringer. ‘With one client, we are on the third rebuild. The earlier rebuilds failed because they keep redoing them the same way. Most rebuilds are feeble — for example, they will just tape the window fins with four strips of tape rather than using a pan flashing.’

“[Building diagnostician Steve] Klossner blames the failed rebuilds on bad diagnosis. ‘Some of these buildings have had major fixes, and they are already failing a second time, because the first time around they didn’t catch all the causative issues,’ says Klossner. ‘At one house I saw, they blamed the windows, so they pan-flashed the windows. The first time, they put it all back together and ignored a major problem — elevated indoor humidity. Now the walls are wet again.’

“Ellringer has seen so many rotting walls that he is prone to glum predictions. ‘I think that eventually the failure rate for stucco homes will turn out to be 80 percent,’ says Ellringer. ‘I also think the rebuilds will fail at the same rate as the original houses.’ ”

A nationwide problem

Although the article focused on problems in Minnesota, similar clusters of wall-rot problems have occurred throughout the country, wherever builders have installed stucco over wood-framed walls.

Although rot problems have occurred in walls with differing details, the most risky wall type appears to have the following layers: stucco, one or two layers of building paperTypically referring to Grade D building paper, this product is an asphalt-impregnated kraft paper that looks a lot like a lightweight asphalt felt. The Grade D designation has come to mean that the building paper passes ASTM D779 (minimum 10-minute rating with the “boat test”) and different products are called out as “30-minute” or even “60-minute” based on D779 results. At times confused with roofing felt, roofing felts and building paper differ in two ways: felts are made of recycled-content paper, building papers of virgin paper; felts are made of a heavier stock paper; building papers a lighter stock. See also roofing felt., OSB, fiberglass-filled studs, polyethylene, and drywall. While it always helps to have two layers of building paper instead of one, or to use plywood instead of OSB, or to omit the polyethylene, switching just one of these details is not enough to prevent a wall from being damaged by moisture.

So — what’s the best way to detail a stucco-clad wall to avoid these problems?

Lesson one: Install at least two layers of building wrap under the stucco

The evidence is overwhelming: one layer of asphalt felt or Grade D building paper is not enough protection under stucco. This fact is now reflected in most building codes. For example, section R703.6.3 of the International Residential Code requires that “exterior plaster” (stucco) be installed over “a water-resistive vapor-permeable barrier with a performance at least equivalent to two layers of Grade D paper.”

Lesson two: Install a ventilated rainscreen gap

Many stucco installers believe that two layers of Grade D paper or asphalt felt creates a “drainage gap” between the two layers of paper. The idea is that when the wet stucco mixture is troweled onto the metal lath, the stucco soaks the outermost layer of paper, which wrinkles when it dries. These wrinkles are said to create a drainage gap.

Although the theory has some merit, thousands of failed walls have shown that two layers of paper aren't enough to prevent OSB from rotting. Whatever wrinkles exist in the paper are not enough to allow water to drain or to allow the stucco wall to dry faster than it gets wet.

Stucco absorbs water every time it rains, and it dries very slowly. To prevent wet-wall problems, it’s essential to include a true ventilated rainscreen gap behind stucco. The gap needs to measure at least 3/8 inch.

If you’re installing stucco over OSB or plywood, here’s how to create a ventilated rainscreen gap:

If you’re installing stucco over foam, here’s how to create a ventilated rainscreen gap:

  • Install 1x3 or 1x4 vertical strapping on top of the foam, screwed through the foam into the studs.
  • Fasten paper-backed metal lath to the strapping, and proceed with a standard installation of three-coat stucco.

If your local building inspector raises questions about the number and size of the screws you plan to use to fasten your 1x4 strapping, you may need to get an engineer to review your details. As long as you have an engineer’s stamp, your local building department should be happy.

Lesson three: Follow best practices for moisture management

It should be obvious that any house needs adequate flashing and moisture management details. However, such details are often missing or installed backwards. If your house has wide roof overhangs, your walls may survive such errors. If roof overhangs are stingy, however, and if your siding is a slow-drying siding like stucco, your walls may be in trouble.

So, here are the basics:

  • Asphalt felt, building paper, or housewrap must be lapped shingle-fashion, so that upper courses lap over lower courses. Any window flashings or door flashings must be integrated with the water-resistive barrier in a way that respects these laps.
  • Windows and doors need pan flashing at sills and Z-flashing at heads. Ideally, the Z-flashing will include end dams.
  • Roof rakes that intersect walls (as when a single-story garage is attached to a two-story house) need kick-out flashing to prevent water dripping off the roof from soaking the wall below.
  • Roof overhangs should be generous, not stingy.

Lesson four: Don't raise the grade around your house

Since stucco siding is made from Portland cement and sand, many homeowners confuse stucco with concrete. That's why it's fairly common for homeowners to raise the grade around their house until the grade is above the home's wood framing. After all, the homeowner thinks, the stucco looks similar to the foundation — so why can't I backfill against it?

Home inspectors often find raised flower beds or deep mulch around the perimeter of a house. Behind the soil or mulch is soaking wet stucco. And behind the stucco is wet OSB.

Some readers are now thinking, "Oh, come on now — no one's that stupid." But home inspectors are just nodding their heads and thinking, "Yup. I've seen that many times."

What about one-coat stucco or EIFS?

The details suggested above apply to traditional Portland-cement-based three-coat stucco. Some builders have switched to more modern versions of stucco — either “one-coat” stucco (a stucco method that actually requires two coats) or an exterior insulation and finish system (EIFS).

One-coat stucco consists of a 3/8 or 1/2-inch-thick layer of “enhanced” Portland-cement based stucco covered with a thin finish coat. Most one-coat stucco formulations include glass or polypropylene fibers and plastic additives (polymers). One-coat stucco can be installed over OSB, plywood, or 1-inch-thick expanded polystyrene.

An EIFS installation always includes a layer of rigid foam. A plastic mesh is laid over the foam, and a synthetic stucco mixture (that is, a stucco formulation that is not based on Portland cement) is troweled or sprayed directly over the rigid foam.

Some builders (and most insurance companies) associate EIFS with a well-publicized cluster of wall-rot failures in North Carolina. After suffering a serious black eye, the EIFS industry came up with new installation details (“water-managed EIFS”) that include a drainage layer between the rigid foam and the wall sheathing. Newer EIFS-clad homes with water-managed details have experienced very few wall-rot problems.

Regardless of which system you choose — traditional three-coat stucco, modern one-coat stucco, or EIFS — any installation that lacks a drainage gap between the stucco and the wall sheathing is risky.

Last week’s blog: “How Risky Is Cold OSB Wall Sheathing?”

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  1. Energy Design Update