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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

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.

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.

For further information on the use of paper-backed metal lath over furring strips, see the statement by Kohta Ueno, quoted in a comment on this Q&A thread: "Stucco over exterior rigid foam - yea or nay?"

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?”

Tags: , , , ,

Image Credits:

  1. Images #1 and #2: Energy Design Update
  2. Image #3: Building Science Corporation

Aug 1, 2015 5:58 AM ET

Edited Aug 1, 2015 6:35 AM ET.

Response to Adrian Jo
by Martin Holladay

It's impossible to determine what's going on without a site visit. I can make some guesses, but they are just guesses.

In any case, if you have stucco that is bulging out 2 inches and cracking, someone is going to need to remove a section of stucco to investigate and repair the problem. Once the damaged area of stucco is removed, you will be able to see what's going on.

Here are my guesses:

1. In Photo #1, ineffective kickout flashing on the roof above the problem area is somehow allowing water to enter the wall. If my guess is correct, the solution will involve flashing corrections.

2. In Photo #2, the mold on the wall is due to spashback from the nearby roof. The splashback occurs when water from the higher roof dumps water onto the lower roof. There may be nothing wrong with the stucco here.

3. In Photo #3, the mold on the stucco and damage to the window casing is due to splashback from the nearby roof. Again, the splashback occurs when water from the higher roof dumps water onto the lower roof. One possible solution to this problem (and the problem shown in Photo #2) is to install a gutter on the higher roof.

One other thing I noticed: the soffit vents are all dark. This probably indicates that you live in a dusty area, or an area with polluted air.

Sep 14, 2015 11:27 AM ET

plaster, lath, studs, boards, tar paper, 1923 stucco ....
by H R

We bought a 1923 stucco bungalow 20 years ago. It came with many layers of old paint under a locally made "waterproof" paint loaded with mica flakes that we were warned would have to be removed once it started to peel (no way to re-adhere it). We got that done properly, including removing the underlying paint down to the original stucco. (Wet hot grit blastng, suction through filters -- the residue was so high in lead it had to be trucked to Nevada for burial -- the bottom layer of paint was upwards of 20 percent lead, apparently the standard in the 1920s.) Expensive, precautionary, glad we did it.

Now, with a 1923 stucco over tar paper -- the question is, what next? Had many of the walls open to install new plumbing, no sign of rot, tar paper surprisingly intact but for screw and bolt holes here and there. One honeybee nest in one wall. House was heated constantly by the previous owners, several generations all elderly (they didn't have a pilot light on their gas heater, it just ran all the time, so the house stayed dry.)

Now what?

Advice is split between regular sulfate stucco and then paint it, or carbonate stucco with a smooth surface that will dry readily.

Sep 14, 2015 11:45 AM ET

Response to H.R.
by Martin Holladay

We need more information.

1. What is your location or climate zone?

2. Do you plan to install any insulation between the studs? If so, what type of insulation?

3. Are the windows in good shape? Do you plan to replace any of the windows?

4. Does the building have generous roof overhangs or stingy roof overhangs?

Sep 14, 2015 12:32 PM ET

Edited Sep 14, 2015 12:35 PM ET.

SF Bay area.Windows are in
by H R

SF Bay area.

Windows are in good shape, some have been replaced with double-pane; old ones kept rainproof by a reliable local painter who does touch-up. Plastic heatshrink film we put up inside the old windows during the winter prevents condensation.

Roof overhang about 15 inches

Stud insulation? not easy -- lots of fire blocks and X-bracing in the walls (as we know from those we've opened up for plumbing -- those, we replaced lath and plaster with plywood screwed and anchored to become shearwall). I've read that blow-in settles leaving gaps on this old construction.

A longtime contractor friend says in this area in old construction, air circulation in the stud bays prevents moisture from condensing inside the walls. He's seen old houses here get rot in the walls within a few years after people put insulation in stud bays, and patch and paint their stucco with vapor barrier acrylic paint, a big mistake apparently since otherwise stucco here dries itself pretty quickly. (That's why we're looking into calcium carbonate stucco without the slow-drying additive)

Proper bathroom and kitchen moisture removal fans, by the way.
But see the note I put in the roofing section -- moisture under a new cool roof is being handled by a dehumidifier in the attic.

We could probably replace the old gas heater with a dehumidifier in the living space -- cutting the humidity is effectively making the living space warmer. Around here high humidity in cold weather pulls heat right off you.

Sep 14, 2015 12:43 PM ET

Edited Sep 14, 2015 12:54 PM ET.

Response to H.R.
by Martin Holladay

It sounds like you have made up your mind not to insulate. I'm not going to try to convince you otherwise.

But your question is rather vague: "Now what?"

I don't know. You could go out for ice cream if you're all done. You tell me.

Seriously, if you don't want to insulate, your walls are likely to stay pretty dry. Just talk to a stucco contractor and have the exterior finished in stucco if that's what you like.

Most water-entry problems are due to rain, so you need to make sure that your window rough openings are well flashed. In most older homes, they aren't.

Your method of avoiding water entry problems -- "old windows are kept rainproof by a reliable local painter who does touch-up" -- is a faith-based method. Trust me -- you can't depend on paint to keep your windows rainproof.

Dec 11, 2015 12:47 PM ET

Siding over stucco
by Jennifer Lane

Thanks for all of the excellent advice in this article and responses to others.
I have a client in central Vermont that has a house built in 1981 with a stucco first floor exterior. The existing wall construction for the first floor from interior to exterior is: 1/2" gypsum, 2x6 wood stud with batt insul. infill, 1/2" PB (which I'm assuming is really OSB) and wire lath & plaster. There is no mention of building paper or any other WRB in the wall detail, but I can't imagine that they didn't use it.
The client would like to cover over the stucco with a wood siding. They're leaning towards horizontal reverse board and batten. I've been scouring the internet for concerns with siding over stucco and wonder if you could offer your advice? My inclination is to install vertical 1x furring secured to the studs, then run the 1/2" thick horizontal battens with the outer boards over those. I would leave a slight gap at the top of the siding to allow air to pass between the vertical furring, and a gap below with bug screen. It seems as though this would work fine to keep air movement over the existing stucco and allow the new siding to breath as well. The other option would be to only attach vertical furring and then have horizontal boards with a 1/8" gap between them for a shadow line, eliminating the horizontal battens completely. This seems like it would offer the best air circulation between the new siding and the exist. stucco, however I'm concerned about the insect/hive building potential between the two.
Any thoughts would be appreciated.

Dec 11, 2015 1:43 PM ET

Response to Jennifer Lane
by Martin Holladay

There are lots of issues here. How much to delve into the issues depends on the owner's expectations and budget.

1. Is the sheathing sound? Or is there sheathing rot? Lots of stucco walls have rot. If you want to explore a little, start by looking under the lower corners of windows.

2. Where's the WRB? If you don't know, it's time to install one.

3. What about the windows? Extending the plane of the siding will require new window jamb extensions and a window flashing plan. This can range from a quickie job using caulk (not recommended) to a conscientious job that involves removing and reinstalling each window.

4. Is horizontal reverse-board-and-batten a good idea? Water will sit on the horizontal siding seams. This is a recipe for rot.

As I said, there are lots of issues here.

Dec 14, 2015 2:18 PM ET

Siding over stucco
by Jennifer Lane


Yes, issues abound, I realize.

1. I have to discuss and dig around a bit more with the client to find out about any issues they might be having with the existing wall construction. I'll suggest we do a little exploring before we get much farther along.

2. If we can't determine if there is a WRB in the existing construction, it doesn't seem correct to add one over the existing stucco...even if we did install a rainscreen layer between the stucco and new WRB and then another over the this correct? That leaves us with removing the existing stucco (windows too) and starting from there.

3. One thought I had was to hold the new siding away from the perimeter of the existing window returns, leaving essentially a picture frame of existing stucco. There wouldn't be a way to flash this, but if I keep an air circulation layer behind the siding would this be enough to allow the water that infiltrates to dry? Tricky detail.

4. They've seen it installed before (in images) and liked the look of the horizontal boards but are wanting a more modern look than clapboards. I suppose you could bevel the top of each board slightly to allow for some run off.

5. My main concern is whether adding any sort of wood (or other material for that matter) directly to the stucco (for instance, the vertical furring I mentioned) would hold enough moisture against the existing stucco that it wouldn't be able to dry out, even with sufficient air space behind the siding itself.
If they went with the scenario of only using vertical furring and horizontal boards (no battens) with an air space, that seems like it would increase air circulation, but it would also allow more water infiltration.
They're intent on covering over this stucco.


Dec 14, 2015 3:26 PM ET

Response to Jennifer Lane
by Martin Holladay

It's better to install a WRB over the existing stucco than it is to cross your fingers and hope there is a WRB somewhere.

In any case, it's a good idea to install a WRB -- so that you have a layer that you can tie your flashing into.

If the clients want horizontal boards, it's up to them -- but if I were doing the installation, I would probably ask them to sign a disclaimer -- "The clients acknowledge that they have chosen a siding that is contrary to the builder's advice, and the builder is not responsible for disappointing siding performance..."

If you include a vented rainscreen between the existing stucco and the new siding, I wouldn't worry about moisture. But you need to work out the window flashing details.

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