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Q&A Spotlight

What Happened In This Roof?

After an unconventional spray-foam installation, stains appear on the side of the house

Spray foam gone bad: After an insulation contractor added spray foam to the roof, these stains began showing up on parts of the exterior. What's going on? Photo courtesy of James Fugate.

James Fugate’s intent was to insulate the roof of his Rochester, New York, home and make the attic part of the conditioned space. What he got instead was a mysterious brown stain that shows up on the outside of the house at certain times of the year.

The project started innocently enough. Where the roof rafters were accessible, the insulation contractor sprayed foam on the underside of the sheathing. Then he hit a snag. As Fugate explains in this recent Q&A post, on the second floor of the house is a 5-foot-high knee wall, above which is a sloped ceiling that rises to the flat ceiling below the attic. Rafters and sheathing were inaccessible in this area.

So, the contractor came up with an unusual plan. He decided to spray the foam from the outside. In order to do this, he removed shingles and sheathing above the sloped ceiling, applied the foam, and reassembled the roof.

“Now,” Fugate says, “every winter I get this brown stuff dripping down the siding of my house. It only happens when we have a thaw after a deep freeze. The dripping never occurs during warm weather. It does not happen everywhere.” There are no signs of water intrusion inside.

Getting the contractor back for remedial work is out of the question. Fugate ended up replacing the entire roof after the contractor did such a lousy job of patching the area he had removed.

What explains this mess? And what are Fugate’s best options for fixing it? Those are the issues in this Q&A Spotlight.

Is moisture condensing above the foam?

Zephyr7 believes because the foam was installed from the roof, there is now an air gap above the newly installed foam in that sloped part of the ceiling, and that the sheathing has become a condensing surface in cold weather.

“My guess is moisture is making its way into that gap, freezing, then thawing and running out the soffit vents which is what you’re seeing,” he tells Fugate. “This is a bad thing. You probably have rotten sheathing which is where that brown coloring is coming from.”

One plan of attack, Zephyr7 adds, would be open the roof up again and make sure that there are unobstructed ventilation channels from the soffit vents all the way to the ridge in every rafter bay. If the roof had been vented, the contractor probably sealed some of the rafter bays up.

But that’s the point, Fugate replies: There are no ventilation channels of any kind in the roof. Once the insulating foam was sprayed on the back side of the sheathing in the upper part of the roof, the assembly became unvented.

Why didn’t the contractor work from the inside?

Vaportranz finds the contractor’s decision to work from the outside odd. Faced with the same situation, Vaportranz had his contractor remove the plaster and lath on the sloped ceiling and spray the foam from the inside.

“The lath and plaster were a mess,” he writes, “but they did a professional job of getting it out.”

Vaportranz wonders what special building features the contractor was trying to preserve. Fugate responds that it was plaster over gypsyum board with curved transitions from the vertical wall to horizontal ceiling.

“Removing the plaster would have disrupted three bedrooms, a playroom and a bathroom,” he says. “Plus the curved transitions would have been hard to recreate. At the time, it made sense to spray from above, but that itself ended up being very problematic.”

Water is backing up behind the sheathing

David B guesses that water is indeed condensing in the roof, as Zephyr7 surmises, and building up until it begins seeping through seams in the sheathing. At that point, the water runs between the roof deck and a layer of tar paper that David B assumes is there before it emerges on the side of the house. The tar paper would explain the color of the stain.

“I’d further hypothesize this as it is clearly coming from behind the gutters and is not subject to the drip edge, again leading me to believe the condensation is coming from under the shingles and on top of the roof deck,” David B says.

Peter Engle is onboard with the condensing moisture theory, but he thinks the brown stains on the side of the house are a sign of structural decay.

“The brown staining is probably ‘wood tea,'” he says, “which happens as wood rots and the water soaks up tannins and other wood solids.”

It’s also possible, Engle says, that the problem is occurring in the eave/soffit area. “Warm/moist air from inside the walls can come up through homes for cables, plumbing, etc,” he says, “if they didn’t foam all the way down and over the top plate. The foam dams the airflow from going up the roof and it vents out the soffits instead, but the whole roof/eave/soffit area is cold because the spray foam inboard of this area.”

Or, adds Mike Theis, “It could be something stupid like a bathroom fan or a disconnected heat duct or a humidifier run wild.” Keep an open mind, Theis says.

Balloon framing may be a factor

The house was built in the 1940s, and as far as Fugate knows builders used balloon framing (an old technique in which studs ran the full length of the exterior wall, from sill to top plate, without breaks at each story).

This leads John Clark to ask whether moisture might be traveling up the inside of an exterior wall where it freezes on sheathing. Eventually, the moisture thaws, follows the underside of the sheathing and emerges at the soffit vents.

“With balloon framing and without proper fire blocking, there are wide open stud bays that can carry moisture from the basement to the roof,” Engle adds. “Holes in the top plate (if there is a top plate) can allow fountains of warm/wet air up in the soffit area.”

Moisture trapped there freezes in the winter, then melts when the weather warms up. “The reason this only happens in certain places can be as simple as those being the only unblocked stud bays,” Engle says. “Or, as Mike says above, it could be a bathroom fan venting into that soffit. We can’t know for certain, but opening up the area for investigation should help.”

Our expert weighs in

Peter Yost, GBA’s technical director, had this to add:

I think we can all agree that the brown stain is the result of air leakage, subsequent condensation, and then condensate picking up tannins or other solubilized content to then run down to the eaves.

Here are some questions whose answers may help us better understand this situation and similar ones I have seen:

  1. What type of spray foam: open-cell or closed cell? The former—because of its 100:1 expansion rate—can lead to voids if the installer “hurries” or loses patience in tight-fitting awkward spaces. The latter—because of its dependence on exothermic heat-of-reaction for proper expansion and surface bonding—can lead to bond break when substrates are below the minimum surface temperature required by the spray foam component manufacturer.
  2. Was the spray foam installer third-party certified and experienced? Spray foam is job-site chemistry, super dependent on quality of installation. I always recommend that spray foam specs include that both the company and the lead installer who shows up at the job site be third-party certified through the Spray Polyurethane Foam Alliance (SPFA) or the Air Barrier Association of America (ABAA).
  3. More information on the difference(s) between areas showing and not showing staining: Differences in roof complexity, differences in conditions during actual installation, differences in cardinal direction or other factors affecting substrate (roof sheathing) temperature.

I have had two recent building investigations involving this same problem: air leakage condensate making its way down to the top of eave wall/soffit. In both cases, we zeroed in on the air leakage pathways by blower door depressurization and interior infrared camera work and with blower door pressurization and theatrical fogging.

The first test can identify the interior end of the air leakage pathway while the second test can ID the exterior end of the same pathway. And that is how this sort of roof assembly seems to work. If you see evidence of condensate in cold climates during the winter, the condensation is happening in a discrete channel rather than more diffuse air leakage, which may wet to the same degree but the expression is spread out through more material with less concentrated expression.

In one case, taking apart the soffit prior to the pressurization/fogging test meant that we could see exactly where the leakage was occurring. Rafter tail extensions sistered to each side of existing rafters were not adequately air sealed and revealed the exterior end of the air leakage pathway.

Will identifying the two ends of the pathway be enough to solve the problem? Good question. If you air seal both ends of the pathways, will that stop the condensation and staining? Prevent mold, mildew, or even rot in the roof assembly?

I think that so long as the damage done to date does not involve serious degradation to framing or sheathing, stopping the air flow and turning the pathways into dead air spaces could be enough. The way to assess this is either:

  1. Using extended probes from the interior to the underside of the sheathing to determine the moisture content of the sheathing and also to get a feel for how solid or punky the sheathing is (see “Extending the Reach of Moisture Meters”).
  2. Conducting invasive inspection of what seem to be the most vulnerable areas of the roof (along the pathway from the roof down to the areas of soffit/eave wall staining).

Or, maybe both.

I’d love to include photos for both of the tests mentioned above, but both cases are current and a bit delicate for the time being.

-Scott Gibson is a contributing writer at Green Building Advisor and Fine Homebuilding magazine. “Our Expert” is Peter Yost, GBA’s technical director and founder of a consulting company in Brattleboro, Vt., called Building-Wright


  1. Tyler Keniston | | #1

    GBA=Sherlock Homes

  2. User avater
    Paul Eldrenkamp | | #2

    I'm skeptical that it's mostly a condensation problem. That's an awful lot of water. My hypothesis is that it's a flashing issue.

    I would get up on a ladder the next time there's a thaw after a deep freeze and observe first-hand what's going on at the drip edge.

    1. Albany Landlord | | #3

      I agree that is not likely a condensation issue due to the volume of staining. I have seen staining like that that a few times due to old integrated gutters above the roofline (Yankee Gutters) leaking and running through the soffit. Whether it is flashing, ice damming, or bad roofing I would say water is coming through the roof. The thaw after a deep freeze could also be consistent with ice damming.

  3. User avater GBA Editor
    Rob Wotzak | | #4

    I power washed my vinyl soffits the other day and had brown liquid dripping down from a squirrel or bird nest above the perforated parts. It would be simple enough to open up those soffits (they probably just pop out by hand) and at least see where the water is running from under them before getting into any more invasive inspections.

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