Timber frame rot (C) Daniel Friedman Timber Frame Building Rot Case Study

  • TIMBER FRAMING, ROT - CONTENTS: Wood rot & structural rot caused by several factors. Foam sheathing, too much moisture, and a broken vapor barrier almost spelled disaster for a four-year-old timber-frame house in southern Vermont. Why Two Inches of Foam Insulation Are Better Than One. Poor Construction Details Can Lead to Dangerous or Costly Rot to Timber Frame and other Wood Structures
  • POST a QUESTION or READ FAQs about the relationship between construction details and the occurrence of structural rot in timber-framed buildings

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This article explains improper construction details combining with foam sheathing, high indoor humidity, and a broken vapor barrier leading to severe rot to a timber frame building. This article includes excerpts or adaptations from "A Rotting Timber Frame", by Steven Bliss, adapted by permission, courtesy of the Journal of Light Construction.

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Case Study of Timber Frame Building Rot Explains the Role of Vapor Barrier, Insulation, & Moisture

Rotting timber frame sketch (C) Journal of Light Construction, Steven Bliss

"If you don’t put in a vapor barrier, your house is going to rot away." You’ve heard this many times, but it’s not so simple. Thousands of insulated houses with no vapor barrier (or a lousy vapor barrier such as kraft paper) have not rotted away. Furthermore, sticking a sheet of poly in the wall is no guarantee against problems. Take the rotting timber- frame house that I visited in southern Vermont one November.

The 1,800-square-foot, 1-1/2-story Cape was four years old. The walls were framed with 8x8 timbers, which were left exposed on the interior of the house. Between the 8x8s the builder framed-in with 2x4s to provide nailing and a place for fiberglass insulation. The building frame was sheathed with one-inch boards, then wrapped with one-inch thick, foil-faced isocyanurate foam insulating board, which was taped and caulked. Wood clapboard siding was installed over kraft paper (see diagram just below).

A printer friendly PDF version of this article is provided courtesy of the Journal of Light Construction. This and other JLC articles about good construction practices can also be found at the Jornal of Light Construction website.

Investigating Rotting Beams on a Timber Frame Home

The owner discovered the problem when a renovation contractor opened up the south side of the house in order to add a sunspace. He found extensive decay in and around the timbers. The rot occurred on the outer face of the timbers-up to two inches deep in some sections—and in the sheathing and 2x4s wherever they touched the timbers.

To learn more, the owner cut out sections of siding and sheathing on all sides of the house and found decayed wood on the north, south, and west. Only one hole was cut on the east side, and showed only minor damage.

There was decay on nearly all the beams looked at—high and low, on vertical posts, and on horizontal beams. Rot also occurred in the 2x4s that were directly nailed to the beams, and in the 1x pine sheathing where it touched the beams. No decay was found in the wall sections between the beams, or elsewhere—although a thorough search was not made of all areas.

The west face of the house (at page top) was cut open in four spots, all revealing severe decay of the timber frame and adjoining wood. The southwest comer (below left) and center holes (below right) are shown close up.

Photograph of  rot on a timber framed  home (C) Journal of Light Construction, Steven Bliss. Photograph of  rot on a timber framed  home (C) Journal of Light Construction, Steven Bliss.

Timber frame rot (C) Journal of Light Construction, Steven Bliss Timber frame rot (C) Journal of Light Construction, Steven Bliss

Further Diagnosis of Structural Rot on a Wood Framed Building

When I visited the house in November, I looked for evidence of high moisture levels. It was a sunny day in the 40s—too warm for condensation to form on the windows. But all the second-floor windows—and most on the first floor—were badly stained from pooling condensation.

The owner confirmed that condensation covered most of the windows for most of the winter. The sources of moisture were many. For the first two years, the house had a wet basement each spring. (This was finally cured by regrading around the foundation.) There were no bathroom or kitchen fans, and the dryer vented indoors. The house is heated mostly by a wood-fired furnace in the basement, which tends to keep the basement warm and drive any moisture upstairs. To this day, the basement houses wet firewood.

Up in the attic, the owner and I found black mold covering the underside of the sheathing on the north side. The wood felt wet. If you find mold in your home, see ACTION GUIDE - WHAT TO DO ABOUT INDOOR MOLD, and when renovating or repairing a home, to avoid future mold problems also see MOLD PREVENTION GUIDE.

What let moisture into the attic were eight recessed lights, along with the usual wiring, plumbing, and framing holes. The attic was vented with two large gable-end vents and small, round, plug-type soffit vents. Judging by the mold, however, the vents could not handle the excessive moisture load. Surprisingly, the home’s interior had no musty smell, and no obvious signs of water damage other than on the win dow sash. All the damage was “safely” hidden from view.

What Caused Structural Rot on This Wood Framed Building?

So what caused the problem? In short, a combination of green wood, a moist house, a cold-side vapor barrier, and a cold climate. The timber-frame, built of 8x8 hemlock beams, had been assembled green in the fall and closed in in the spring. Since wood does not dry well in the cold, it was probably still quite wet when wrapped in foam the following spring. The water in the green wood gave the decay fungi a head start the first year.

Why didn’t the beams dry toward the inside of the house over the summer? They did—at least near the inside faces, which became severely checked. But when winter came, the high moisture levels in the house drove the moisture back into the beams toward the sheathing, where it condensed.

The large gaps in the 8x8s provided an easy path for moisture into the wood, which is quite permeable anyway. Moisture could also penetrate the wall along the sides of the beams. Other interruptions in the vapor barrier—at floors, ceilings, and electrical outlets— let more moisture into the wall cavities. The inside face of the foam was below the dew point of the moist interior air throughout much of the winter (see DEW POINT TABLE - CONDENSATION POINT GUIDE).

The exposed inside sections of the beams dried, but the wet outer sections festered. Enough water got into the wood each winter so that warm spring temperatures caused decay before the wood could dry out. The foam kept the wood from drying outward, and kept the sun from drying the wood inward. By midsummer, perhaps, the wood fell below saturation levels, stifling decay growth. But the next winter the cycle would repeat.

What Is the Treatment for Structural Rot in a Building?

To prevent further deterioration, the wood must be kept dry. A building consultant, Bill Lotz, recommended a three-pronged approach:

  1. Keep household moisture levels down by adding fans and venting the dryer outdoors. Also see VENTILATION, WHOLE HOUSE STRATEGIES.
  2. Seal the checking in the beams and the gaps around the beams with caulking. Then seal the beams with a clear finish.
  3. Replace the foil-faced sheathing at the beams with beadboard to allow some drying at these points. It is possible that just reducing the moisture level would do the trick. In fact, most of the damage may have occurred in the first two years when household and wood moisture levels were highest. But several layers of defense is the best approach.

Conclusions About Rot, Condensation, Moisture, Insulation, Vapor Barriers on Wood Frame Houses

Timber frame rot (C) Daniel FriedmanUp to a point, wood-frame houses are forgiving; they can safely store winter condensation and get rid of it in the spring. But if you push things too far. watch out.

This house violated too many rules. It combined too much moisture with too little ventilation, too cold a condensing surface, and too few opportunities for the wood to dry out. The moisture balance was tipped the wrong way, and the consequences were severe.

But how far is too far? What precautions should you take?

If you like to live dangerously, you— or an engineer—can make an educated guess about how wet a given wall system will get in a given climate, and how fast it will dry. But there are always unknowns.

To play it safe and allow for a margin of error, you should design for dry wall and ceiling cavities. Keep in mind that:

  1. You can’t control how a home owner will run a house, but you can reduce the likelihood of high moisture levels. At a minimum, install kitchen and bath fans, continuous soffit and ridge vents, and build a dry foundation. Inform the customers that if they have condensation all winter long on double- glazed windows, they need to reduce moisture levels.
  2. A vapor barrier with gaping holes (like the beams and recessed lights) is no barrier at ail. To keep moisture out of the walls and ceiling, seal all significant seams and holes in the air/vapor barrier. If you use recessed lights, put them in a dropped ceiling or use IC-type units.
  3. In cold climates, keep the exterior of the wall more permeable than the interior, or keep the sheathing temperature warm enough that condensation there is rare. In practice, this means don’t use foil-faced exterior insulation at all, or use a lot of it-at least two inches. Better yet, put it on the inside. 4. And for remodelers: Don’t weatherize a house without solving moisture problems first.

Also see DEW POINT TABLE - CONDENSATION POINT GUIDE for a sidebar to the original of this article: an explanation of how to calculate or look up the dew point - the temperature and relative humidity at which condensation will occur in building cavities or on a building surface.

-- Adapted with permission, from material by Steven Bliss and appearing originally in the February 1987 issue of The Journal of Light Construction. A Printer friendly PDF version of this article is provided courtesy of the Journal of Light Construction. This and other JLC articles about good construction practices can also be found at the Jornal of Light Construction website.


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