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 Mobile ViewMobile ViewSTRUCTURAL INSPECTIONS & DEFECTS ADVANCED INSPECTION METHODS BEST CONSTRUCTION PRACTICES GUIDE BUILDING SAFETY HAZARDS GUIDE CHIMNEY INSPECTION DIAGNOSIS REPAIR CONDENSATION or SWEATING PIPES, TANKS COOLING LOAD REDUCTION by ROOF VENTS Cracks, Checking or Splitting Beams & Log Homes CRAWL SPACES CRAWL SPACE DRYOUT PROCEDURES CRAWL SPACE GROUND COVERS CRAWL SPACE INSULATION RETROFIT CRAWL SPACE SAFETY ADVICE CRAWL SPACE VAPOR BARRIER CRAWLSPACE MOLD ADVICE MOLD CLEANUP by MEDIA BLASTING MOLD ON DIRT FLOORS SUMP PUMPS DECK & PORCH CONSTRUCTION DECK COLLAPSE Case Study DECK FINISHES COATINGS PRESERVATIVES DECK FLASHING LEAKS, ROT Case Study DECKS, ROOFTOP CONSTRUCTION DEFINITIONS of Mobile Home, Doublewide, Modular, Panelized DEFINITIONS of ENGINEERED WOOD OSB LVL etc CONDENSATION or SWEATING PIPES, TANKS COOLING LOAD REDUCTION by ROOF VENTS DEHUMIDIFICATION PROBLEMS DEW POINT CALCULATION for WALLS DEW POINT TABLE - CONDENSATION POINT GUIDE DISASTER BUILDING INSPECTION & REPAIR SAFEY EARTHQUAKE DAMAGED FOUNDATIONS ENERGY SAVINGS in buildings FLOOD DAMAGE ASSESSMENT, SAFETY & CLEANUP FLOODS IN buildings-mold FLOOR, ENGINEERED WOOD & LAMINATES FLOOR FRAMING & SUBFLOOR for TILE FLOOR TYPES & DEFECTS FLOOR TILE ASBESTOS IDENTIFICATION FLOOR TILE HISTORY & INGREDIENTS FOOTING & FOUNDATION DRAINS FOUNDATION BULGE or LEAN MEASUREMENTS FOOTING & FOUNDATION DRAINS FOUNDATION CRACKS & DAMAGE GUIDE FOUNDATION WATERPROOFING FRAMING DAMAGE, INSPECTION, REPAIR FRAMING DETAILS for BETTER INSULATION FRAMING DETAILS for DOUBLE WALL HOUSES FRAMING & SUBFLOOR for TILE FRAMING MATERIALS, Age, Types FRAMING METAL STUD PERFORMANCE FRAMING METHODS, Age, Types FRAMING SIZE & Spacing, Age, Types FRAMING TABLES, SPANS for DECKS FRENCH DRAINS FROST HEAVES, FOUNDATION, SLAB FRT PLYWOOD HOUSEWRAP INSTALLATION DETAILS I-JOISTS, Wood Roof Floor KIT HOMES, Aladdin, Sears, Wards, Others LEED GREEN BUILDING CERTIFICATION LOG HOME GUIDE Antique & Old Log Cabins Cracks, Checking or Splitting Beams & Log Homes Condensation & Moisture in Log Homes Energy Efficiency of Log Homes Leak Diagnosis & Cure for Log Houses Log Wall Height Changes R-VALUES & THERMAL MASS in LOG HOMES Sealants, Caulks, & Coatings for Log Homes Shrinkage In Log Home Walls Slab Log Cabin Siding Spline & Gasket Designs for Log buildings Spline Gaps & Gasket Omissions Types of Log Houses - Log Choices Vertical Log Walls on Cabins & Homes Wall Insulation Values in Log Homes PORCH CONSTRUCTION & SCREENING PRE-CUT & KIT HOMES Preservative-Treated Framing Lumber MODULAR HOME CONSTRUCTION MOBILE HOMES, DOUBLEWIDES, TRAILERS MOISTURE CONTROL in buildings ATTIC CONDENSATION CAUSE & CURE Attic Moisture or Mold Sources BASEMENT CEILING VAPOR BARRIER BASEMENT HEAT LOSS BASEMENT LEAKS, INSPECT FOR Chimney Leaks CONDENSATION or SWEATING PIPES, TANKS CRAWL SPACE DRYOUT PROCEDURES DEW POINT CALCULATION for WALLS DEW POINT TABLE - CONDENSATION POINT GUIDE DRYER VENTING DRYWELL DESIGN & USES EFFLORESCENCE, Salts & White / Brown Deposits FLOOD Damage Assessment & Repairs FLOOD DAMAGED FOUNDATIONS FLOOD VENTS & FLOOD PORTS FLOODS IN buildings-priorities FOUNDATION WATERPROOFING GRADING & SITE WORK, EXTERIOR GUTTERS & DOWNSPOUTS HUMIDITY CONTROL & TARGETS INDOORS HOUSEWRAP AIR & VAPOR BARRIERS LOG HOME Leak Diagnosis & Cure LOG HOME Condensation &Moisture MOISTURE in BUILDING WALLS, EFFECTS MOISTURE in CELLULOSE INSULATION MOISTURE CALCULATIONS MOISTURE METER STUDY MOISTURE PROBLEMS: CAUSE & CURE MOLD in buildings Mold in Fiberglass Insulation Mold on Books, Book Conservation Mold on Fiberboard Insulating Sheathing MOLD PREVENTION AFTER FLOODING MOLD RESISTANT CONSTRUCTION ROOF VENTILATION SPECIFICATIONS Soffit Intake Vents & Attic Condensation WATER ENTRY in buildings VENTILATION, WHOLE HOUSE STRATEGIES MOLD INFORMATION CENTER ROOFING INSPECTION & REPAIR ROT, FUNGUS, TERMITES TERMITE SHIELDS vs TERMITICIDE ROT, TIMBER FRAME SHEATHING, Gypsum board SHEATHING Homasote & Other Board SHEATHING, OSB SHEATHING, Plywood SHEATHING, FOIL FACED - VENTS SINKHOLES, WARNING SIGNS SINKING BUILDINGS SLAB CRACK EVALUATION SLAB INSULATION, PASSIVE SOLAR Splits in Structural Wood Beams STAIRS, RAILINGS, LANDINGS, RAMPS STONE CLEANING METHODS STUCCO WAll FAILURES DUE TO WEATHER STUCCO WALL METHODS & INSTALLATION STUCCO OVER FOAM INSULATION STUCCO PAINT FAILURES VENTILATION in buildings WALL FINISHES INTERIOR WATER BARRIERS, EXTERIOR BUILDING WIND ENERGY SYSTEMS WIND TURBINES & LIGHTNING WIND WASHING INSULATION At EAVES WINDOWS & DOORS WINTERIZE A BUILDING Wood Burning Heaters Fireplaces Stoves ZONE VALVES More Information |
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. Also see these key related articles, MOISTURE PROBLEMS: CAUSE & CURE and HOUSEWRAP AIR & VAPOR BARRIERS and also VENTILATION in buildings. 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. © Copyright 2012 InspectAPedia.com, Daniel Friedman, Steve Bliss, Wiley & Sons, All Rights Reserved. Information Accuracy & Bias Pledge is at below-left. Use page top links to major topics or use links at the left of each page to navigate within topics and documents at this website. Green links show where you are in a document series or at this website. This Case Study of Timber Frame Building Rot Explains the Role of Vapor Barrier, Insulation, & Moisture"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).
Investigating Rotting Beams on a Timber Frame HomeThe 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.
Further Diagnosis of Structural Rot on a Wood Framed BuildingWhen 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:
Conclusions About Rot, Condensation, Moisture, Insulation, Vapor Barriers on Wood Frame Houses
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:
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. Questions & Answers regarding this article. Ask a Question or Search InspectAPediaHTML Comment Box is loading comments...
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