Derailing the Ice Dam: Moisture Management is Key in Colder Climates

Ice dams have plagued roofs in cold-climates for years. When accumulated snow melts and flows down a slanted roof, it remains liquid, insulated under a blanket of snow. When it reaches the eves, it encounters freezing air, and ice accumulates forming a dam. This ice dam impedes the proper drainage of melt water and precipitation from the roof, and may result in leaks, and damage to ceilings, walls, and insulation.

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Weather conditions conducive to ice formation, limited ceiling insulation, and inadequate air leakage control have resulted in formation of ice dams on this home. Control of heat and moisture flows is an ongoing area of research at FPL.

But ice formations aren’t only a threat to the building — the dangers ice can pose to occupants and bystanders are real. In 2010, for example, falling ice killed five and injured 150 people in St. Petersburg, Russia, following a particularly cold winter.

Roof ventilation is the most commonly used strategy for the prevention of ice dams, but researchers aren’t convinced that ventilation is a silver-bullet to halt ice dam formation. A comprehensive review of studies concerning roof ventilation by various research organizations throughout North America was completed by former Forest Products Laboratory (FPL) researcher Anton TenWolde and Bill Rose of the University of Illinois. That review, which was published 13 years ago, still serves as a good guide for how to limit the likelihood or severity of ice damming.

The full article, published in ASHRAE journal, is available here. After a careful evaluation of the selected studies, TenWolde and Rose offer the following conclusions on how homeowners, designers, and builders can help derail the dams:

1. Indoor humidity control should be the primary means to limit moisture accumulation in attics in cold and mixed cli­mates; we recommend attic ventilation as an additional safe-guard. 

2. To minimize the danger of ice dam formation, heat sources in the attic and warm air leakage into the attic from below should be minimized. The need for venting to avoid icing depends on the climate and the amount of insulation in the ceiling. How­ever, ventilation is necessary in climates with a lot of snow to prevent icing at eaves, regardless of insulation level. 

3. We recommend venting of attics and cathedral ceilings in cold and mixed climates. However, if there are strong rea­sons why attic vents are undesirable, unvented roofs can perform well in cold and mixed climates if measures are taken to control indoor humidity, to minimize heat sources in the attic, and to minimize air leakage into the attic from below. However, ventilation is necessary in climates with a lot of snow to prevent icing. 

4. Ventilation should be treated as a design option in cold, wet coastal climates and hot climates. Current technical infor­mation does not support a universal requirement for ventila­tion of attics or cathedral ceilings in these climates. 

In summary, for each of the most commonly cited claims of benefits offered by attic ventilation (reducing moisture prob­lems, minimizing ice dams, ensuring shingle service life, and reducing cooling load), other strategies have been shown to have a stronger and more direct influence. Consequently, the focus of regulation should be shifted away from attic ventila­tion. The performance consequences of other design and con­struction decisions should be given increased consideration.

Composite Products and Moisture Management.

Composite wood products posses different characteristics than lumber or timbers. Whereas lumber and timbers are solid wood, engineered composites use adhesives to bond various layers, wood chips, or particles together. In terms of moisture and durability, composite products deserve special consideration.

According to Build Green: Wood Can Last for Centuries, composite products are an increasingly common building material used in building construction. Products such as oriented strand-board, commonly called OSB, are composed of large wood chips compressed and held together with adhesive. Composites are an efficient way to use smaller pieces of wood that might otherwise go to waste. Products such as I-joists and rim boards make up a growing proportion of materials used in home construction.

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I-joists are set above structural timbers to provide support for floors and roofing.

Knowing about the special needs of composite products can save homeowners from costly repairs. It is very important to minimize wetting and to maximize drying when composites are exposed to water.

Composite products for exterior applications, such as marine-grade plywood, composite siding, or exterior grade OSB, may be treated with preservative chemicals. Wax added to the adhesive provides additional protection from water. Composite products, such as OSB or plywood sheathing and subflooring, can absorb water more readily than solid wood, particularly from cut edges that are wetted repeatedly. Absorbing water causes these products to swell (or in the case of plywood, to delaminate); once these products have absorbed enough water to swell, they dry very slowly and are vulnerable to decay. When they eventually dry, they do not return to their original dimensions and consequently lose strength.

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Close-up end view of OSB (left) and wetted OSB (right) that has absorbed water and swelled. (Photo provided by Steve Easley, Steve Easley & Associates, Inc.)

 

Water and Wood Decay: A complicated relationship

As discussed in Build Green: Wood Can Last for Centuries, water is the main culprit in wood decay. Because of this, buildings should be designed to minimize wetting of wood or to maximize how quickly wood dries when wetted by rain.

As an example, cliff dwelling ruins of the ancient Anasazi people at Kiet Siel (“broken house” in Navajo) in northern Arizona date back to the 13th century. Dwellings in this semi-arid region are sheltered from occasional precipitation by cliffs overhead. Wood beams and superstructures have remained sound for centuries because they have always been too dry to decay.

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The cliff dwelling ruins at Kiet Siel in northern Arizona date back to the 13th century.
(Permission from the Wetherill family.)

Dry wood will last indefinitely. It may come as a surprise then that wood can also be too wet to decay. Just like all living organisms, fungi require oxygen to live. When wood is submerged in water, air is driven out of all the cells, and decay fungi cannot grow. As an example, the remains of 34 Byzantine ships dating from between the 7th and 11th centuries were uncovered below sea level in Istanbul, Turkey. The wood remained intact because there wasn’t enough oxygen to permit wood-decay fungi growth (see below). Wood too wet to decay is not likely to be an issue for the homeowner, but these historical examples illustrate the point that wood-decay fungi need the right mix of air, moisture, temperature, and food source material in order to thrive.

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Wood Decay: Homeowners must remain vigilant.

Harkening back to our Lab Notes post on the report, Build Green: Wood Can Last for Centuries, let’s discuss what causes wood to decay.

What Is Wood Decay?

 A close-up look at wood through a microscope reveals that it consists of many thick-walled cells that are like hollow tubes running through the wood. The arrangement of the cells varies to give different types of wood different properties, such as appearance, strength, or resistance to decay.

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Hyphae (pronounced “highfee”) are thread-like fungal structures that are capable of moving from wood cell to wood cell when they grow within a piece of wood. As they grow, fungal hyphae use the wood for food, causing structural damage called decay. To see a larger image visit the FPL Flickr site.

Decay is caused by microscopic thread-like fungi that attack the thick cell walls of wood. To live, decay fungi need four things: favorable temperatures, moisture, air, and suitable food material. If the other conditions are right, fungi get their food from wood. Controlling their growth usually results from depriving fungi of the one condition that is simplest to control—water. It is important to know that water by itself does not decay wood. Moist wood is more likely to decay because the spores (like seeds of a flowering plant) from which decay fungi grow and attack wood cells are everywhere in the environment. If provided with enough moisture, these fungi will destroy cell walls and weaken wood.

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Photo by Stan Lebow, FPL.

In the advanced stages of decay, fungi produce fruiting bodies (mushrooms) on wood. Each fruiting body produces billions of spores that when released are blown about by the wind. If they land on moist wood, they may begin growing and start the decay process again. You can see why the homeowner must remain vigilant.

 

Build Green: Wood Can Last for Centuries

One of FPL’s most beloved and long-lasting publications was the 1976 brochure by Rodney C. DeGroot, “Your Wood Can Last for Centuries.” In 2012, folks in FPL’s Durability and Wood Protection Research Team decided that this historical publication needed updating and revision.

The colorful and practical report that resulted was Build Green: Wood Can Last for Centuries by Carol A. Clausen and Samuel V. Glass. This report explains why wood decays, alerts the homeowner to conditions that can result in decay in buildings, and describes measures to prevent moisture-related damage to wood.

Wood is our Most Valuable Renewable Resource

The use of wood in home construction affects our environment in ways that are not obvious to most homeowners. Efficient use of wood as a green building material promotes healthy forests that, in turn, clean the air of greenhouse gases and purify drinking water. Wood is not only a versatile structural material, but its use for home construction also reduces the effects of climate change by storing carbon for as long as the home exists. Thus, the longer the service life of the building, the greater the benefit to the environment. One limitation that can shorten the service life of a structure is wood’s vulnerability to moisture and decay. Yet wood buildings can last for centuries without decay problems.

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The Fairbanks House in Dedham, Massachusetts, circa 1636, is the oldest reported frame house still standing in America today. (Permission from the Digital Archive of American Architecture.)

Why do some homes built of wood last for centuries while others develop decay soon after construction?

The reason is that wood is a biological material. When it is used properly, wood does not deteriorate. However, when misused, wood succumbs to the same biological process that decomposes dead trees in the forest. In other words, it is rotted by fungi or eaten by termites, or both! In the forest, decomposition is a necessary and worthwhile process, but to a homeowner it means costly repairs.

FPL scientists explain why wood decays and alert homeowners to conditions that cause decay in buildings. Being alert to decay hazards can prevent future damage to your current or future home and construction projects. Often, you will find that simple procedures provide remarkable protection. Other times, more drastic repairs are necessary to correct damage and prevent recurring problems. Whatever the damage, it will surely get worse unless you locate the problem and correct it. Yet, historic wood-framed structures illustrate that properly constructed wood buildings can last indefinitely.

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The Marrs Log House near Harrodsburg, Kentucky, was built in 1793. (Photo provided by the Library of Congress.)