Using modern wood for historic restoration

by Katie Daniel | September 8, 2015 10:58 am

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by Laura Ladd
When it comes to historic preservation projects, architects and installers can find themselves at a loss. Wood is the most traditional material, but also notoriously unstable. It has a tendency to warp and becomes vulnerable to rot, decay, and insects. Some replacement products are more durable, but far from historically accurate, such as aluminum-framed windows. However, a growing number of design/construction professionals are discovering a material that allows them to be historically accurate without compromising on durability—acetylated wood.

Though only recently made commercially available, acetylation has been studied for almost a century. A proprietary process modifies wood at its molecular level by increasing the natural compounds already found there—in other words, nothing is added to the material that is not naturally present. The result is a more durable and dimensionally stable solid wood.

More specifically, acetylation targets free hydroxyls, an abundance of chemical groups contained within the wood. Free hydroxyl groups readily absorb and release water according to changes in the climatic conditions to which the wood is exposed, which is the main reason why wood swells and shrinks.

The acetylation process modifies the wood to its core. Therefore, unlike pressure-treated wood, it can be cut or profiled without exposing unprotected surfaces. Further, since acetylation is non-reversible, there is no risk of leaching or loss. Due to these characteristics, acetylated wood is appropriate for a number of exterior applications, including:

• decking;
• cladding;
• windows;
• outdoor furnishings; and
• structural projects, such as bridges.

The technology behind the acetylation process is different for each species; it is currently being used for radiata pine.

Better wood through chemistry
The acetylation process alters the wood’s reaction with water by permanently replacing those free hydroxyls with stable acetyl groups that will not bond with water. With a higher level of acetyl molecules, the shrink/swell cycle is improved by 70 percent or more.

This can make it ideal for manufacturing historically accurate wood replacements, says Brooks Gentleman, owner of Re-View, a window replacement manufacturer.

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“One thing we really enjoy about acetylated wood is that it’s very dimensionally stable, meaning it doesn’t warp and bend to the changes of humidity,” he explains. “So when we’re manufacturing windows, we don’t have to worry about boards that are bent and warped or unsuitable for use.”

He adds the ‘waste factor’ found with other woods such as mahogany is absent, which saves additional money.

The acetylation process also results in a product unrecognizable to insects, and proven to be an effective barrier to rot and decay. This is particularly important to those who work in climates where termites and other wood-boring pests are a constant threat.

“Termites don’t see it as a food product so you don’t have any additional concerns there,” says Hastings Read, vice president of Oakleigh Custom Woodworks (Mobile, Alabama). “When we specify acetylated wood we know the customer is not going to have any problems. [Also,] you’re not departing from the basic idea and authenticity of the original building. It lets the architect comply with historic standards.”

Acetylated wood acts as an effective barrier against a broad spectrum of fungi, including cellar, wet rot, dry rot, soft rot, white, brown, and pore fungi. It is verified by American Wood Protection Association (AWPA) E 10, Standard Method of Testing Wood Preservatives by Laboratory Soil-block Cultures.

The wood carries a Class C fire rating under ASTM E84, Standard Test Method for Surface Burning Characteristics of Building Materials; it can be treated to meet higher requirements. Fire treatments have been found to bring it to Class A without compromising the material’s attributes after acetylation.

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In Europe, acetylated wood carries a Class 1 durability rating under EN 113:1997, Wood Preservatives: Test Method for Determining the Protective Effectiveness Against Wood-destroying Basidiomycetes. This is the highest available and comparable to the best tropical hardwoods. In fact, a recent durability test [4]in New Zealand showed the wood product to be more durable than even old-growth teak.

“We’ve converted a number of different projects from mahogany to acetylated radiate pine, because this is a material that will last like the existing windows,” Gentleman says. “This is a 100-year product.”

The durability and, particularly, stability provide a much more stable substrate for wood coatings. Numerous outdoor and accelerated coating tests demonstrate wood coatings last significantly longer on acetylated wood—film-forming opaque coatings, for example, have been found to endure up to three times longer. Therefore, maintenance needs are dramatically reduced.

“It is a stable substrate for coatings and has good resistance to weathering,” says Roger Rowell, professor emeritus at University of Wisconsin-Madison. “These properties give the acetylated wood a much longer usable lifespan without the use of toxic preservatives.”

Sustainable attributes
Acetylation is an environmentally benign process—no toxins are added to the wood, and the chief byproduct is acetic acid, which is nontoxic and biodegradable. This byproduct is either recycled back into the compound used for the acetylation (i.e. acetic anhydride) or sold into the substantial acetic acid merchant market. Besides a very small amount of natural resins isolated from the wood, there is very little waste, and greenhouse gas emissions (GHG) are minimized.

A recent lifecycle assessment[5] (LCA), conducted by Joost Vogtlander of Delft University of Technology in the Netherlands (and verified by Andrew Norton or Renuables UK), concluded acetylated wood has a much lower environmental impact compared to many commonly used building products, including metals, concrete, and tropical hardwood.

The radiata pine used to make the material can be sourced from Forest Stewardship Council (FSC)- or Programme for the Endorsement of Forest Certification (PEFC)-certified forests. Further, at least one manufacturer has achieved Cradle-to-Cradle Gold certification—a first for wood products under the McDonough Braungart Design Chemistry (MBDC) program.

Acetylated wood can also contribute to multiple credits under the U.S. Green Building Council’s (USGBC’s) Leadership in Energy and Environmental Design (LEED) v4 program, including:

• Materials & Resources (MR) Credit 1, Building Life Cycle Impact Reduction;
• MR Credit 2, Building Product Disclosure and Optimization−Environmental Product Declarations;
• MR Credit 3, Building Disclosure and Optimization−Sourcing Raw Materials; and
• MR Credit 4, Building Product Disclosure and Optimization−Material Ingredient.

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Acetylated wood and window replacements
A look at two projects making use of acetylated wood will provide a good sense of how the material can be employed in applications. With respect to window retrofits, the venerable Minnesota State Capitol is undergoing a $272 million, four-phase restoration plan. The project includes restoring the regal exterior designer Cass Gilbert envisioned when the building was completed in 1905.

A major part of the project was the restoration of the capitol’s original wood windows, which had been covered over with aluminum windows 30 years ago. The aging units—242 in total—suffered from fogging, failing glass, air leakage, and broken balances that rendered the windows inoperable, said Gentleman, whose firm was selected to restore the windows.

“They were boxy, and they didn’t look like the original. They didn’t have the same kind of detailing as the original windows. Plus, the glass was failing and the windows were leaking air because the weatherstripping had failed,” he explains.

The aluminum replacements were removed, revealing the original wood window frames, which were then restored using restoration epoxies and replicated wood parts. Gentleman and company used an original complete wood window that still remained in the building as the basis of design for replicating new sashes with acetylated wood, which will last well beyond the glass.

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On the interior surface, recovered pine—older than a century—from the Idaho Lakes were laminated so the interior surfaces of the windows would match the existing woodwork in the building. Sashes were glazed with laminated insulated glass (IG) units with low-emissivity (low-e) coatings to improve the energy efficiency, security, and sound transmission.

Since some of the individual double-hung windows were about 2 m (6 ft) wide by 4 m (13 ft) tall, the sashes weighed in excess of 115 kg (250 lb). Gentleman’s team engineered a system of weights and pulleys to make the massive windows easy to open.

A combination of historic metal and modern weatherstripping was incorporated in order to seal the operating windows, which produced an impressive outcome. After an independent agency tested many of the installed windows for air and water infiltration, they were shown to be twice as tight as the published ratings for modern replacement windows.

Gentleman credits use of acetylated wood for much of those results.

“Acetylated wood as the base material is very stable; it doesn’t warp and bend like a lot of other woods,” he says. “That allows for more consistency and therefore better performance.”

Restoring Roosevelt’s summer home
Home to the 26^th president of the United States, Theodore Roosevelt, the Sagamore Hill National Historic Site (Oyster Bay, New York) is undergoing a three-year, $7.2-million rehabilitation project, including upgrades to the mechanical and electrical systems and lighting dating back as far as 1918, as well as restoration of historic windows, doors, siding, and porches to historic preservation standards. After years of patchwork renovation to the Queen Anne-style home, the current full-scale rehabilitation will help preserve the historic landmark while helping to avoid ongoing smaller-scale sitework in the future.

Originally built in 1885, Sagamore Hill is a wood-frame-and-brick Victorian structure comprising 23 rooms. It sits on 33.5 ha (83 acres) that also house the Theodore Roosevelt Museum at Old Orchard, icehouse, stables, gardens and sheds, pet cemetery, and a visitor center and nature trail. The home was often referred to as the ‘Summer White House,’ as Roosevelt would host dignitaries during his presidential years from 1901 to 1908. For three decades, Roosevelt and his wife, Edith, raised six children in the house.

For replacement of the wraparound porch, the National Park Service (NPS) selected acetylated wood.

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“In caring for such an important property, we needed to ensure we used the highest-quality, most environmentally friendly products,” says Susan Sarna, museum curator for NPS. “In addition, given the negative impact renovation works have on visitor experience, we wanted to use a product that would allow us to reduce ongoing maintenance at the site. Acetylated wood is longer-lasting than competitive products, is less likely to warp or split, and due to its increased stability, paint coatings last three to four times longer than on traditional wood. Using acetylated wood was therefore an easy decision to make.”

According to Tom Ross, the superintendent at Sagamore Hill, the “project represents a significant investment by the American people, ensuring the Roosevelt home and its irreplaceable collection are protected for future generations to appreciate, gain inspiration and learn from.”

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Overcoming challenges
While all the qualities listed in this article make acetylated wood an ideal choice for historic preservation, architects experienced with the material say it does come with some minor limitations and considerations. The biggest of those may be cost, as acetylated wood often comes in around the same price as mahogany and other tropical hardwoods.

Along with overcoming the cost hurdle, architects and installers must take care to use only stainless steel fasteners with the wood since its acidic quality may corrode other metals, say Read and Gentleman.

Though acetylated wood takes most finishes well, Gentleman worked with a major paints/coatings provider on testing and discovered oil-based primers and acrylic latex are the best choices for finishes.

Read discovered the wood can be brittle, such as at a thin window corner, so care should be taken when handling. He finds the best glue choice to be the epoxies, though they take longer to set up and are harder to use. He also recommends using epoxy to seal holes made for locks or other hardware to prevent off-gassing damage.

Possibly the biggest limitation to working with this particular material is in the mind of architects and installers themselves who are not accustomed to acetylated wood.

“We are a product of our education and experience. Most professional woodworkers are so used to wood moving as the moisture changes and decay issues that they have to work with acetylated wood before they can accept the new material,” Rowell says. “But once they do, most are converted to the new type of wood and thinking.”

Laura Ladd has more than 12 years of market experience and deep knowledge of the timber industry, including four years with Accsys Technologies, where she currently heads up worldwide marketing for the Accoya and Tricoya brands. Ladd began her career at Weyerhaeuser Europe Ltd as an internal sales assistant for Medite MDF before joining Coillte Panel Products’ marketing department in 2007. She has completed the U.K. Institute of Wood Science (IWSc) foundation course. Ladd can be reached at laura.ladd@accsysplc.com[10].

Source URL: https://www.constructionspecifier.com/using-modern-wood-for-historic-restoration/

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