Sustainability and siding
Natural eastern white cedar shingles. Note the beginning of aging patterns. Photo by John Hession.
An architect's guide to choosing siding for your house
By Chris Briley
I’ve heard it many times.
“I like wood and I want to be as sustainable as I can, but I’m tired of painting. I also want my siding to be affordable, attractive, and require little maintenance. What are my options?”
Maibec® white cedar shingles (with water-based stain) and Maibec® prefinished pine vertical siding. Photo courtesy of BRIBURN
This is a question that is not easy to answer. Every word that is in bold is subjective, and every homeowner places a slightly different priority on each. Because of this, there is no one “slam-dunk” answer. But the good news is that there are many options, and this article will review some of the more readily available.
Costs and availability of siding vary widely with fluctuations in the market and from region to region, so note that this article was written in 2019 with a primarily New England market in mind.
At the end of this article, a comparative chart has been provided to help summarize how all the below materials rank across these different criteria.
natural wood
Nature has bountifully provided us with one of the world’s most technically advanced building materials. It is strong, light, easy to work with and highly durable if installed correctly and properly maintained. Not all woods are suited for exposure to the elements, however. In fact, most common American hardwoods do not fare well when subjected to weather (though the heartwood of some do surprisingly well). Coniferous woods (softwoods) tend to do better and are more affordable. Luckily for the consumer, market forces have culled the offerings of naturally rot-resistant wood to some of the more affordable species.
Eastern White Cedar
Possibly one of the most sustainable choices for siding in our region is eastern white cedar. Cedar is naturally rot-resistant thanks to the oils and acids (polyoxyphenols) contained in its heartwood. This is also what gives it its pleasant and strong aroma. In fact, it starts to become evident that the more pungent and “piney” the coniferous wood smells, the more of these natural phenols it has, and the more rot-resistant it becomes.
It should be noted that sap wood and heartwood are very different. There is no such thing as naturally rot-resistant sapwood; it is really only the heartwood, the harder centers of the tree, that can be truly rot-resistant, and this is also why you might have a random shingle (or part of a shingle) weather faster than its adjacent shingles when it’s from the same batch, even the same tree.
Eastern white cedar is a smaller tree and so is often used for smaller pieces, i.e. shingles. If the shingles are thick enough and made from the heartwood of the cedar tree, they can be installed over a rainscreen matrix, be left completely untreated and perform beautifully for decades.
If the cedar is also sustainably harvested (meaning it is from a managed forest that is growing as fast or faster than it is being harvested), then this is an extremely environmentally responsible material.
Though harder to come by, white cedar can also come in shiplap, board and batten, and any variety of clapboard profiles. Its life can be lengthened by the application of sealants, stains and a vast array of other coatings, though many products come pre-finished. Companies such as Maibec® manufacture pre-finished cedar shingles and pre-finished pine siding. Their factory finishing process applies a penetrating solid stain on all sides of the wood product that comes with a 10-15-year warranty and performs exceptionally well.
Weathering and aesthetics
An observant road trip through Maine reveals very well now natural wood (usually cedar) weathers. Where it sees moisture, it turns gray, losing the pigment of the intact cells as they open and close, rupture and wear. Where it sees both water and sun, the UV rays of the sun can work on the damaged cell structures and react with the tannins, which accelerates this process to create a darkening effect. Thus, when looking at a classic New England cape: under the eaves where the wood is better protected, it is less gray. Where it actively sees water, it is more gray. Where it sees the most sunlight, say on the south side of the building, it darkens.
FSC Certification
The Forest Stewardship Council has set up a standard by which a chain of custody is established for lumber. If a piece of siding is certified, then it comes with paperwork that traces it back to the forest from which it came, and that forest has been certified as a sustainably managed forest (meaning that it grows as fast as it is harvested). This is a beautiful concept that helps us, the consumer, know that our purchase is a sustainable one. Here in Maine, the most forested state in the nation, we are blessed in that most of our lumber forests are sustainably managed.
Why, then, is FSC so hard to come by? Why is there often a premium to pay for purchasing FSC lumber? Because the lumber industry is not set up to handle such administrative paperwork. Lumber is also treated like a commodity: harvested, milled, shipped, stored, mixed, bought, sold, shipped, stored again, sold again.
The author hopes this certification process gets streamlined and easier somehow in the very near future. If we want to reduce the embodied carbon in our buildings, we need to support a carbon sequestering process like sustainable forestry. A tree grows for decades, building itself from the carbon it has pulled out of the air; to then take that carbon (wood) and store it (build with it) successfully sequesters that carbon, and that wood is actually carbon negative if, and only if, the forest from which it came is growing as strong and sustainably as when it was harvested.
Western Red Cedar
Like its eastern cousin, red cedar is naturally rot-resistant. It is a much bigger tree and yields more useable clapboards and shakes. As its name implies, the wood has a reddish, warm hue. Because of this and the higher level of tannins in it, it tends to age to darker tones than eastern white. (Depending on the exposure of the wood, it is not uncommon to turn completely black.) One drawback for us here in the East is that this species must be transported across the country, adding to its carbon footprint and cost.
Pine (Spruce, Pine, Fir, Hemlock, Larch)
There are numerous other species of affordable woods available in siding profiles (clapboards, shiplap, etc). These are typically meant to be sealed or painted — and should be — to greatly extend their life span. When installed properly, with ALL sides coated, and over a rainscreen layer, these wood sidings can be very durable, especially if the coatings (finish paint, sealer and/or primer) are factory-applied.
Redwood and Cypress
These are two other popular, naturally rot-resistant wood species that deserve mention. For us here in Maine and the Northeast, these are not indigenous species and are therefore used far less often, as they tend to cost more and require more energy and cost to transport. Each is beautiful and offers great durability, so if you happen upon a batch that has somehow already made its journey here and is looking for a home, it might be worth consideration.
Exotic woods
If one’s value set is focused more on durability and appearance than on cost or sustainability, then exotic woods may enter the picture. There are numerous species available for decking and siding, primarily mahogany, teak and ipe.
Most of these come from South America and most do not come from sustainably harvested forests. In fact, most contribute to the deforestation of tropical rainforests, where up to 50 acres of forest are clear cut for every one good “marketable” tree. This statistic is arguable, but regardless, harvested exotic woods from South America simply do not play a role in the sustainable low-carbon future.
Treated Wood
There are a variety of different products on the market today that treat wood with proprietary chemicals and processes to enhance the durability of natural wood and in many cases, enhance the appearance as well. Our architectural office has used a few of these with substantial success.
Acetylated wood
Acetylation is essentially the pressure treating and kiln drying of wood (primarily radiata pine), only instead of using toxic chemicals and copper to treat the wood, it uses acetic anhydride. This sounds science-y and like a highly synthesized chemical, but in fact is basically industrial strength vinegar. It is non-toxic, according to the manufacturer, and boasts many sustainable accolades. The only treatment plant currently making this product is in the Netherlands, so even with all its sustainable virtues, it still represents a larger carbon footprint due to the energy required to transport it.
Thermally modified poplar being installed over a rainscreen—vertical wood strips that create ventilation channels behind the wood siding.
Rainscreens
If a porous material such as wood is able to dry, it will last a long time. This is why most sidings should be applied to a rainscreen, which typically is comprised of vertical strapping (or strips of wood) that are affixed to the sheathing (usually through a weather-resistant membrane). This creates an air gap between the siding and the weather barrier of the house. It is usually screened at the top and bottom to prevent insects from nesting in the cavity but also to allow air to flow freely behind the siding. In the case of shingles, a thick mesh sheet is applied to the weather barrier and provides a similar function. Without this, any failures in the siding are compounded by the fact that any water that gets in can’t get out, and the siding stays wet, aging much faster. Anyone who has owned a wood-sided house lacking this important feature knows that painting becomes an every-other year chore.
Torrefied (thermally modified) wood
Also called thermally modified wood, this is a process where wood is heated at extreme temperatures in an anoxic (oxygen- free) kiln so the wood won’t ignite. This process transforms the cellular structure of the wood and cooks off the natural oils, sugars and resins within it. Some manufacturers capture the steam and smoke from the process and reignite it to heat the kiln, essentially using the wood to help cook itself. The result is a wood that has been robbed of its energy, making it resistant to mold, rot, pests and decay. I often liken this process to that of beef jerky. A steak in the fridge will only last a few days, but after being cooked and dramatically dehydrated, rendering all the high energy fats out, the resulting jerky can last weeks, even months.
Shou sugi ban
Torrefication has its roots in the ancient Japanese method of charring wood to preserve it. Here in the West it is known as “shou sugi ban;” in Japan it is actually known as “yakisugi,” loosely translated as burned cedar (or cypress). This method entails burning or charring Japanese cypress and then saturating the charred surface with a natural oil. The result is a handsome charcoal-black siding that is rot-resistant and pest-resistant. Thanks (in part) to this method of preserving an already hardy species of wood, in Japan you can find wood structures over 1300 years old.
This technique has recently gained in popularity, driven mostly by aesthetics, the desire for natural products and by the hardy do-it-yourselfers who happily trade off labor for cost.
Because of the many techniques and variations of this method, Yakisugi possibly deserves its own article, but there are some important things to note. First, results vary with different woods, and the act of charring wood does not automatically make it more durable. Second, the charring is not permanent. It will wear away with the elements, leaving a darkened aging wood that is no longer black (though to some this is even more beautiful). Lastly, the process is labor- intensive and messy.
Composite Siding
The market’s quest for more affordable and durable siding has led to the creation of several composite siding products. As with any synthesized material, its chemistry is complex, and with complexity comes many pros and cons.
Fiber cement siding
This type of siding has grown tremendously in popularity in the last decade or so, and it’s easy to see why. It is affordable and often the same price as (or even cheaper than) painted pine clapboards. It comes in a wide range of styles, finishes, textures, sizes and configurations, and is readily available. It is made from cement, wood pulp and silica (or fly-ash for some products). It is highly durable, rot-resistant and, when installed correctly, moisture resistant and able to hold its finish for a long time.
Fiber cement siding requires a bit more care in installation. Its flexible nature allows it to follow the contours of its substrate, so any imperfections or variations in the sheathing or strapping will translate through and be visible in the finished product. When cutting fiber cement with a saw, it erupts into a cloud of very fine silica dust that is hazardous to inhale (though not directly toxic). Most manufacturers recommend cutting shears for this reason. Some builders, for ease of construction, choose saws and (hopefully) masks anyway.
In almost all cases, fiber cement is a painted product, and while it absorbs far less moisture than wood, it is still absorbent. This means that it moves with humidity and frost, and when detailed improperly — say, installed too close to an adjacent roof slope, or too close to the ground, or if the cut ends were not painted or sealed — it can rapidly eject its paint coating, swell and begin to flake and deteriorate. This is not to say that it isn’t durable, but care must be taken to ensure its predicted lifespan.
Fiber cement siding is typically a resource-efficient material (being made from abundant and sometimes recycled materials) but is very high in embodied energy and embodied carbon. As we come closer to the year 2030 (the year sustainable architects aspire to have all buildings be net zero) and 2050 (when the aspiration is for all buildings to be carbon neutral), the embodied energy of materials is playing an ever-increasing role in our decision matrix for selection. This weighs heavily on fiber cement products.
The more popular product brands are James Hardie®, Certainteed®, and Nichiha®. Our office has, over the years, used each, though we tend give preference to Nichiha as it has a higher recycled content.
Fiber-cement clapboards and fiber-cement panels. (Note the new white cedar rails. They will turn gray and match the house in a year). Photo courtesy of BRIBURN
Smartside® and Truexterior®
There are newer players to the stage with different composites to offer. LP makes a composite lap siding branded as SmartSide®. It is made from wood strands, wax and a binding resin, and coated with a zinc-borate primer. This combination avoids the embodied energy issues and silica dust clouds of fiber cement and also offers better toolability. Our office has some reservations about its proprietary “resin” as they will not disclose what it is, though it is not PVC. It has been around a while and is proving to be durable, but it is still too new to know its exact durability.
Another composite product that is new to the scene is TruExterior® by Boral. The product is made of “polyash” composed primarily of fly-ash, a byproduct of the coal industry, mixed with a proprietary polymer as a binder. Like SmartSide’s resin, Boral’s polymer’s full chemistry is not publicly known. It is more difficult to tool than Smartside, being very hard and “chippy” but easier than fiber cement. TruExterior made its entrance to the market as a trim material, where it has been very successful. Being a polymer-based composite, it absorbs even less moisture than the others mentioned in the composite category. Its siding products are very new to the market, and so its true durability remains to be seen.
From a sustainability standpoint, vinyl gets a failing grade. While it does not require paints and coatings and has low embodied energy, vinyl, or PVC, is toxic to make and difficult to recycle.
Vinyl Siding
Vinyl siding gets a bad rap, and it deserves it. Mostly. Many have argued that this is an affordable material that lasts forever, and for those on a tight budget, it is the only option. It is true that it is fairly durable and certainly cheap. Typical vinyl siding, however, does not have the durability that its proponents might claim. It gets brittle, it breaks, it fades or chalks, it slumps.
That said, good installations of vinyl siding do exist, and many people are very pleased by the “curb appeal” of fresh vinyl on their house. One positive attribute is that no rainscreen installation is required — it is self-venting. Of course, this is also a downside because wind-driven rain and moisture easily finds their way behind it (though not in bulk quantities). The substrate of a vinyl siding project needs to be weather-resistant and detailed well.
From a sustainability standpoint, vinyl gets a failing grade. Its pros are that it does not require paints and coatings and that it has low embodied energy. But vinyl, or PVC, is toxic to make and difficult to recycle. Here in Maine, where we have some excellent recycling systems in place, when vinyl siding is removed it invariably ends up in the mainstream trash. If not disposed of carefully, it can end up being incinerated (50% of Maine’s garbage is incinerated), in which case the dioxins of burning PVC will be released into the atmosphere.
This also brings up the point that it is extremely hazardous under fire conditions. It burns very hot and emits highly toxic smoke.
There are some new products on the scene such as Celect® from Royal that take vinyl siding to the next level. It is a foamed PVC (similar to Azek®) and coated with a kynar- based paint finish. This approach certainly takes the “cheap” feel out of vinyl and adds to its durability, but the problems associated with PVC still remain.
Site-formed pre-finished steel siding, FSC Philippine mahogany, fiber-cement panels with exposed fasteners. Photo by Sandy Agrafiotis
Metal Siding
Once relegated to commercial projects, metal siding is beginning to find a market share in the residential sector, usually on modern homes. Most metal sidings are thin steel sheets (20-24 gage) that metal sidings are thin steel sheets (20-24 gage) that
have been coated in a highly durable paint such as Kynar® or an alloy coating such as zinc (galvanized) or galvalume (an alloy of zinc and aluminum). This siding either comes in panels or it is formed in place, much like a standing seam metal roof. There are truly countless options in color, pattern, gloss, orientation, size and fastening technique. It is a highly versatile medium for the architect and usually requires a higher level of detailing. Because of the variation in types of metal siding, the details are far less standard than what one would find in traditional carpentry.
From a sustainability standpoint, metal siding is not a bad option. Steel is relatively high in embodied energy. However, its durability, recycled content and recyclability are also high.
Uncoated metals
It is very rare to see copper as a siding these days; in fact, I only know of one residence in Maine clad in sheet copper. CorTen® steel – or weathering steel – however, is gaining popularity and is starting to be used as a naturally weathering (rusting) siding.
True Cor-Ten is not available anymore as it was exceptionally difficult to detail at its connection points. It was an alloy of copper and steel (two metals that have no business being combined, as they are on opposite ends of the galvanic chart). The second it was made it needed only the smallest amount of oxygen and moisture to aggressively begin rusting, to the point that the rust itself formed an airtight coating around the metal, and the rusting stopped.
Today’s weathering steel is far less “touchy” than its progenitor, but the principle is essentially the same. You might install it and it will look like steel at first, but the goal is for it to coat itself completely in rust. The end result is a rich, deep red-brown material that looks aged and comfortable with its age.
Above, you’ll find a chart that gives each siding material marks in each category. They are given a score of 0-5. The green marks are good (more green = better); the red marks are bad (more red = worse). Hopefully this article and chart have given you a good foundation from which to build your research for the perfect siding for your project.
ABOUT THE AUTHOR: Chris Briley is a partner and principal architect at BRIBURN, specializing in energy-efficient, environmentally-friendly design. A Certified Passive House Consultant, he is a founding board member of PassivhausMAINE, a blogger at greenbuildingadvisor.com and co-host of the Green Architects’ Lounge podcast.
This article first appeared in the Fall 2019 edition of Green & Healthy Maine HOMES. Subscribe today!
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