Which type of siding is greener? Which type of roof?

Asked by Carl

I am planning to build a new home. Which would be greener choices for me to make: * Cement fiber lap siding vs. real cedar siding? * A standing seam metal roof vs. a composite material that contains plastics, fibers and tire-derived rubber? There are also components that protect it from UV degradation and provide the necessary coloring to replicate the look of cedar shakes.


Mick Dalrymple

Answered by Mick Dalrymple

Scottsdale, AZ

Eco-Friendly Building Center

May 1, 2008

Great questions that get to the heart of one of my driving passions today: the development of a broad, consensus-based, comprehensive, transparent green products rating system. There are no single answers to your questions because there is no industry consensus on what constitutes a green product. There are efforts afoot to get there, though, and we can look at the pros and cons of your options.

While green product certifications in the market today tend to focus on single (or a few) attribute(s), like indoor chemical emissions or whether wood comes from a sustainably harvested forest, the newer efforts are focused more on Life Cycle Assessment (LCA), attempting to measure the total environmental and human health impacts of the product starting from when the raw materials are harvested or mined and ending with end-of-use impacts. Systems and data to measure these impacts are still very imperfect. I believe that a true sustainability-oriented certification should also look at the whole "triple bottom line" (people, planet and prosperity) and include impacts such as fair trade and fair labor conditions. For instance, manufacturers who protect against child labor and slave labor from their suppliers should be recognized because they are helping avoid injustice and potential social unrest that has a definite cost to sustainable society.

With some of the imperfect information we do have, we can look at the trade-offs of the products you are considering.

Cement fiber lap siding is more durable than real cedar siding and requires less maintenance but also has a much higher embodied energy level. The cement industry has worked to reduce energy consumption significantly in newer plants over the past several years, but the extraction and manufacturing process is still much more energy-intensive than cutting down and sawing trees that utilize the sun as their "manufacturing" energy source.

Trees also sequester carbon, whereas cement production emits large amounts of greenhouse gases. However, clear-cutting forests to create siding creates enormous environmental damage and eliminates the forest’s ability to sequester carbon. Always look for wood that is certified as coming from sustainably managed forests, which is identified by the FSC (Forest Stewardship Council) label. FSC certification includes some social equity criteria.

So, while from a durability and maintenance perspective, cement fiber siding is the way to go, I would argue that FSC-certified real cedar is still a better choice from an overall sustainability perspective. While you’re at it, utilize environmentally friendly stains or paints. They are better for those applying the coatings, better for the planet, and better for end-of-life considerations.

The roof options present additional trade-offs. Most metal products in the United States already contain a high percentage of recycled content, so you are minimizing extraction of virgin materal from the earth. The embodied energy of recycled metal is less than for virgin metal, but still high. Besides having a very long life, metal roofs are also recyclable at the end of their life. Newer coatings technologies allow you to have a spectrally selective roof in a wide range of available colors that acts as a "cool roof" in that it reflects heat away from the home. Look to see if the products you are considering have a high Cool Roof Rating Council rating. Even as the cost of metal soars with growing world demand, its use as a roofing material is a good choice.

The composite roofing option presents some interesting questions. Recall that the composite decking industry had issues early on with mold and rot of fibers embedded in the composite. This may not be an issue if the fibers are cedar or if the manufacturer has overcome the problem in another way and has a strong warranty. Another consideration is whether the plastic is recycled or not. Virgin plastic means fossil fuel ingredients and intensive energy to produce. Composites have the additional end-of-use issue of being composites – you can’t separate the ingredients to recycle them.

I would consider using a roofing tile made of 100% recycled HDPE plastic that looks like a wood shake or slate tile. It eliminates the potential issues described above and diverts plastic bottles from the waste stream. We know those are a plentiful resource. Stick with a lighter color to reduce heat transfer. Both that and metal are good choices. Both have the added benefit of being much lighter than their environmentally challenged cousin, the asphalt shingle.

Tagged In: fiber cement siding, metal roofing, wood siding, composite roofing

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