A Review of Chemical Additives, Toxins, and Offgassing in Siding/Cladding
Those avoiding toxins or who have chemical sensitivities will want to consider the initial chemical offgassing, toxins in the dust, as well as upkeep of the siding. How often the siding needs to be repainted, and how healthy the paint is could be a critical consideration.
The most common types of siding (also called cladding or facade) are wood, engineered wood, fiber cement, metal, vinyl, and stucco.
This post will consider all siding types and how healthy and “green” they are.
This post contains affiliate links to relevant products that I use and recommend. Upon purchase, I earn a small commission at no extra cost to you.
Mold Preventative Siding
Those who are building mold preventative builds can use almost any type of siding as long as it’s installed right. Almost all types of siding require a vented rainscreen. I will note the few types I think will not work for a mold preventative build.
While some siding types like stucco and engineered wood are commonly installed wrong, and therefore make the building prone to mold, they are not in and of themselves wrong materials to use.
Top Choices for Non-Toxic Siding:
Top Choices for the Severely Sensitive: wood that doesn’t require a sealer/paint, most metal, traditional stucco, and possibly plastic.
Top Choices for the Moderately Sensitive: the above options plus HardiPlank, cement stone veneer, wood that requires paint/sealing, bark (like cork), all metal and plastic.
Top Choices for the Less Sensitive: the above options plus engineered wood, and possibly other composite options listed.
1. Wood Siding – Finishes and Offgassing
Types of Wood and Finishes
Wood types used for cladding include redwood, cedar, pine, spruce, fir, cypress, oak, chestnut, and ipé.
Most wood types that require finishing have to be refinished every couple of years, which is a downside for the chemically sensitive. Plus the more “eco” stains need to be redone more often.
But not all wood siding needs to be sealed/finished.
Cedar, Douglas Fir heartwood and Ipé do not need a finish. Ipé though is very expensive (and not very environmentally friendly). Cedar is the most common choice – it’s rot-resistant and does not require a sealant or paint.
Oak does need a finish. Oak is pricy and is more commonly used as siding in Europe
Durability of Wood Siding
Wood siding is more vulnerable to decay, rot, and vermin that non-organic types of siding. Using the proper wood species, care (no sprinklers hitting the building), maintenance (if required), and installation are the keys to keeping the wood from rotting.
Some wood types are more prone to insect attack than others. It also depends on where you live.
If you go with a wood that needs painting, it can still be rot-resistant if the paint finish is re-done on schedule and you take care not to have sprinklers hitting the siding (which they never should anyway).
My post on exterior paints outlines the best brands.
Rot Resistant Wood Siding Types
Redwood, Cedar, Ipé, and specially treated woods (listed below) can be very rot-resistant.
Cedar is the most obvious choice for wood siding. It doesn’t need a sealer. Although it’s not as inexpensive as some non-wood options, it’s less expensive than Shou Sugi Ban, Ipé, and specially treated wood.
Ipé siding is very expensive but is the most durable type of wood. It looks cool too. It does not need a sealer or paint which is great for those avoiding unnecessary toxins.
Rot Resistant Treated Woods
Shou Sugi Ban is wood (usually Cedar) that is charred, and then finished with an oil. I have seen both tung oil and Penofin used. My post on sealers goes into detail on natural oil finishes. You will want to test that out for your tolerance. My post on tung oil goes into great detail on this oil.
In Japan, they don’t traditionally re-oil the siding. In North America, most people do re-oil it every 5-10 years to add longevity to it. But it lasts a long time without this maintenance. This finish makes it more resistant to rot and pests.
Like Thermacork and thermally modified wood, it does smell burnt, which might not be desirable for those sensitive to odors.
Thermally Modified Wood
Thermally modified wood is wood siding treated with heat. It provides durability and protection against rot and termites. No chemicals are used in the treatment. It has the smell of smoked wood. The siding can be stained or left natural.
Thermory is heat-treated wood, they say it lasts as long as a tropical hardwood like ipé.
There is a wax to seal the edges of Thermaroy. Other than those edges it comes finished and you don’t have to refinish this again.
Their shou sugi ban look is not charred – it is cooked the same way as all Thermaroy, the pattern is indented and then it’s stained black.
This is a great video showing its application by Corbett Lunsford. Corbett says in the video if this is used for decking it can “get weird with water”.
Accoya wood (Radiata pine and alder) is treated with acetic anhydride. The treatment renders the wood harder, more dimensionally stable, and immune to insects. Some acetic acid is left in the wood and it does smell like vinegar.
Kebony is another modified wood. Furfuryl alcohol is impregnated into the wood and is polymerized. I’m a little more hesitant to recommend this type of wood due to more unknowns with this chemistry.
2. Metal Siding – Finishes and Offgassing
Steel and aluminum are the two main types of metal used for siding. The benefits to metal siding are that it will last a long time, it doesn’t need to be repainted for decades. There is a 30-year warranty on some products.
The finishes tend not to offgas, or offgas very little VOCs. You should get a sample as there are different finishes.
Kynar 500 a popular high-end long-lasting finish is a polyvinylidene fluoride resin.
Metal siding can have a modern smooth look or it can look like wood.
Copper and zinc can also be used for siding but they are unusual and very expensive. They don’t have a paint or coating on them, which is a plus side for the chemically sensitive.
I’m wary of insulated steel siding due to the fact that it appears to be made to be installed without a rainscreen behind it. MIAQ building science experts say that insulated vinyl impairs the bulk water management that is inherent in regular vinyl siding.
3. Types of Exterior Stucco Siding – Additives and Toxins
Types of Stucco:
Stucco is cement-based. It is made up of sand, Portland Cement, lime, and water. Traditional stucco can be mixed by the contractor using these three ingredients plus water.
Premix stucco, like LaHabra brand, contains some proprietary ingredients. These might be polymers. The severely sensitive may want to mix stucco themselves. If you are not severely sensitive it’s unlikely that these additives will affect you.
The stucco can be tinted with pigments in the mix. Mineral oxides might be used. As with all products that contain silica (in cement), and metal/mineral oxides, take great care with the product when it’s in dust form. Use an N95 mask or better. Once it’s mixed it’s safe.
Traditional stucco breathes but it does require a rainscreen to do so.
It is not very flexible, so anywhere there is movement (of the soil, foundation, and house) can result in cracks to the stucco and water can get in there.
Synthetic stuccos are made from acrylic polymers and designed to be completely waterproof (not breathable). They are flexible so they are less prone to cracking.
Synthetic stuccos are pre-colored from the factory.
They are also called polymer-modified, acrylic, or elastomeric stuccos. For most people, the offgassing of VOCs from acrylic and other polymers is minor. Those with moderate to extreme chemically sensitive should proceed with caution.
They are installed with the EIFS system.
Exterior Insulation and Finish System (EIFS)
Face-sealed systems should not be used if you are concerned about mold prevention. Be careful if buying any property that has that system.
Drainable EIFS systems can work well according to BuildingScience.com.
One Coat Stucco
One-coat stucco is mixed with fiberglass (applied over metal lath). A newer method that very sensitive folks may want to avoid due to the fiberglass which will offgas some VOCs.
Spray on Stucco
Some stucco homes built after 1945 used a spray-on stucco. These homes could have problems with cracks and stucco falling off.
6. Fiber Cement Siding – Is it safe for the Chemically Sensitive?
Fiber cement siding is made of wood pulp and Portland Cement. Possibly with fly ash or silica added.
The most popular brand is James Hardie – the product is Hardiplank. Hardiplank siding does not contain fly ash.
It can look like a wood grain or be flat.
The James Hardie factory-applied paint has a 15-year warranty. But it can fade before that time. It may need to be repainted due to fading as early as 5 years. Plan to re-paint in 5-8 years.
You might need to do touch-ups on-site during installation with paint, so you might want to test that paint which you can buy through Sherwin Willians.
I got a sample of Jamea Hardie with ColorPlus (which is the prefinished type). When the samples arrived the samples did smell a bit like paint but it quickly dissipated. This would be low-VOC enough for almost everyone to use on the exterior.
You can also purchase a primed version that you paint on-site. You use a typical exterior acrylic paint. Sherwin Williams Emerald is recommended. If you need to re-prime it, you can use Sherwin Williams Masonry Primer.
I think most people with chemical sensitivities would do well with the ColorPlus after giving it a short while to offgas. But you do have to consider the paint as you will likely be re-painting it in 5-8 years.
It costs less than wood siding, more than vinyl. It’s very durable and is not going to rot. You don’t need to caulk the butt joints like with engineered wood.
Hariplank is one of the top choices for healthy homes.
4. Bark Siding – A Natural Solution?
Cork Facade Siding
Cork is a bark that can be used as a siding. It has an insulation value and it works well as a sound barrier. Thermacork facade can be installed with and without a rainscreen.
Thermacork siding is all-natural, it is only cork heat-pressed together with no adhesives. It retains a natural smoky smell due to this process (which does fade).
When using this without a rainscreen be sure that this is discussed with your architect and building science team.
My post on a mold-preventative tiny house build has some more details on Thermacork.
It’s not inexpensive, but if it’s doubling up as your exterior insulation the cost can start to make sense.
Poplar Bark Siding
Poplar bark siding is made by a few different companies. I used poplar wood (not bark) in my tiny house and I did not find it to be rot-resistant.
But barks are quite different than the wood. I don’t have experience with this siding (it’s not very common). They do claim that this is insect repellant.
5. Engineered Wood Siding – Adhesives and Offgassing
LP SmartSide is one brand of engineered wood siding. It is made of flakes of aspen glued together, similar to OSB. It is made with resins (ie adhesives), the glue here is MDI (like OSB). Zinc borate and wax are added. There is a paint finish on top.
Composite wood siding contains binders, biocides, and stabilizers. The binder in this one is MDI glue, the biocide is zinc-borate, and the stabilizers are not listed.
You will need to use their paint on-site to do touch-ups, especially to the edges.
For those chemically sensitive, you will want to check this product out in person before committing to it. Be sure to check out the paint as well.
Because it’s a composite wood product if it does get wet due to any installation mistakes (which are common) or if it does get hit by sprinklers or excessive water, then it’s just not likely to hold up well. Here are some common mistakes.
Matt Risinger has stated that he would personally not use an engineered wood siding due to durability concerns and that there is a lot of maintenance with the caulking.
Ecoclad Engineered Wood Siding
Ecoclad is another engineered wood made of bamboo and paper mixed together with a copolymer resin (adhesive).
Richlite Paper-Based Siding
Richlite, the same material used to make countertops, is made from paper, with a 0-VOC adhesive. I can pick up the formaldehyde for the first few weeks or so.
This is very water-resistant and essentially not absorptive. It’s very different than the LP product above, which is more or less OSB.
NewTechWood Wood Plastic Composite
This is a brand you can find at Home Depot. It’s more plastic than wood, but is a mix of both. It’s made of safer plastics like PET. I like this siding more than engineered wood, and more than vinyl.
It doesn’t have the main drawbacks of each one. Even compared to real wood, it’s easier to maintain.
7. Brick or Stone Veneer Cladding
Real Stone Cladding
Real stone cladding is 100% natural stone. Granite, limestone, and sandstone are normally used. Marble and slate can be used as well. Some stones may be sealed, consult with your architect.
This is the most expensive option of them all.
If your walls are not made of brick themselves, you can add brick as the cladding. When brick is used for cladding, it requires a rainscreen.
Brick is often not treated with any sealer. Sometimes silanes/siloxanes water repellants are used. Consult with your mold-aware architect on whether the brick requires a coating.
Limewash can also be used to coat exterior brick if you want it to have a white look. RomaBio makes a breathable paint for this purpose.
Concrete casts can also be made to look like brick, similar to the stone look concrete casts.
Cement Stone Cladding
There are many cement-based sidings made to look like stone. Certainteed Stonefacade is a cement-based product that looks like stone. It has an integrated rainscreen.
Borel is another panelized stone veneer that has an integrated rainscreen.
These veneer stones are made of Portland cement, aggregates, and iron oxides formed in molds to look like stone.
Faux veneer stones are a different product, these are a plastic product. (There is a similar product for a brick look as well).
Do not fully adhere a faux stone right to the WRB. That will not be best practice for mold prevention.
8. Vinyl/Plastic Siding – Are they a Healthy Choice?
Vinyl siding is a common low-cost option. It comes with the color embedded and does not need to be painted. Though it can fade with time and you may choose to repaint it for that reason.
Some is hollow, and some in insulated with EPS foam (polystyrene). Vinyl can look like wood and even cedar shake.
I got samples from KP and I did not detect any vinyl offgassing. I suspect these samples had already offgassed because I can detect offgassing from hard vinyl windows and LVP flooring.
If you are extremely sensitive you may want to track down a piece of this from the hardware store that is as freshly produced as the product you will be using on the house.
I am wary of vinyl siding due to the debate over whether it can go over furring strips to create a rainscreen. In almost every situation, I would use siding that has a rainscreen. But vinyl without foam backing has a lot of air movement, and many mold-aware contractors and building science experts think there is enough air movement behind it.
Insulated vinyl (with foam backing) seems particularly designed to go right up against the WRB/sheathing with no rainscreen (which I wouldn’t do in any mold-preventative build).
For those who want to steer clear of vinyl, KP also makes polypropylene siding. This plastic is safe for almost everyone.
I have the same concerns over the installation that I have with vinyl.
9. Fly Ash and Polymer Siding – Toxins and Additives
Most folks avoiding toxins tend to stay away from fly ash because it can contain lead, arsenic, mercury, cadmium, and uranium.
However, as an exterior siding, that you don’t normally touch, and with extreme caution with the dust during application, it’s not necessarily a hard no for everyone.
Certainteed icon composite siding is made from fiberglass, polyurethane, and fly ash. Fiberglass and polyurethane do offgas.
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Cheryl Ciecko courses and materials.
Green Building Advisor https://www.greenbuildingadvisor.com/article/can-vinyl-siding-be-applied-over-furring-strips