There are two main types of hardwood flooring, solid and engineered.
Solid hardwood flooring is a solid plank of wood.
Engineered wood contains real hardwood as the top layer and a core of another material.
Usually, this is wood (plywood, fiberboard, or solid slats), but it can also be a core of plastic.
Both are good choices in a healthy home.
Solid wood is one of my top three choices for least toxic flooring and engineered wood is not that far behind.
Though it does by definition contain glues within the flooring.
There are benefits to engineered wood over solid – cost of course, but also stability in a wider range of temperatures and humidity.
With engineered wood, you can re-finish it if the wear layer is thick enough to do so.
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1. Solid Hardwood Flooring
Solid hardwood flooring is a great natural and non-toxic choice.
It works for many chemically sensitive folks since it’s free of glues.
There are some chemically sensitive people who have become sensitive to the natural odorants of wood, however.
Wood contains natural odorants that I talk about in this post.
Aromatic woods like pine have higher natural volatile compounds than maple, as an example.
Solid Hardwood flooring can also contain anti-sap stain chemicals, which could explain why a few people react to wood used in buildings and not wood in the forest.
Wood also has a higher possibility of harboring mold than less porous materials.
To prevent mold, you should make sure your wood has been kiln-dried and kept dry at the store and on-site.
In normal conditions, mold is not an issue.
Wood Finishes
There are four main finishes used on solid hardwood flooring.
One is the “prefinished” flooring which is almost always a UV-cured polyurethane finish discussed below.
You might also find an oil-based finish (solvent-based) on prefinished flooring, but that is rare.
Some of them are UV-cured.
If you want to finish it yourself, you can find unfinished solid hardwood flooring through the major outlets, and there are two main finishes that those avoiding toxins can consider:
Natural oil finishes, which are described in detail in this article on natural oil floor finishes, and 0-VOC or ultra-low VOC synthetic floor finishes, which I tested side by side here.
Prefinished Hardwood
Prefinished hardwood flooring is usually finished with aluminum-oxide-infused polyurethane and cured under UV lights.
This finish is typically very well tolerated by chemically sensitive folks once cured. It is zero-VOC or close to it.
This finish has a few benefits: not having to finish it in-house, which is both easier and reduces your exposure to the curing of any finish, it’s super durable and will last a long time.
Also, the finish partially blocks the wood odor for those who are sensitive to wood odors.
You will find that almost every brand offers a UV-cured water-based polyurethane finish (with aluminum oxide).
The risk of the aluminum exposure is outlined here.
Installation of Hardwood
Nail-down installation is less toxic than glue-down.
You may need a small amount of glue on some pieces, or you may be able to face-nail.
The best practice for wide planks is to glue and nail.
Consider the width of the planks at the planning stage if you want to avoid glues.
Specific glue recommendations are here.
My post on non-toxic underlayments includes silicone-backed paper (for the most sensitive), Rosin paper, and more weighty options.
Use solid wood baseboards – they come in both primed and unfinished (I wouldn’t recommend MDF (medium-density fiberboard) or PVC (vinyl)) for sensitive folks.
Wood Floor Glues
For subfloor glues, my top pick is definitely AFM Almighty Adhesive, which is safe for most people and highly tolerable.
My post on construction adhesives has more info.
There are no VOCs (volatile organic compounds), no HAPs (hazardous air pollutants) wood fillers –Mohawk Fil-O-Wood and ECOS for the nail holes.
I find the standard Minwax wood putty to be too potent.
The Dynamic putty pencil is very benign.
2. Engineered Wood Flooring
The most common type of engineered wood is a flooring with a real wood layer on top and an engineered wood layer as the base.
I am also going to list below what is called a hybrid floor, this has a real wood layer on top and a plastic layer as the core.
There are three types of construction and two main finishes to consider when choosing typical all-wood engineered wood flooring.
When looking for a zero-VOC, healthy engineered wood floor, the most important part is the construction type.
The lowest VOC types are those with a plywood base or solid slate base. The lowest off-gassing finishes are the UV-cured water-based finishes.
Certifications like GreenGuard Gold don’t matter to me here because the highest off-gassing engineered wood can meet this level.
The lowest offgassing options are zero-VOC.
Three Types of Engineered Hardwood Flooring
Engineered flooring can be divided into three types based on the construction of the core.
This influences how much glue is used and the off-gassing levels.
1. Plywood Construction
This is the most common type of engineered hardwood floor
A plywood base with alternating solid layers of pine/spruce/fir or acacia and eucalyptus wood is one of the construction types you should look for, as these floors can meet zero VOC.
Though they do contain glue between the layers.
There is also plywood where the alternating layers are MDF (fiberboard) which contains formaldehyde. I would avoid this if possible.
The real wood top layer can range in thickness from 1mm up to 6mm.
The brands that claim zero VOC with this type of construction are listed in my article on engineered wood.
2. SPF Quarter Sawn Construction
This is another great option for those sensitive to glues/VOCs.
I prefer this option to the plywood base because it has less glue.
Many of these brands are 0-VOC.
This is a far less common construction type.
It has a core of quarter-sawn spruce, pine, and/or fir (SPF) softwood wood species.
The wood is cut and laid in “pickets” (i.e. it’s strips of solid wood) which run perpendicular to the solid hardwood top layer.
While the hardwood top layer is still, of course, glued on, and there might be a stabilizing layer below the quarter-sawn layer that is glued on, there are no glues within the main core layer.
This core can also be called “solid cross slat core” or “lumber core” at LL Flooring.
The brands with this type of construction are listed in my article on engineered wood.
3. HDF Core Construction
The last type of engineered wood flooring has the same top level of real hardwood but in this case, it’s glued to an HDF (high-density fiberboard) core.
(Or sometimes MDF, medium-density fiberboard core, which is very similar).
Sensitive folks may not be able to use most brands of this type of engineered wood flooring because of the formaldehyde binder in the fiberboard core (HDF/MDF).
There is significantly more formaldehyde here than in plywood and therefore it’s my least favorite engineered flooring type.
However, there are two brands that use an HDF core that is not made with formaldehyde resins (no added formaldehyde) – Kahrs and Mohawk, including Pergo engineered wood floors like Pergo Defense + Engineered Hardwood.
And the VOCs are extremely low in these two, so I would consider these good options in terms of off-gassing.
Fiberboards, however, are far less resistant to spills and water, and they swell up easily (though some brands have added plastics or waxes that can make up for that vulnerability).
Engineered & Solid Prefinished Wood Flooring Finishes
Aluminum Oxide
The vast majority of engineered hardwood floors have a UV-cured polyurethane finish with aluminum oxide.
Sometimes it’s simply listed as “aluminum oxide” or just as “UV-cured finish”.
This finish has no or extremely low off-gassing and is very durable.
The exposure risk of the aluminum is outlined here.
Polyurethane without Aluminum Oxide
A similar finish is a water-based polyurethane (or polyurethane/acrylic mix) without aluminum oxide. These are not as durable as an aluminum oxide finished floor but they can be more natural-looking.
Kahrs Water-Based Finish Engineered Floors have this finish.
Oil-Based Finishes
Oil-based finishes usually have some off-gassing of VOCs, and you also have to make sure you are OK with the maintenance oils.
Many of these finishes, if not all, are solvent-based, though some claim to be natural (and they still likely contain a solvent).
They are often UV-cured, which helps them to cure faster than normal, but I do still pick up some off-gassing odors from the oil-finished options I have sampled.
Note! You can also find unfinished engineered wood flooring and finish them yourself with the options listed under solid wood floors above.
Installation of Engineered Wood
There are four main ways to install engineered wood.
Glues are listed here.
Similar: Hybrid Flooring
Hybrid flooring could be considered an engineered wood flooring.
Like all engineered wood, it does have a real hardwood layer on top. In this case, it’s quite thin (and you can’t re-sand it).
The core of the boards is not wood-based – it is an SPC core (limestone-PVC composite). This is the same SPC core that you find in luxury vinyl plank (LVP) floors.
I like this type of flooring if you want something waterproof, you want to cut down on plasticizers used in luxury vinyl by having a real wood top layer, and/or you want to cut down on the amount of wood in the floor.
Since plasticizers are semi-VOCs, they generally are not being released in gas form, but rather by leaching. Therefore, by having a physical layer of wood on top of the vinyl layer it could act like a block to the leaching.
Other Options
Bamboo flooring is not technically a hardwood flooring, but it is similar in appearance. My article on bamboo flooring goes over the ultra-low-VOC brands.
Corinne Segura is an InterNACHI-certified Healthy Homes Inspector with certifications in Building Biology, Healthier Materials and Sustainable Buildings, and more. She has 10 years of experience helping others create healthy homes. You can book a consult here.
Join the top Substack on healing from MCS and mold!
Erin
What would you recommend as the best option of non toxic wood flooring for families with babies and young children that will be constantly playing and crawling on the floor? It would seem that a real hardwood would be my first choice. However, because it doesn’t hold up to spills well, I’m considering an engineered wood like Stuga, but am not fully convinced of the saftey of aluminum oxide for infants and children who will be in constant contact with the floor. Can you help point me in the direction you would go with. Thank you!
Corinne Segura, Building Biologist
me and a toxicologist did a super detailed review of the aluminum exposure from floors. there is also Kahrs which has a line without al if you prefer. otherwise solid wood does reasonably well with spills, depending on the species, the finish and what it is you are spilling.
Carmen
Hi Corinne. Kahars have some floors advertised as lacquered and other as olied? Does lacquered mean water based or UV cured? I can’t see these words on their specifications so I am confused. Thank you
Corinne Segura, Building Biologist
lacquered for Kahrs is water-based
Linda L.
Hi Corinne,
Our local flooring installer recommended a new product he has been using for glue assist in installing hardwood floors that he says doesn’t give off an odor that he has noticed. It’s Bona Quantum R851. I was wondering if you have tested it before or heard anything about it?
Thanks,
Linda
Corinne Segura, Building Biologist
i talk about this adhesive type in the article on construction adhesives
Linda L.
Hi Corinne,
I am looking to book an email consult with you looking for guidance on choosing the best option for solid hardwood flooring brands for the upstairs of our home. Also, we will be redoing our bathrooms that have mold, and im looking for guidance on that as well as i am highly reactive to mold. I am not seeing any availability for bookings from now into the summer at least, we will be doing this work soon since i am having reactions often, are you still doing consulting? If not, is there someone else that you could recommend?
Thanks,
Linda
Corinne Segura, Building Biologist
The top of the consulting page explains my availability.
Jade
hello, I came across this article when I was looking for the least toxic solid hardwood flooring as we’re planning to redo our floor. would you please name your top brands for the solid hardwood flooring (we live in northern California and hopefully can find them)? Also I’m wondering if we can find a list of Greenguard Gold solid hardwood flooring? Thank you!
Linda L.
Hi Corinne,
Under hybrid flooring you mention that plasticizers are generally not being released in gas form but by leaching. Does that mean generally that someone that is sensitive to chemical offgasing giving breathing problems, that they should be ok with waterproof luxury vinyl flooring like provenza maxcore flooring? it wont offgas much?
Appreciate your thoughts,
Linda
Corinne Segura, Building Biologist
The article on vinyl plank covers that
Linda Lintow
Hi, thank you for this website. I have developed throat swelling allergies and have become sensitive to chemicals. I am looking g to change out our carpeting to the least likely to cause a problem option which I am leaning towards hardwood. Is there a difference between solid hardwood that gets nailed down and solid hardwood that is click togrther as far as being the least likely to offgas or be a problem?
Thank you for your consideration.
Corinne Segura, Building Biologist
I would look for hidden mold in the home
Susan
We called a hardwood flooring company and they told us they did use a microban product in the hardwood flooring. They couldn’t tell us what the chemical was or if it was bioavailable or sealed in-would you avoid this? Is microban in flooring a problem, like does it migrate out? Thanks!
Corinne Segura, Building Biologist
See the article on microban and what all the microban chemicals are
Susan
It’s O.I.T. (2-Octyl-3 (2H)-Isothiazolone). Is this a bad one/dealbreaker ?
Corinne Segura, Building Biologist
isothiazolones are definitely common, often the raw wood is dipped in those before it gets made into floors. and it’s a common preservative in other products. whether you want to avoid it in applications that are avoidable is up to you.
Susan
Would it migrate out of the wood? I would ideally like to avoid unnecessary chemicals as we have a young child with eczema and chemical sensitivity. I’m going crazy calling every flooring company that’s an option for us with our build and they all say antimicrobials are in the urethane finish.
Corinne Segura, Building Biologist
You can finish floors yourself
Susan
We called a solid hardwood flooring company and they told us they did use a microban product in the hardwood flooring. They couldn’t tell us what the chemical was or if it was bioavailable or sealed in-would you avoid this? Is microban in flooring a problem, like does it migrate out? Thanks!
Lindsay
Thank you so much for your detailed research. I am in the process of trying to pick new engineered hardwood floors but I am so overwhelmed with the variety of finishes. Is it better if the floor has been cured or has a UV application? Would you steer clear of certain finishes? Any opinion of these finishes:
UV aluminum oxide
UC cured urethane (treffert)
UV matte finish
UV lacquer
Water based matte lacquer
I wish you could just pick the flooring for me
Corinne Segura, Building Biologist
i wouldnt use oil based, UV cured water based is good, UV curing means it cures in factory not in your house, all those descriptions ar likely for the same thing
Lindsay
Thank you for the response- I am still confused as to what water based actually means.
Are aluminum oxide, urethane, and lacquer water based?
Corinne Segura, Building Biologist
in this context, it’s likely all of those refer to aluminum oxide infused polyurethane that is UV cured. but you should ask as there are also UV cured oil based finishes.
Tracey Houser
Such a great article! You always provide invaluable information for homeowners trying to make the best decisions possible!
For my situation, buying prefinished hardwood floors makes the most sense. If I go with a hardwood flooring that is finished with aluminum-oxide-infused polyurethane and cured under UV lights, do I need to still consider if the wood was tested with a *cide for mold or the stain that is used? Wondering if this type of finish neutralizes these other two issues in terms of off gassing/VOCs/SVOCs. Thank you for your consideration!
Corinne Segura, Building Biologist
it would likely block it from the top side. whether you want to consider the anti sap stain chemicals is up to you. some of them are just borates.
Renee
Hi! I am looking to have a floating floor installed in my bedroom. I like the fact no glue is used. Is there a brand you can recommend? Mohawk? I love your website! You have a wealth of information and I have found it very helpful.
Anita Keetch
Hi! Thank you for your website! We are building a new home and trying to make it as healthy and non-toxic as possible. We were going to lay a waterproof laminate in all rooms except bathrooms and laundry room. However a friend told me today she regrets getting laminate because s glass of water spilled in the living room and the water we t through the cracks/junctions and got beneath the laminate, which would lead to mold growth. My question is, does solid wood or engineered wood better at preventing water going through the seams?
Corinne Segura, Building Biologist
After hundreds of home inspections Ive never seen mold growing under floors if the subfloor is wood and there is another space below. It can dry out so it doesn’t mold from a spill. Hardwood is the most durable to mold and not using a plastic underlayment will allow it to dry to both sides. Some species of hardwood are more durable to mold that others and solid wood will hold up the longest to spills if there is prolonged water.
Robert Stephens
Every house will have moisture coming into it. Water comes in windows, from the ground below, through air exchange, even from people exhaling water vapor.
If a building is properly built, this moisture will be removed through proper ventilation, appropriate insulation, drainage, and barriers etc.
Yes, you could seal any floors to make them watertight. This is a double-edged sword, because materials should be breathable to allow moisture in/under them to escape. So creating moisture barriers to stop water can potentially make more problems by preventing the release of moisture.
So, I’m not sure what the right answer is 😉