Updated Winter 2024
Testing caulking and sealants was the hardest part of building my tiny house since I did this when I was highly sensitive.
This article will help you narrow in on the best caulking types. I cover the least toxic options which are low-VOC, low-odor, fast to offgas, and are suitable for chemically sensitive or anyone wanting to reduce VOCs.
You’re going to need them for repairs and renovations as well. I would suggest finding out which are non-toxic and which ones work for you before that urgent repair is needed.
I’m not interested in certifications like GreenGuard Gold here. I’m interested in the official VOC level and how fast it is to offgas. In this type of product, a high initial odor/VOC level could offgas faster than one that is lower VOC, and that can be better for the user.
There is no such thing as natural caulking though some materials like silicone are partially naturally derived.
This article contains affiliate links. Upon purchase, I earn a small commission at no extra cost to you.
Top Non-Toxic Caulking/Sealant Brands
All caulks have a chemical odor when wet. I tested them at 24 hours, 48 hours, one week, and two weeks.
If you are chemically sensitive I would recommend having a non-sensitive person put a test amount into a jar and sniff then when it’s cured.
Which Caulks are Mildewcide Free?
Note that caulks labeled for bathrooms or as mildew-resistant contain a mildewcide. Currently GE I is mildewcide-free, GE II contains a mildewcide. Any caulk labeled aquarium safe is free of mildewcides, including DAP Aquarium caulk and others listed below. AFM Safecoat and Chemlink Durasil are also mildewcide-free.
1. Paintable Interior Caulk (White Polyether)
AFM Caulk is one of the top choices and is one of two main caulks to have on hand in every build and renovation.
I used this in my bathroom, you can use this around the shower and sink. It can be used around doors and windows and around baseboards and trim.
It’s used indoors anywhere where it interfaces with a paintable surface and is the non-toxic replacement for acrylic latex caulk.
My testing: Not the best when wet, not the best at 24 hours, but the best at one week.
Where to use AFM Caulk:
It can be used inside and outside. Around windows, cracks, tub and shower enclosures, backsplash, siding, walls and wood (like sealing molding), and general maintenance. It does not hold up as long in the bathroom as silicone with a mildewcide.
It is polyether resin (90-95%) with amino silane (1-3%) that does not contain solvents (it’s not acrylic, latex, silicone, or polyurethane). It is non-shrinking, and zero-VOC once cured.
Mildewcide: none. Elongation: 35%. Color: white. Paintable: yes
Where to buy: You can buy this online from Green Design Center.
Alternatives: ChemLink NovaLink 35 is a similar paintable elastomeric silane-modified polyether but comes in different colors (if that is needed) PSI of 145; Chemlink Durasil in white; 3M Marine Adhesive Sealant 4000 also seems very similar (polyether 15-40%, white pigments 35-70%, plasticizer 10-20%, silane 1%), PSI 220-240, and is paintable. (It retains its rubber-like odor for many months).
This video goes over the least toxic caulking options:
2. Non-Toxic Silicone Caulks
Silicone caulking is non-toxic and highly tolerable once cured. But if you are going to be around during the application (or even within two weeks of application for the super-sensitive) be sure to pick the least toxic option.
For most new builds and renos you want one paintable caulk (polyether, below) and one silicone caulk from this list. Silicone is generally for kitchens and bathrooms where you don’t need a paintable surface.
Neutral cure silicone is the least toxic type of silicone – specifically oxime or alkoxy neutral cure silicones – they offgasses alcohols or methyl ethyl ketoxime, not acetic acid. Oxime silicones have good adhesive strength and suitability on different materials.
Which of the two is better (chemically) depends on your specific sensitivities. There is also a small chance you could find an acetic acid cure better after waiting out the cure time, which I would say is 18 days.
Oxime or alkoxy-type neutral cure without added mildewcides are the least toxic types of silicone caulking.
- ASI 388 – is neutral cure (oxime type) without a mildewcide from Amazon. Lowest odor when wet.
- DuraSil – is nuetral cure (oxime type) without mildewcide, from Green Design Center. It says not for use on tub and tile applications, this is because it has no mildewcide.
- GE 2 Advanced – is neutral cure (alkoxy type) for kitchens and bath, and it’s easy to get from the hardware store or Amazon. It does have a mildewcide. Very close second place to lowest odor when wet.
- Silco RTV 4500 – acetoxy cure silicone, more typical type for kitchens and bath. Very potent when wet. Though possibly preferable after 18 days of curing. No mildewcide (food safe).
Testing by the Chemically Sensitive:
Preferences between polyether caulk (like AFM Caulking, Eco-Bond, and Chemlink Clear) and silicone:
- It is possible to prefer polyether caulks (in the sections below) over silicone. You can easily have a different preference when it’s wet, compared to a few days and compared to a few weeks. Polyether can retain more of a rubber smell even after a few weeks compared to silicone.
Preferences between the two top choices of silicone:
- The choice between alkoxy and oxime would be very individual. Alkoxy (GE Advanced) smells more rubbery to me and oxime odor is difficult to describe, though it’s very subtle. Mild to moderately sensitive folks can probably choose oxime and be fine (ASI 388).
Preference between oxime and alkoxy and the more typical acetic cure silicone:
- Acetic acid/acetoxy cure silicone is VERY potent at first. It can be extremely challenging for the chemically sensitive when wet. However, if we want to get into the minute details, I found that at 18 days Silco RTV4500 food-safe acetoxy cure was totally odorless, better than the options like oxime cure (that are far less potent when wet).
Where to use 100% silicone: Windows, doors, siding, vents, around wires/pipes, in the bathroom, under countertops, around sinks, in the shower, and more.
Mildewcide: Some brands have mildewcide and others don’t. Color: Clear, white, black. Paintable: No.
You can also find silicone caulking at Home Depot.
Where to buy:
Buy ASI 388 (the lowest odor option without mildewcide) from Amazon.
Buy Silco RTV 4500 (food-safe, strong odor at first, possibly preferable after curing) from Amazon.
What About Aquarium Caulk?
When I was building my tiny house I read that aquarium caulk (a type of silicone caulking) was the least toxic because fish are exposed to it and they can’t handle chemicals (especially biocides). It turns out that fish are a little different than people. Aquarium caulk is usually an acetic acid cure, one of the strongest odor types when wet.
It’s priced higher only because it’s marketed differently, it’s not any different than other acetic acid cure silicone without mildewcide.
I tested two brands, Aquarium Silicone Caulker (acetic acid cure) and one called Marina. Extremely potent!
Though one super sensitive person preferred the Aquarium caulk Aqueon (even though it’s an acetic acid cure) to GE I or II and Eco-bond (clear polyether). And another did well with DAP aquarium-safe silicone (another acetic acid cure, too strong for me). It’s possible to prefer these when totally cured to the options that are less potent when wet.
Marine caulking is not the same as aquarium caulk (it’s for marine applications, not fish tanks). I tested GE Marine Silicone Sealer. It had that same pungent odor though. Acetic acid smells like vinegar times a million.
It did, however, offgas quite rapidly. After one week it was at the same point as GE 100% Silicone, and at two weeks it was just as good (virtually odorless).
3. Colored Interior Paintable Caulking (Polyether)
Chemlink makes an elastomeric caulk for sealing concrete and masonry called NovaLink35.
I haven’t tested this one, but it’s a great brand and many chemically sensitive folks do well with it. This is a polyether that comes in various colors (white, beige, limestone, grey, and black) and can be used as a replacement for AFM Caulking Compound.
4. Clear Polyether Caulking
Clear polyether caulks can be a replacement for both clear silicone or paintable caulking like typical acrylic caulks (like ALEX brand) or white polyether.
The brand Eco-Bond (which went out of business for several years but now appears to have the website back up) was a top favorite among the chemically sensitive. It’s also a pet-safe formula.
The closest formula to Eco-Bond is Chemlink Clear, a similar clear polyether that is low odor.
Where to Buy: Green Building Supply.
Caulking Materials – What Caulks are Made of:
Polyurethane caulks – Usually contain isocyanate. This type smells strongly like typical fresh paint, and takes quite a long time to offgas. They can be made without solvents. If they do contain solvents it would typically be mineral oil, toluene, or xylene. I tried Loctite PL S40 which was really strong in my opinion. I also tested Sikaflex 1A which is probably the lowest VOC polyurethane caulk on paper but is slow to offgas (it took a full 3-4 months to really reach odorless to my nose).
Acrylic Latex caulk – They have the mildewcide octhilinone or carbendazim in them as well as a benzoate plasticizer (which has largely replaced phthalates) and naphtha (source). They are often water-based but still could contain mineral spirits, mineral oil, and propylene glycol (source). Painters caulk are usually acrylic latex and are sandable and paintable.
I tried DAP Dynoflex 230 latex caulk and DAP Alex Plus, an acrylic latex with some silicone. At one week they both had more offgassing than the silicones, though they had different chemicals offgassing. At two weeks Alex was still clearly offgassing (at three weeks I could still pick up the odor, at 5 weeks it was very mild), and Dynoflex was close to done at two weeks.
Silicone – While silicone itself is not toxic, the chemicals added to keep caulking in liquid form are. For silicone caulk, look for 100% silicone. There are different types listed below that offgas different chemicals. Once they are cured, you may find them to be essentially the same.
The chemical additives in “100% silicone” are not usually listed. Silicone doesn’t usually have a solvent but some have traces of benzene and toluene (source). Plasticizers are typically added. The biocide, if it has one, is likely to be Phenoxarsine oxide or Octhilinone according to Pharos.
Acoustical Caulk – this non-hardening sealant prevents sound leakage. They are mostly water-based acrylic. They are about 20% ethenylbenzene. In smaller amounts, they typically contain plasticizers, naphtha (as the solvent), ethylene glycol, and formaldehyde.
Sil-Terminated Polyether – specialty silicone replacement caulks are often polyether. My top pick is AFM Caulk. These are some of the most tolerable caulks but the polyether rubber does have an inherent rubber odor. They do contain phthalates.
Butyl Caulk – I tested C.R. LAURENCE White 777 Butyl Rubber Sealant. This was very potent at first (more potent than say Sikaflex 1A at first), but it offgassed faster. At 1-2 months it was really odorless to me.
Solvents common in caulking (of all types) include acetone, methyl ethyl acetone, toluene, xylene, and naphtha (Source).
Types of Silicone Caulk and What Chemicals They Offgas:
1. Typical silicone caulking is acetoxy silicone and it offgasses acetic acid. On the SDS you will see “odor: acetic acid”.
2. Neutral cure silicone, also called RTV, a less common type, is a lot more tolerable when wet. Look for oxime cure or alkoxy cure which offgas alcohols or methyl ethyl ketoxime.
3. Aquarium caulk is usually acetoxy silicone without mildewcide. It might be labeled aquarium caulk or you can just get GE 1 if you are avoiding mildewcide, which will be less expensive.
4. Paintable silicone caulk is a mix of silicone and acrylic latex.
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