Tuesday, April 30, 2013

Borax: Stopping Mildew and Rot on the Cheap... and How the EPA Can Make Everything Complicated

rev. 4-20-2014

Websters' version

Borax: 1. a white crystalline compound that consists of a hydrated sodium borate Na2B4O7·10H2O, that occurs as a mineral or is prepared from other minerals, and that is used especially as a flux, cleansing agent, and water softener, as a preservative, and as a fireproofing agent.

EPA's Version

Borax: pesticide products containing boric acid and its sodium salts (borax) are registered in the U.S. for use as insecticides, fungicides and herbicides. As insecticides, some act as stomach poisons in ants, cockroaches, silverfish and termites, while others abrade the exoskeletons of insects. As herbicides, some cause desiccation or interrupt photosynthesis in plants, while others suppress algae in swimming pools and sewage systems. As fungicides, several are wood preservatives which control decay-producing fungi in lumber and timber products.


While generally considered safe--still used in many big-name laundry detergents--if I claim a cleaning formulation containing borax keeps mildew away, I have to register it with the EPA before marketing. But I can sell you a box of borax without registration. Go figure. Is it really about toxicity? I don't really think so. It is about twice as toxic as washing soda (LD50 borax and boric acid are about 2500 mg/kg BM. Although borax is suspected in certain reproductive problems in laboratory animal testing, it is not associated with cancer and does not bio-accumulate.).

It seems that anything that works must be poison, or at least regulated. That is governments purpose.

As part of a future Practical Sailor article I began exploring fumigating agents and anti-mildew products. I truth, most projects I take on are because I've had some troubles related to the subject in question and thus have some understanding and some additional motivation. Every boat has at least one damp spot prone to mildew, and in my case, I have a basement prone to wet carpets every few years. Not flooding, but mildew potencial.

I began exploring the formulations of some successful products. Concrobium is one, dredfully over priced at the local hardware, particularly considering you can look up the underlying pattents (EP 1104450 B1) and learn that each quart bottle ($18.00) contains nothing but:
  • 1 tbs baking soda (sodium bicarbonate)
  • 2 tbs washing soda (sodium carbonate)
  • 2 tbs TSP (trisodium phosphate)
Simple and effective. The chemistry makes perfect sense; it is applied without rinsing and thus leaves behind a thin alkiline coating that repells bacterial and fungal growth (fungi require acid conditions) and provides no food for growth (soaps contain fatty acids and make great fungi food). More is not better because it is used without rinsing, the limitied concentration is important.

That got me thinking, so I began trying other variations including my favorite, also in 1 quart:
  • 2 tbs baking soda
  • 2 tbs borax
  • 1 tbs TSP
I've been testing all three on some mildewed carpet sections, cleaning by scrubbing lightly and then extracting with a vacuum. Which is best? After 6 months they are both perfect, although the borax version killed the smell a bit faster.


Other Applications

Basement Carpets and Walls. We've had problems with a wet basement and biannual flooding for 20 years. I've washed and dried a lot of carpets. Learning to use this formulation as the rinse was a revelation. No more mildew smell, no more mad rush to get them dry. No more black stains. Simply use this as the rinse water and the carpets stay fresh.

Preventing Wood Rot
Borax is VERY effective in preventing wood rot. I've used it myself mixed with ethylene glycol (Goolge it) to preserve a common pine totem pole in damp soil and remain impressed; it's staying as though it were pressure treated, 6 years and counting. West Systems Epoxy has posted on this subject. The National Park Service posted this on preserving totem poles in the PNW with borate/glycol.

Obviously, they can't stand boric acid. One of the most common extermination products, particularly around kitchens and bedrooms (works on matresses). It's not going to work on the flying pests, though, the only ones I have trouble with. Darn.

Wooden Decks
Seems like a good cleaning choise. Should help keep the algae away. Limit the TSP if you wish to be bay-friendly. Try it on your home deck for a little boost to the pressure treatment.
Combine it with some bleach as needed.
A less alkiline variation is well known and should keep the mildew away. Reduce the dose:
  • 1 tsp baking soda
  • 1 tsp borax
Oh, the power of good marketing. I swear I'm not just trying to be trying to be cheap. Furthermore, I'm a chemical engineer and have no phobia regarding synthetic chemicals. I'm not pushing this because of some hidden green agenda or because it is less toxic. The strength of these formulations is basic:
  • No organic mildew food
  • Mildly alkiline film
  • Borate as mildewstat
I'm also reviewing some nice complex synthetic formulations that promise to be more water resistant. We'll see.


  1. Bravo!

    Keep up the great work Drew!

    s/v Eolian

  2. My back yard totem pole project is what got me interested in borate as a wood preservative. The simplicity and effectiveness (3rd party testing) of Concrobium got me thinking about basic, marine-targeted solutions.

    I like knowing what is in a cleaner, because then I understand what it can and cannot do, and what materials it should be safe around.

    I clean so little, I like lasting solutions.

  3. I like your post, thanks for sharing..

  4. Thanks, good info. BTW what is "TSP"?

  5. TPS is trisodium phosphate, available as TSP at Home Depot and most paint and hardware stores. But not "TSP Substitute."

  6. Drew, thanks for this great info and your blog in general, a wealth of information. I'm considering buying a boat that has a mold problem. Once the boat is thoroughly cleaned, wood repaired and accessible areas treated with these substances, do you think that longterm mold issues are more likely on such a boat than one that is very clean at the outset? Can the mold be eradicated such that they end up being more or less the same, all else being equal?

  7. Starting clean is always better. That said, If you can remove the rot that is structural, treat the remaining questionable wood with the borax/glycol formula, and clean the rest, I don't see why it should come back more quickly.

    Dehumidify. This is the first step. Buy or borrow a household unit (>30 pints) and run that for a few weeks until the output drops. Then get a small unit ( 2-4 pints, like the Evadry unit I discuss in some other post) and leave that 12-24 hours each day for months. While many folks simply ventilate to prevent mold, you will need to be more proactive for a while. You need to get the boat really dry.

    Clean with bleach where practical, clean with other alkaline surface cleaners everywhere else. Launder what you can and use the above formula as a rinse before drying.

    You may need to replace foam. The borax/washing soda/backing soda formulation followed by drying in the sun may work... or not.

    Do NOT leave any cleaning formula to dry on clear plastic; it may water spot.

    Consider using one of the tea tree oil formulas (Kanberra, Forespar) for a while. While test results have been inconclusive, there is some reason to believe it will reduce spread. Use 2x as much as they say, with the dehumidifier on and the boat closed up. But this is a polishing step, after everything else has been done. It WILL NOT kill established mold or help if things are damp; it has failed those tests.

    But the main thing is getting the boat dry and leak-free. Mildew cannot grow without humidity. A little stick-up humidity may help; try to stay below 60% at all times.

  8. This is very very helpful, thank you!

  9. Does the mixture simply work by creating an alkaline environment that mold can't grow in?

  10. Certainly creating a buffered alkaline environment is much of it, but boron is also thought to play a role, particularly with algae and fungi, less so with bacteria.