Hi, We were considering finishing our baseboards with just linseed oil until we came across a few people indicating they had a mildew problem. (Apparently mildew *loves* linseed oil.) So if mildew finds our baseboard, it will setup shop. But what is the probability of it being present in our house (New ICF construction, radiant floor, HRV, Toronto, ON: humidity ranges from 20% -> 100%)? Is it the case that mold is everywhere and just doesn't usually find an ideal environment in which to thrive? Thanks, Colin

Apparently many things love to eat linseed oil. Most deck stains/sealers are linseed based then filled w/ UV inhibitors and also algaecides/fungicides etc., which eventually wash out (into the groundwater) and lead to more mildew than untreated wood after a couple years or less. There is very little truly antique southern furniture (i.e. a couple hundred years old) because the owners who meticulously rubbed linseed oil on where left with sawdust as various bugs ate the linsoid oil in the wood. Tung oil reportedly has some anti fungal/mildew properties and is very safe, effective and beautiful. Use orange solvent (not orange cleaner, which has solvent and detergents added)as it's non toxic and won't off gas cancer causing compounds like turpentine, laquer thinner, MEK, etc. The orange solvent is pretty cheap ($15 or $20 a gallon) and may have better carrying properties than many chemical solvents. The tung oil is kind of spendy but worth it. Find real 100% tung oil, hopefully fresh, not lacquer with some oil mixed in.

We've now heard good things about Hemp oil (is there nothing hemp can't do?!).
(Apparently it is naturally anti-fungal/mold.)

Anyone have any thoughts/experience there?

Thanks,
Colin.

Interesting. I'm not saying mildew issues with linseed oil don't exist. But traditional oil paintings are made with a mixture of linseed oil, varnish, and turpentine, and I've never had any mildew issues (I'm a painter). Nor has mildew been a prominent issue in my experience with museums regarding conservation (but of course in museums the humidity level is carefully controlled). I'm wondering if maybe the issue is with linseed oil in combination specifically with wood?

We're trying to stay away from varnish, turpentine and other items that would reduce indoor air quality. So you can imagine yummy linseed on its own could easily be a treat for mold spores.

Solely out of curiosity, how does turpentine reduce indoor air quality? It is after all, a natural product made from pine trees, isn't it? Wouldn't its 'badness', if it has some, dissipate after awhile?

Turpentine contributes immensely to poor indoor air quality, so much so that on the first day of painting class I always warn my students that they will have to drop the course if they are pregnant. It's extremely detrimental to brain and nerve tissue, acting like a powerful solvent against them.

This is of course when the turpentine is still wet, but after it is dry the threat diminishes to almost nothing. It does however take a long time for it to dry out, depending on what it is mixed with (for example linseed oil and varnish mixed with turpentine can literally take years to dry).

All volatile organic chemicals "...dissipate after awhile...", but that doesn't mean they're good for you (even if they're "all natural".) Turpenes are considered an air pollutant when outside (the "smoke" of the Smoky Mountains), and CERTAINLY are indoors in unventilated/low ventilated areas.

IIRC the half-life of turpentine at room temperature is on the order of several months as it polymerizes into something that won't readily vaporize. Figure on a dozen half-lives before it's down to the undetectable range in air sampling. (Got 5-10 years?)

Linseed oil (fresh) is also volatile and not-so-great when inhaled. The stuff in finish is usually boiled, rapidly accelerating the polymerization process, but it's still pretty potent when freshly applied.

Linseed oil is fairly sterile (in fact linoleum floor coverings were standard practice in hospitals 80-100 years ago because nothing would grow in it.) I've never heard of problems with mildew growing on linseed oil finishes in conditioned space. It typically takes relative humidity over 70% for extended periods to get most mold/fungus issues going on surfaces. The wikipedia entry on linseed oil only mentions mildew on linseed oil finishes as an outdoor problem (garden furniture. See: http://en.wikipedia.org/wiki/Linseed_oil ) FWIW: I have furniture and wood trims in my central MA house (~7000 Fahrenheit heating degree days, ~300 cooling degree days- humid summers) finished with linseed oil only &/or linseed-oil/lacquer or linseed-oil polyurethane conversion varnishes, and have yet to see any mold issues related to it despite summertime indoor RH that hits 70-80% at times. Even the slightest amount of air conditioning/de-humidification should be able to keep the RH under 70% in Toronto in the summer, and you'd have to be actively humidifying the air to get it there in the winter with an ERV running 24/7.

For health & comfort keeping the indoor RH between 30-60% year round is usually recommended. ASHRAE recommends the upper limit at 65% for limiting microbe/mold growth. People with dust-mite allergies are advised to keep it under 50% (below which point mite populations die off). Pick any of those numbers and I'm sure you'll never have a mildew problem on your kickboards (or anywhere else) unless you have chronic slow rain/ice-dam/plumbing leaks. And if you let your indoor RH drift up to 80% and stay there for weeks, you're kind of asking for it no matter what finishes you use. (This is a negative for using whole-house fans in humid areas/time-periods. When the dew points outdoors are 17-18C and the indoor temps are 25C, pulling in outdoor air sets the indoor RH over 60%- which feels lousy and increases the mold hazard. Even 27-28C room air mechanically dehumidified to 50% feels great by comparison, and inhibits mold growth.)

Thanks Dana! That's a heck of a lot of great information.

I'm a bit concerned about what will happen with our humidity levels through the summer and figure I had better invest in a few humidistats to track the levels in various parts of the house. We don't have A/C so we'd have to rely on portable units, which would be unfortunate.

I think we'll probably try out the hemp oil (we have some here that's got a green tint to it, so we'll have to see if the product we'd use for the wood has the same quality) but it's good to know we could safely fall back on linseed (under the right conditions).

-Colin.

I used to read a lot about antique boat building practices(I'm weird like that). The books say linseed oil got a bad wrap for mildew because in its raw state it takes weeks to months to polymerize, and makes great fungus food untill it does. Previous poster was right, boiled linseed oil only takes hours to days to set, not much of a risk for mildew. Also, this only applies to heavy coats, not rubbed finishes, rubbed finishes mostly soak into the wood anyway.

I don't know anything pertinent to wood finshes about hemp oil, maybe your stuff has stabilizers but you may want to research a little before trying, most organic oils go rancid after a while. I've heard people espouse the idea of olive oil, but it smells pretty funky after a couple months.