Resistance to biological growth on paraffin-treated wood fiber board versus mineral wool board (not batt)
Conventional wisdom is that mineral wool board, being inorganic, has infinitely better moisture resistance than paraffin-treated wood fiber board.
What didn’t occur to me before is–which one is better at resisting the growth of stuff on it as a substrate?
I can’t find the video, but I remember there was some video where a builder said they’d been using the same piece of wood fiber board as a kind of ‘doorstep/mat’ to a site entry for a while, where it sat out in the rain and received no apparent damage or growth — does anyone else remember seeing this?
Also, there was a comment on a video by Michael Horgan ( https://www.youtube.com/watch?v=KB7lAlqRQGY ) that said “I did, however, leave a piece of Gutex and a piece of Rockwool outside my house for almost a year, accidentally. Just dropped them on the ground after a presentation and forgot them. A year later, I can say that one of those samples (about 1′ x 1′ pieces) help up just fine, and the other was peeling apart in fibers. The woodfiber board was fine, and the Rockwool or Roxul was pretty mildewy. To be fair…neither of those products should be left outside exposed as they were, so it’s not a real-world situation. But it was interesting…”
He didn’t specify if it was Comfortboard or batt Rockwool though.
Does anyone have any experience that could shed light on any differences in terms of potential for biological growth on these insulation boards?
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The simple reality is that "stuff that grows" needs food. Food is a mix of interesting organic compounds. Mineral wool is an INorganic material, so it can't be food. Paraffin and wood both CAN be food.
That doesn't mean that stuff can't grow on mineral wool though, it just means that the mineral wool material itself can't be the food source for the stuff that grows. There has to be some kind of dirt (dust, etc.) on there containing the organic compounds that the growing stuff needs to eat, to, well, grow... In the case of materials like wood, the wood itself can be that food source.
The lesson here is that dirty mineral wool can grow stuff. Clean wood can also grow stuff, even if this guy's informal test didn't show any growth on the wood material. It's also entirely possible that the paraffin coated wood fiber, since the manufacturer KNOWS that CAN be food for stuff that grows, incorporated some kind of biocide into the material to prevent stuff from growing on it. That's another possibility here.
Bill
https://www.youtube.com/watch?v=68s7asgZFbc
Based on this video, I'm inclined to believe that the buffering capabilities of a material may also make a difference. It seems like liquid surface water is the key factor in biological activity, so a material that can absorb water may actually grow less mold than a material that cannot, depending on how fast each one dries out after wetting.
Living stuff needs water too, that's why the "life as we know it" usually means in areas where temperatures are right for liquid water to exist.
Living stuff can get at moisture absorbed into stuff too. Wet wood will rot, dry wood won't. Water on the surface isn't the only concern.
The big thing with mineral wool is that is simply isn't food -- living stuff can't eat rock. This means mineral wool will ALWAYS be less prone to stuff growing on/in it compared to any wood materials. Wood CAN be food to some kinds of living stuff.
Bill
Yes, I'm wondering if there is a borate treatment that is a factor. The other factor I could think of is the molecular properties of the paraffin-coated fibers versus the mineral wool fibers. It could be that one is more likely to adsorb food sources or moisture than the other. Even if the wood fiber and paraffin can be inherent food sources, biofilms (that would be growing using adsorbed dust, etc) tend to behave differently than growth that penetrates substrates.