"Greg G." wrote in message
news

I think the problem is that MDF and other 'engineered' products have
an affinity for moisture. I don't believe it's the glue, but the
broken fibres in the product that absorb water molecules from the air
and hold them like a sponge. As seen in this photo, mold is growing
on a router jig in my garage. Above ground, absolutely never been
'wet'. But there it is...

http://www.thevideodoc.com/Images/moldy_mdf.jpg


I think you and Doug might well spend some time reading about cellulose
adsorbing (that's the proper spelling) moisture from the air. It is pretty
much the same as the sugar in the bowl getting crusty, because cellulose is
formed from sugars (and starches in bread). It's why and how _all_ wood
moves.

Depending on the type of mold, of course, very little moisture may be
required. Mold grows in the desert. If there's nourishment available,
there's almost always something that will consume it. Whether by design or
accident.

At 85% RH, the EMC of wood (cellulose) shows ~18% by weight. Too low for
spalt, but certainly high enough for mildew (`70%RH).
http://www.epa.gov/iaq/largebldgs/graphics/appenc.pdf
From your pictures, you have black mildew, which will grow on concrete, on
gypsum, grout and many other substrates. I'm betting that the
urea-formaldehyde glues used are not particularly appetizing, even if the
formaldehyde has fully outgassed.