Re:Theroretical Drywall Question?

July 6, 2008

Todd,

The discoloration is probably just house dust that was trapped from

small air flows by the roughness of the surfaces. (This is commonly

seen at the edges of light-colored carpets.) If you are worried about

it, I would agree with Carl that tape sampling would supply the answer

to your question. Be carefeul where you send the sample. One lab

identified soot, a possibility if you burn jar candles, as

Stachybotrys mold!

I would disagree with Carl's expert about drywall. There are millions

of homes and commercial buildings that have been built with drywall

where walls and ceilings do not have mold problems. Lots of folks even

put wallpaper (with starch glue!) over plaster walls to no ill effect.

Mold grows because of excess dampness, whether from leaks or high

humidity. You don't even need paper for this to happen. Mold will grow

in settled dust on steel and glass if the humidity is high enough. And

dust is everywhere. This is why mold grows on foundation walls in

basements and in carpets on concrete. Neither concrete nor plastic

carpet fibers are biodegradable, but the dust is.

So the key is to monitor the relative humidity. Therma-Stor now sells

an " Alert " thermo-hygrometer that measures temperature and relative

humidity (RH) and records the hours the RH is above a value that you

set; it even rings a small alarm if the RH exceeds the preset value:

http://www.thermastor.com/HA-DEH3000/HA_manual.pdf

You can place one of these for a few days or weeks where you have

concerns and see what is happening. Most molds can't grow under about

75% RH and don't really grow much until the RH is over 80%.

Don't be confused by advice that I and others have given about RH, as

you will hear that 50% RH is a recommended indoor value in a basement.

This is because you want a large safety factor in the basement air;

keep in mind that if the RH is 50% in the middle of a basement, it is

likely to be higher at the cold foundation walls near the floor

because RH INCREASES as you cool air and the air is colder at the

floor perimeter, especially in the corners. Keeping the RH close to

50% also keeps the mite populations down.

But indoors, above grade) there is no problem, other than comfort,

with summer RH at 70% (because the walls and floor are not colder).

Of course, in the winter, the indoor RH on any surface should not

exceed the dew point, or you will have condensation. So it's risky to

have the RH over 35-40%. (Harvard University is about to tear down a

17 year-old addition to the Fogg museum:

www.boston.com/ae/theater_arts/articles/2008/06/28/falling_down) due

to moisture condensation in walls.

Most indoor air investigators rarely see Stachybotrys spores in the

air but this mold growth is most often seen on the paper of wet

drywall. I and others believe that the spores are in the paper to

begin with (from manufacturing processes), and are not from the air.

It makes no sense that a mold whose spores are rarely seen in the air

would be present inside every wall cavity.

There are many more foolish practices than using drywall with paper.

Some " green " builders are constructing houses with straw bales! (The

first one was torn down after several years due to mold growth!) Other

builders are creating greenhouses (to collect solar heat) and

below-grade spaces (to cool air or store heat from passive solar) as

" pre-conditioners " forthe return air in heating or cooling systems.

(In this case, dust accumulates on the storage material and gets moldy).

To reduce mold problems, keep building materials dry and monitor the RH.