"ASHRAE standard limit was 65% RH @ 76F, (the threshold of a mold explosion) though most health professionals bound it at 50% over the dust-mite issue."

Exactly so and I am more comfortable in the warm, dry heat and it is all about me...ehehehehjeee

I am far from an expert relative to mold, but I believe that you need BOTH moisture and a food source (paper, wood, or other organic material) to create mold. I don’t believe just a moist concrete wall will create mold…unless you also build a wall assembly with it that also provides the food source. At least that is what the Oregon CCB hammers into our heads with their annual mandatory Building Exterior Shell Training (BEST) training requirements.

I trust what the experts at GBA and what they have to say about AAC and humidity/moisture issues. I don't believe they have an agenda and would purposely mislead people about AAC.

What I know personally about CMU is that it absorbs water/moisture like crazy. I've seen CMU walls get wet from a rain and then the temps drop to below freezing and the freeze-thaw does its damage to the wall. Building Science

My experience has been that mold is happy to grow on moist walls where there is no apparent food source. I suspect that dust in the air is sufficient.

ICF buff/professional(?) Lbear is fond of asking critics if they have ever been inside an ICF home. Well, I have $20 that says the "experts" at GBA have never seen AAC let alone had firsthand experience with an AAC home. In fact, the folks who had experience in those threads were witnesses for the defense. Old timers here know that I am quite happy to discuss my mistakes, which are manifold. AAC was not one of them.

The wife and I both have severe mold allergies thanks to a rental home in our past. (A persistent dry cough in the summer is nature's way of telling you to buy a mold test kit.) The thing about parge-coated AAC is that it won't grow mold where you can't see it.

Posted By jonr on 17 Apr 2013 08:18 AM
My experience has been that mold is happy to grow on moist walls where there is no apparent food source. I suspect that dust in the air is sufficient.

This flies in the face of everything I know and have been told about mold. I suppose if a building was contaminated with organic dust this would be possible. However, once the food source is gone, the mold would soon be gone even if moisture remains. So the building would have to continuously get exposed to organic dust to have a long-term mold problem.

In Chris' example, the water leaching out of CMU cavities would bring food with it from the accumulation of detritus therein. But I lived in cmu country at one point, too, and the mold was probably growing on drywall nailed to furring strips over the CMU. Again, there are no cavities in AAC block, and I conducted several experiments/accidents that suggest moisture can't traverse the blocks in any weather event short of Noah's return. Consider that morning, pre-rubber roofing over my garage and post noreaster, when I discovered that I had bug proofed the weep holes through the parapet too well, and the roof had two inches of standing water. On the bright side, I am reasonably confident that the roof will hold any snow load that comes along.

My understanding is for mold to grow, four things have to be present:  mold spores, moisture, food and temperature.  Remove any one item and mold will not grow.  Am I correct?

Posted By Lbear on 17 Apr 2013 12:06 AM
I trust what the experts at GBA and what they have to say about AAC and humidity/moisture issues. I don't believe they have an agenda and would purposely mislead people about AAC.

What I know personally about CMU is that it absorbs water/moisture like crazy. I've seen CMU walls get wet from a rain and then the temps drop to below freezing and the freeze-thaw does its damage to the wall. Building Science

I've seen that too, but in practice it's pretty rare except in bulk-moisture wetting conditions in US zone-6 or colder climates.  But I have also seen freeze/thaw spalling on exposed brick & concrete walkways/porches even in climate zone 4 where it had substantial & chronic roof-drip wetting.  While masonry is generally tolerant of moisture, it isn't immune from moisture damage when that moisture is cycling between frozen & liquid states.  But it still takes substantial moisture content to get there.  (This is a bigger issue in Canada than in the US.)

Thanks for the BSC link- as it turns out friend of mine did substantial work on this project (which was second on the list), and I hadn't seen the writeup on it previously.

I do not remember who posted, but, I hold a BS environmental biology degree and have done considerable work in labs using microscopes. A person carries around enough spores in hair, boots and clothing to repopulate an area with spores. That is unless you are in one of those clean rooms the computer chip makers use. At a construction site I doubt you can make it spore proof.
DanD

Nothing short of a silicon-fab clean room or bio-weapons labs are truly spore-proof. But it's not impossible to keep from feeding the spores. Some bacteria and lichens can feed on limestone/concrete, but not any mold species that I'm aware of. It's other things that might be on the masonry wall (by design or incidental) that feeds the molds, not the masonry itself, and those are a bit easier to control than the spores themselves.

Posted By Alton on 17 Apr 2013 11:58 AM
My understanding is for mold to grow, four things have to be present:  mold spores, moisture, food and temperature.  Remove any one item and mold will not grow.  Am I correct?

Yes, that is precisely my understanding as well.  You can't do much about spores or temp.  However, you should be able to do something to eliminate moisture and food...and just eliminating one of these will prevent mold.

Perhaps some people are confusing mold with moss...  We have plenty of moss growing on most everything in the PNW

Posted By sailawayrb on 17 Apr 2013 03:41 PM

Posted By Alton on 17 Apr 2013 11:58 AM
My understanding is for mold to grow, four things have to be present:  mold spores, moisture, food and temperature.  Remove any one item and mold will not grow.  Am I correct?

Yes, that is precisely my understanding as well.  You can't do much about spores or temp.  However, you should be able to do something to eliminate moisture and food...and just eliminating one of these will prevent mold.

Perhaps some people are confusing mold with moss...  We have plenty of moss growing on most everything in the PNW

Being a native of said region, that may explain the fuzzy texture on my face!?!

...and our webbed feet and gills

This has been a particularly bad year. Spring automobile washing consisted of three complete rounds of scrubbing.

1) Removal of mold/moss colonies outright - hose, and scraper.
2) Moss/mold removal from crevices - hose, soap and stiff-bristled brush
3) "Regular" all-over scrub with scrubbing wand.

Next year, I need to remember to park the vehicles so as to alternate North/South exposure. That tends to reduce the colonies' growth.

New GBA article about AAC. http://www.greenbuildingadvisor.com/blogs/dept/qa-spotlight/does-autoclaved-aerated-concrete-make-sense

Lbear,

Thanks for posting the link.  Interesting read.  I agree with its conclusions.

AAC has a major market share in almost every developed country around the world --China, Japan, Australia, New Zealand -- except the US and Canada. I daresay more AAC homes are built in a day than are ICF houses in a year.

The difference in North America is forests in private ownership and a corresponding vigorous and innovative wood industry. I can't make an energy case for any concrete structure, and I haven't read a convincing one here.

The moisture business is BS. Darwin Australia is one of the most humid cities on earth. You'll see here that an award-winning Darwin builder uses AAC 200 block. http://newabode.com.au/

I established earlier that Martin Holladay's "horrendous bridging" is also BS. Ditto for his diss of effective R value being marketing hype. I turned the mini split off in mid March, even with morning lows this week at 32-45 degrees. (My overhangs have reduced the sun to a 3 foot strip on great room floor.) My power bill for March was $37. I you want, I'll post a picture of it.

OK so, you parked the mini-split, but for completeness, how much did you burn in the wood boiler in March?

Not running yet. I'm setting a record for the slooooowest house build in Pa.