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Application of gas laws to home mold problems

  1. Jan 16, 2013 #1
    I'm a mold inspector looking for an answer to gases from mold growing in wall cavities of a house. The issue is whether gases penetrate into room air (which I can test for) and what the best approach (short of gutting the house) is to reduce exposure to mold gases in the house.

    The two options are to dilute any gases via air exchange (heat recovery ventilation) or to make a positive pressure in the house (Therma-Stor Sahara-like unit).

    My question is this: Would interior positive pressure keep gases in the wall cavities from migrating into room air -- or would the concentrations of the various mold gases still migrate to the interior because each gas is seeking its own equilibrium? Would the positive pressure do any good?

    Thanks for any light you can shed.
  2. jcsd
  3. Jan 16, 2013 #2

    Simon Bridge

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    Welcome to PF;
    the short answer is that it depends.
    In general, positive interior pressure will tend to drive diffusion towards the outside.
    However, the gasses have to be able to get outside and, perhaps, be convected away.

    I'd have thought there is a third option: adjust the cavity so mold is not favored.
    Is the problem "how best to remove the mold gasses from a room" or "how to test for mold in a wall cavity - given the strategies employed"?
  4. Jan 18, 2013 #3
    Simon, thank you for your response. Yes, driving diffusion of fresh air with a positive pressure is envisioned, with indoor gases (carbon dioxide, etc.) being driven out as doors are opened and closed.

    The question is whether the indoor positive pressure would have any effect on gases in the wall cavities.

    This mold may have been left over from the time of construction, i.e., maybe lumber was rained on. The family does not have the funds to gut the house, so another approach is needed. Further, the house may not need to be gutted. I am thinking that they may be fine, once the positive pressure equipment is installed and electric receptacles are changed to the energy-efficient type, to stop air infiltration from the wall cavities. Then, in the humid summer, under worst-case scenario conditions, we will test for mold gases in room air and then in the wall cavities, to see what we learn from that.

    Best, Mayz
  5. Jan 18, 2013 #4
    I guess you have not thought about how wind can increase the air pressure on a wall leading to infiltration on that side and exfiltration on a lower pressure wall. If the wall with the higher pressure has mold on surfaces, spores can be driven into the living area. There is also what is called the stack effect for a building. This comes from the buoyancy effect of warmer air rises while cooler takes its place. For a heated dwelling, cooler air will enter at the lower part while hotter air will exit through the roof and upper part of the wall. During the summer, if the interior is at a lower temperature than the outside, either naturally or with the usage if an air conditioner, the airflow is reversed - infiltration at the upper levels and exfiltration at lower levels.

    There is also the airflow from one living area to the next, a particlular aspect of each dwelling, taking into consideration the heat and humidity of bathrooms, kitchens, laundry rooms.

    A positive pressure within a home may not have the capacity to deal adequately with all kinds of situations. A newly built tight house would adjust better than a lossy older house, I would presume.
  6. Jan 18, 2013 #5
    265 bits - Thanks for your input. Interestingly, culture plate air samples were taken in every room of the house, as well as in about 10 wall cavities. Paecilomyces was found in all the wall cavities and in none of the rooms. Thus, while your scenario would apply to many houses, it does not seem to be the scenario for this house. Spores are not infiltrating from the wall cavities to the rooms - and have not infiltrated over the past dozen or so years.

    It would seem that if there is any health influence, it would be mold gases. Our hope is to keep gases from infiltrating the house (if indeed there are mold gases in the wall cavities). It is not a good time of year to test for gases in the wall cavities. We'll wait for more humid weather. - Mayz
  7. Jan 20, 2013 #6
    Why is mold growing in the wall cavities? Moisture.
    The 'Therma-Stor' whole house dehumidifier may be needed, but a smaller unit might suffice. Try tracking the temperature and humidity for a while. This cheap little logger http://www.manafont.com/product_info.php/usb-temperature-and-humidity-data-logger-kg100usb-p-7311
    helped me solve an attic cold weather humidity problem that developed after replacing a 30-year-old breathable tar and gravel roof with a new plastic membrane roof -- that didn't breathe at all, so it trapped moisture.

    Over a year the humidity showed, beyond seasonal and daily cycles, a steady increase and some white mold showing up on the cold north side of the roof. We put a dehumidifier in the living space, dropped that from 70 percent, and within a month the attic dried out as well. I'd guess this also was happening in wall cavities.

    This also happens when insulation and air conditioning are added to older buildings that had neither when built.

    http://www.buildingscience.com/docu...ng-cold-weather-condensation-using-insulation is a good summary.
    This is a great overview: http://www.buildingscience.com/docu...different-this-time?topic=resources/retrofits
    Last edited: Jan 20, 2013
  8. Jan 21, 2013 #7
    Thanks, Hank - but my question was about whether a positive pressure in rooms would keep mold gases in the wall cavities from infiltrating into room air. It was a question about the behavior of gases. Any ideas about that one?

    I'm glad you solved your attic issue and thank you for the citations you provided. I'll check them out. Thanks, too, for the dehumidifier info. What is good about the ThermaStor unit is that it can be modified to bring in filtered fresh air to make a positive indoor pressure, plus being a source of oxygen.
  9. Jan 21, 2013 #8
    Hank, I reread your reply and see that I misread the beginning - about logging temperature/humidity. At this time, the concern is just potential MVOCs in room air. The homeowner wants to get the HVAC technology in place and then next summer, when it's a better time to test for MVOCs, we'll work with UL-Air Quality Sciences to test for MVOCs in room air and then in wall cavity air, to see if there are correlations with MVOCs.

    The question we face is what is the best technology to use - positive pressure with the ThermaStor unit or HRV with dehumidification.

    Thanks, May
  10. Jan 21, 2013 #9
    Hank, since you know about mold - I've been keeping my eyes lately on the subject of Paecilomyces in wall cavities. I've been finding it in quite a few, where I do culture plate sampling at leak areas. The experience at this one house, where the Paecilomyces has been in virtually every wall cavity has me wondering if I should be doing control sampling in wall cavities... and then I wonder how common the Paecilomyces will be in wall cavities. What if a lot of houses have this issue?

    And then, where did the Paecilomyces come from? Could the houses have been rained on during construction? I want to bring this up at the next chapter meeting of the IAQA, but so far I've not seen anything written on this subject, other than "Paecilomyces can grow in wall cavities."

    There is also the real estate disclosure and the gutting-of-a-house issues... neither one to be approached casually. The stakes could be high - Mayz
  11. Jan 21, 2013 #10
    I've already told you more than I know (grin).
    This is the best overview I've seen; the point I was trying to make is that you'll get unexpected changes if you change just one thing, and may create new problems:


    "... We began to insulate .... The hangover from all this activity hit in the early 80’s. Rot and mold joined bad music as the signature symbols for that decade.

    ... In retrospect, the obvious happened. When you change one thing it changes something else. And when you change that something else the law of unintended consequences rears its ugly head. We discovered that buildings, houses in particular, are interrelated systems....."

    I'd suggest looking at diffusion rates -- I know some gases will diffuse "upwind" quite effectively, and trying to create positive pressure inside is going to produce a lot of different pressure variations in different areas at different times with different activities and different weather. I'd seriously doubt if a slight internal overpressure is enough to push 'mold gases' away -- and what got pushed out would likely swirl around the house and get back in.

    Dry out the cavity spaces and the mold quits thriving.

    I recently heard from a neighbor with some expertise that large companies that have older solar thermal (hot water supply) systems are now replacing them with simple solar PV panels and running the electricity directly to hot water heaters --- no plumbing, no valves, no freezing -- because the price of solar electric has dropped to where it's cheaper to get hot water that way.

    If that works for residences, it would be the simplest possible retrofit for the old pre-1970s house building design --- where you dump cheap heat into the building, and natural air circulation keeps fresh air coming in and the humidity low. Just use any surplus energy collected as throwaway heat.

    The benefit -- no airtight building, no damp in the walls, no mold problem.
  12. Jan 21, 2013 #11
    Thanks, Hank. I've run off the Lstiburek piece to read. I understand the point you are making, and my question has always been whether the positive pressure would keep any mold gases in place in the wall cavities - perhaps you are correct, that it probably wouldn't - but I can't get any answer on this other than an opinion - which is why I made the post originally.

    We haven't established that the wall cavities are damp. Maybe it is just the relative humidity of any unconditioned wall space in the neighborhood. Paecilomyces was found in the wall cavity of an interior wall, too.

    Maybe the Paecilomyces is just left over from the construction process, where it rained on the framing. Who knows? Maybe there isn't much growth there, or gases, and it would be ill-advised to make too big of a thing of it. But we don't know that for sure at this point.

    If adding heat and opening windows during dry weather would do the trick, then we're home already.

    What would you say to my clients if you found Paecilomyces in 10-12 wall cavity samples? They just want to make sure their house is healthy. - May
  13. Jan 21, 2013 #12
    I'd say I know nothing about this, but that mold spores are found everywhere, all the time, but molds only grow in specific conditions. Some like high humidity and near-freezing temperature, and prefer refrigerators; others like high humidity and cool temperatures; I don't know of any that grow in normal to dry areas.

    Google often can be reassuring:

    turns up http://www.jmedicalcasereports.com/content/6/1/86
    Journal of Medical Case Reports 2012, 6:86 doi:10.1186/1752-1947-6-86
    says only six people -- worldwide -- have had sinus infection from those.

    suggests it's not a good idea to let the mold grow on wallpaper colored green with arsenic-containing paint, and that humid conditions favor it.

    I'd guess that whatever sampling you did just found spores and got them to grow out in favorable conditions -- rather than poking a hole big enough to shine a flashlight in and look for actual fungus growing. You can use a rifle "boresight" or something like that, the cheap version of what doctors use to look inside people through small openings, to check.

    If the report was from getting a sample to grow in a lab, seems to me you can probably do that from any dirt anywhere, or any dust that contains dirt blown in the wind.

    I didn't find any suggestion that there's any other "mold gases" problem with this species -- why would you think there is?

    Remember -- I know nothing about this, just curious myself.
  14. Jan 21, 2013 #13


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    Mayz, As a homeowner I'd be nervous trying your approach without addressing the reasons that mold are present in the first place. Rain during construction is common and usually doesn't cause problems by itself. Your mold is growing in enclosed, warm, continuously damp spaces--and those are what I'd expect you to mitigate if you were working for me.

    One cause is poorly executed "energy tight and green" building practices. Leaky window installations, sheathing, etc., coupled with waterproof membranes that keep moisture in place, lead to mold issues. These were especially common in houses built in the 1980's, when tight construction techniques were being developed but had not been perfected. A leaky roof or plumbing is another common cause. Engaging a qualified building/construction specialist to investigate the causes would be money well spent.

    If the cause is excessive interior moisture migrating into the wall cavities, then dehumidification is a good remedy as hank already mentioned.

    You might also provide other information. Where is the house located and what kind of climate is it in? When was the house built? Has the house (roof, plumbing, etc.) been well maintained?
  15. Jan 22, 2013 #14
    Also -- no offense to anyone here, but I found this guy on the Internets who has this to say:


    "What You Should Know About Certified Mold Inspectors And Certified Mold Remediators
    ... Not only is there no regulation, anyone that wants to become a certified mold inspector can pay $95, take a 30-question online exam and VOILA!, they are instantly a Certified Mold Inspector. Now I am not saying that all certifications are worthless. On the contrary; I have nearly a dozen myself....."
  16. Jan 22, 2013 #15
    Thank you for your response. Here are couple of points:
    A boroscope is fine for black mold but often misses green mold, which is what Paecilomyces is. Further, if it's not a problem (as confirmed by MVOC testing next summer), the homeowners would rather not know about it.
    Paecilomyces gives off VOCs.
    I'd expand your comments on buying inspection & remediation certificates to the whole mold industry. Even with the best training, inspectors are not trained to work with microscopes on-site. They go out to investigate something that is often invisible, without the tool that reveals the invisible. - May
  17. Jan 22, 2013 #16
    Marcus, thank you for your response. Yes, you said you would want to know as a homeowner and to have the condition (if there is a condition) remediated. The homeowners would feel the same way -- if they were convinced it was an issue. Since we know that spores are not penetrating into room air from the wall cavity, the only other potential health issue would be gases from the mold. If gases (after testing with a certified laboratory) are ruled out next summer, then why investigate further, when the house has already stood the test of time?

    This house is an average, 50 year old house, not in a damp area. It is well maintained. Because the Paecilomyces was also found in an interior wall cavity, we suspect something like rain during construction rather than an ongoing moisture issue. Further, there are two types of siding - vinyl and stone. The same finding was found behind each. Maybe a low-moderate level of Paecilomyces is common in houses. Who knows? Who has investigated? Probably no one. We're talking about a fringe area of investigation here. Thanks for sharing your thoughts--- May
    Last edited by a moderator: Jan 23, 2013
  18. Jan 24, 2013 #17
    > two types of siding - vinyl and stone. The same finding was found behind each.

    Is there insulation in the wall cavities?

    Unless the vinyl and stone have an air space behind them for water that condenses to drip down and out, those are waterproof outside walls, so any moisture that condenses on the spaces up against the cold outside wall will stay there.
  19. Jan 25, 2013 #18
    PS from May - Just reread my recent post and see it needs a correction - Paecilomyces grows out yellow on a Petri dish with MEA - not sure what color it would be on wood, if visible. Because it looks like Penicillium under the microscope, my mind jumped to "green."
  20. Feb 8, 2013 #19
    Chemical diffusion is all about chemical gradients, not pressure. If there are holes in a wall, then pressure is an issue. Pressurising to prevent gases diffusing is pointless as it won't make much difference to any chemical gradient.
    Sounds like you have a huge problems on your hand if the wall cavities are affected.
    I'm not too sure what "gases" you are testing for. Sometimes the most toxic emissions are at very low levels and hard to pin down.
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