I have been doing a good deal of research into the topic of mold that has grown on exterior walls (and appears to be spreading).


I have had clients that asked me about how to deal with mold. I tell them to deal with it themselves if it is surface mold created by indoor humidity (such as mold in bathrooms without exhaust fans). I then strongly recommend that they install an exhaust fan.


But, if it is mold that has appeared on exterior walls, and the problem appears to be related to outdoor moisture entering the wall cavities, then I tell them not to touch the mold. The next step is to open up a section of the wall(s) and see what the hell is going on behind the walls.


I believe this is the right process to take, but I wanted to hear your take on this.


Thanks in advance!



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First it all depends on your climate - down here it is caused by humidity & is generally only on the surface which requires a good power washing

As for area's up north, I would recomend taking a look via IR & possibly pressurizing the house to see what issues might show up on the exterior 

Back to bathrooms - I would recomend they install a fan with timer or humidistat no matter what, not to many people will throw open a window when it is freezing outdoors

Thanks Sean.


Would you consider the Washington D.C. area North or South? My take is that it's more of a northern than southern climate.

Mixed bag in the N / S (more northern than southern) - but being in a coastal area & humid if memory serves me correctly can lead to surface issues seen in the south


I've seen cases where there are 2 or more layers of siding, and when some of the most recent layer of siding is removed and it reveals that the existing layer(s) are deteriorating. The same can be said for the sheathing in some cases as well. In such a case, it appears obvious to me that the moisture issues have to do mainly with excessive air leakage through the walls.


If the envelope appears somewhat airtight and the mold does not appear to be spreading, then I would probably recommend a simple solution. But, in cases such as those described above, I feel it might be irresponsible (if not straight up dangerous) to just tell homeowners to scrub it down and hope for the best. Do you see where I'm coming from on this?

Most definetly and that is the problem with trying to find a one size fits all diffrent scenarious (like your inside mold one - as that is a major depends for me on amount, location, what the real issue maybe).

Based off what you wrote - while interior moisture might be an issue, that almost sounds like a major installer error & not getting the flashing right or trapping existing water.

Yep, my thoughts exactly Sean.


My sense is that the right move in such a case is to recommend removing all existing layers of siding and starting from scratch (i.e. install new sheathing, insulate the walls, install building wrap and tape seams, and install new siding). Any thoughts on this?


I run into a good deal of houses with no exterior wall insulation, where the siding is nearing the end of its useful life.

I think you need to be very careful when dealing with "mold" (or is it a fungus, or maybe mildew, or something else?). Any sign of moisture on the interior needs to be investigated carefully as to possible causes, but it can be very hard to determine what's happening unless the owner can you give an accurate history. Unless you're trained for it, telling owners what the growth is and what they should do about it is completely out of range, as far as I'm concerned. There is a good EPA handout you can give them about mold.

The only thing I'll comment on is obvious mildew in an obvious place for an obvious reason, such as at the bottom of a window jamb on the interior in a bathroom that four teenagers use and the fan is so noisy you know they never use it. Other than that, they should consult an industrial hygienist or other separate expert if they want to know more.

I'd say limit yourself to your area of expertise. Interview them and find out what occupants habits they may have that are contributing to moisture buildup. Inspect the exterior for problem areas (roof-to-wall flashing areas, kickout flashings, etc). Measure the interior humidity (including over a period of time if possible). Of course do a blower door test, measure the exhaust fans, etc. Make sure that people know that there are quite a few things you cannot inspect without destructive investigation, and there are definitely times when that is warranted.

If there are excessive levels of mold, do you want to perform a blower door test? Would that not pull the mold into the living space and cause a potential health hazard?



In such sitautions I have already performed a blower door test. I have thoroughly investigated the the wall cavities and siding on the exterior. I have begun to record indoor humidity levels, and I am then actively working with a client to find a resolution.


One of my big questions is "What might it cost these folks to remediate mold, etc. the right way?" If the cost is deemed too high they might just go for the old scrub it with bleach solution. I want to avoid this outcome, if possible.


My sense is that the best approach is to get to a point where we have eliminated any bulk water intrusion. Once this is achieved, move on to ensuring a tight envelope. Tear out and replace interior finishes within the affected area(s). Install mechanical ventilation. And, hope that the problem is in the past.


Am I missing the point?

I think your approach is generally correct, but it sounds like you may be putting yourself in the position of telling clients how to remediate mold. The answer to "what might it cost these folks to remediate mold, etc. the right way?" is a difficult one. IMO only a person with significant training can tell you.

In reality, contractors often tear out water-damaged materials without any formal identification of the mold (or fungus or whatever) and simply hope for the best (if they even stop to notice the potential bio-hazard). I've done it myself, bathroom remodels where there has been water getting on the floor or into walls for long periods and there's black stuff growing--cut it out with a sawzall, throw it in the dumpster, and replace with new material.

I've seen a few extreme cases where an industrial hygienist was retained to inspect and sign off, but that isn't common due to the cost and the fact that a contractor who wants to proceed with caution ($$$) will often be replaced by a competitor who is willing to dive in and git 'er done.

The horror stories that we've all heard usually happen in situations where really big mistakes were made in waterproofing, flashing, etc., and the walls are filling up with mold very quickly. There are plenty of stories of condo buildings like that. I've rarely seen it in the typical houses I deal with, but I have seen a few.

I have dealt with mold on multiple occasions..

​As a Mold Inspector Charlotte NC, i recommend Vinegar Spray more helpful in removing Mold from your exterior walls in limited time..

Executive Restoration

5437 Cedarmint Drive, Charlotte,NC




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