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Mold in attic after roof replacement.

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I had my roof replace in 2006. They also replace much of the decking because of mold. My baths were vented into the attic. I had many more soffit vents put in, my baths were vented to through the roof and more insulation blown in. I made sure that the vents were not covered with insulation. I had a roof leak recently (small) and it was repaired; but I now see some mold (far from the area that was leaking). It is less than 10 square feet. What should I do about this? There is so much information about doing something myself or contacting professional I need some advice.

Thanks. Joe Pedersen

asked in Attic Area by joeped (120 points)
edited by darekrudy
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1 Answer

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Hi Joe,
I can’t really provide you with any information or solution because I know nothing about your house’s attic and its ventilation.
Things like:
  • you location / climatic zone
  • roof shape (single open area, dormers, and attic dividers)
  • attic's floor sq. footage
  • types of vents installed
  • vents positioning
  • combined air flow area of the vents (lower and upper vents separately)
  • if you have soffit vents: are their grills clean, type of insulation / is there a vapor retarder underneath
  • details of the mold location (what’s underneath: bedroom, bathroom, 2 story open foyer, etc., what’s above the roof in this area: tree branches / shaded area, or open space)
  • are there open / functional vents below and above the contaminated area?
You can go over my articles in the "Attic" section of my website - http://checkthishouse.com/category/attic/ but I will need some answers to mine before I can answer your questions. Just click “comment” below to answer.
answered by darekrudy (21,730 points)


Thank you. Here are my answers:
Location: 35 miles west of city of Chicago
Climate: Very hot to very cold
Roof: Single Open Area
Soffit Vents (16 in front and back)/Ridge Vent all across the roof The soffit vents are approximately 16" apart in front and back/the grills are clean and free of any insulation or obstruction Sq. Footage of Attic: 400 sq ft I'm pretty sure there is no vapor retarder under the insulation
The mold is on the north side of the roof. It is opposite of a bathroom; however the bathroom is vented to the outside. It is directly over a bedroom on the north side of the house. There are no trees here. I did have a small leak in my roof that was repaired. It was on the southeast side. Should I have taken out the insulation? I see that some of the insulation has some black in it. Could this also be mold? And if so, does it have to be removed?
I also have a whole house fan that is just south of the mold in the attic.
It is in the located in the hallway just outside the bedroom with the mold.
Could this be a source of humidity for mold to grow? During the winter I do put insulation between the fan and the ceiling so that none of the heat escapes to the roof. I am just so concerned after replacing my roof deck and shingles just 6 years ago.
With the leak in the roof, the roofer fixed it as it was under warranty for
8 years. I am just concerned as to whether I need an inspector in here to determine the cause of this mold. He may even have an answer for things to do to remedy the issue. Are you doing any inspecting in Illinois? I live in Naperville.
Joe Pedersen


Hi Joe,
I’m in Vernon Hills, an hour or so north from your place but I quit inspection business almost 2 years ago. I still maintain my website and answer questions but that’s about it, no more inspections smiley.
I’m surprised that there’s no vapor barrier beneath the insulation… there is a possibility that drywall underneath has an aluminum lining (unless you have an older, plastered house and the only vapor retarder would be your ceiling paint).
Since you didn’t provide the air flow area of your vents I still don’t know if you have enough air flow. You can use this simple calculator to get an idea (this is for NO vapor retarder) – http://tamlyn.com/Calculator.html. For a 6/12 roof pitch and 400 sq feet you’d need at least 11’ of a ridge vent and 22 feet of continuous soffit vent (9 sq.in. of free air flow per foot – 198 sq inches total for the soffits).  You need to calculate that for your type of vents, I don’t know what the air flow is for each one of them.
Next thing I would do is to check if there’s a sufficient cutout at the ridge line or board along the entire ridge vent (approximately 1.5” – 2” wide if there’s no ridge board, or 3/4” to 1” on each side of the ridge board).
The rest of your investigation is best to perform at winter when the ambient temperature drops below freezing. To determine if your whole house fan is not leaking air, simply put some plastic foil over it (from your attic side) and look for condensation after a few hours.
You can do the same with the attic floor area directly underneath that mold contaminated section. Let it stay overnight (it has to be cold) and look for signs of condensation on its underside – make sure to remove the foil after test.
Any condensation would suggest that you don’t have sufficient amount of insulation or that missing vapor barrier is your problem. If you notice any condensation or frost on decking surface it means that there’s no sufficient air flow between the soffits and the ridge vent. The two solutions would be 
  1. To increase air flow area of vents (upper and lower should be divided equally)
  2. To install vapor barrier and / or increase insulation amount. 
Sufficient ventilation would take care of warm attic but you’ll still be losing energy. 
As far as the mold… without testing its spores in a lab you cannot say that it is toxic or that it presents any hazard to the occupants. However, since you have a whole house fan there is a possibility of those spores getting into the living space while the fan is operating. 
Remember that the output of the whole house fan is huge and the existing ridge and soffit vents may not be able to handle it. Any excess of air that is not able to escape to exterior will be pushed back into the house through the vent shutters carrying insulation, dust, and possible mold spores particles. 
There are general rules for attic ventilation but each house is slightly different and sometimes they need to be stretched to accommodate those differences.