Mold quiz

here is a diagram of a house i looked at today that has a pretty bad mold issue and some of the wood was damp to the touch (not caused by leak). the roof is 2 years old (not mine) but looked good. probably about r19 insulation in the attic with clear airflow to the soffitts. i have come to a conclusion of the problem and what to do to fix it. i just want to see what all of you would come up with in this scenario. sorry my diagrams are’nt as good as "ranchhand"s.

WTF , i dont know if you guys can read the diagram…the site shrank it to fit the stupid ads on the right side of the page :evil:

I am guessing that the whole house fan is dumping more air and humidity into the attic than the vents can exhaust.
The gable end vents are shorting out the ridge vents, this in addition to the whole house fan forking up air circulation.

Assuming that the soffit vents are not plugged up with dust or painted shut, and the ridge slot is cut open/large enough.

That is my stab in the dark. :slight_smile:


I would have the mold tested before I did anything.

There are a lot of guestions I ask before for making any conclusions.

I would close off the whole house fan. If the mold is the rare dangerous kind. It needs to be kept in the attic.

there is no ridge vent.

the mold has been tested.


With just the diagram. The moisture is coming from the fan. Close it off.

Or eliminate the source of the moisture in the house.

I would concur with lefty here,spray nine will kill most molds and fungus,if you didn`t know,definitely call in a pro ,and use a mask/respirator to be safe

my conclusion was a combination of

kill the house fan

block the gable vents

add a ridge to balance the intake…

i think the intake is much higher than the little exhaust from the gable end vents and needs a ridge (havent run the numbers yet for the fha 1/300 thingy) to properly balance things out.

 a seperate company is going to do the mold killing. i recommended they wait a few months after the venting is fixed to let the moisture in the wood dry out. they are also going to replace all the insulation in the attic.

I’ll bet the gable fan only comes on when it’s hot. All other times, there is little air movement in the attic and moisture from the house is going past the whole house fan and condensing in the attic. Naturally this problem is worst in the winter.

Ithought eliminating the gable vents went without saying :wink: ,I wouldnt wait on the mold as its a serious health issue

If the whole house fan is necessary to alleviate excess heat in the living space;

a. make a temporary cover that will close it off in the winter but allow for removal in summer

b. install a humidistat on the gable end power vent

c. install ridge vent

d. leave the gable vents alone (that short circuit thing is an old wives tale)


The whole houe fan is forcing all ot he exhaust to go through the one gable vent illustrated.

The side without a gable vent, (unless the drawing got cut off), is being short circited. If there were a gable vent on the opposing side as well, the enire attic moisture content would be able to exhaust from both ends.

The better solution, would be to seal off the gable vent and install a continuous ridge vent similar to the Shingle Vent II.

Contrary to the evidence and the air flowage, Dennis still will not allow this to conclude short-circuiting, even in the face of a real worls example.

The proper air-flowage will cause the mold growth to die off naturally, because the mold needs stagnant moict air in a dark cavity and a source of food to live and rejuvenate.



I do not think the system is short circuted.

The mold is in the direct path of the forced air. That means the moisture is from inside the living space.

The present attic ventilation is working fine until you introduce the inside air. Remove the whole house fan. Remove the problem.

I doubt ridge vent will do anything to relieve this problem.

If you are going to leave the whole house fan. Put a powered exhaust right above the whole house fan. Take the moist air right out of the house.

Do not reintroduce it into the building struture.


I would also tell the homeowner my thoughts. Then tell them that they need to get an engineer to design a system to be installed. I am only an installer with practical experince, you need an expert in ventaltion to design the system. Then I will install exactly what they draw and design.
I would not take any credit for what is going to be done.

the system is already short circuited. there is way more intake than exhaust. there are gable vents on both ends. the house now has central air …so no more need for house fan. there is currently very moist boards and the home owner hasnt run the fan in months. so i think it is a combination of unbalanced venting and humidity periodically from the house fan.


The intake is exactly the same as the exhaust. The exhaust pulls the air out. The intake does not push air in. So the system is balanced.

If there was mold all over. You may have a case for ridge vent. There is only mold in one area. 3/4 of the house is fine. If ridge vent was the problem the mold would be all over.

I agree with lefty about the whole house fan,it`s the same as dumping a bathroom fan into the attic w/out the tube and roofvent attached to it

screw it all…he took my knowledge and estimate and handed it to “low bidder joe” and said “can you do this (hands him my bid) for 1/2 the price?”

you must have a sweet pc marshall.
thats a clear picture.

as far as mold goes
doesnt seem to bother me.
course some call me crazy.
maybe it makes ya crazy.


gweedo…thats not me in the pic (i could only hope for a sweet mostauche like that). i just google “angry man”.