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Residential Energy Efficiency Blog

Saturday, August 15 2020
Black Mold on Attic Side of Roof Plywood


Why does Mold Form on Plywood?

Typically, condensation in the attic starts due to a moisture issue created by introducing two completely different air masses.  Here are the two possible reasons this problem is happening in your attic if you are seeking answers to the problem.

1. No air sealing at the attic floor to stop conditioned air from escaping into the attic (warm air/ winter issue) coupled with a lack of adequate attic ventilation creating conditions that are right for condensation that adheres to the plywood and eventually forms mold.

2. Leaking ductwork in an attic and other HVAC related inefficiencies that can allow air to leak out and interact with the attic air.


How Summer Condensation is Forming in This Attic

Uninsulated ducts Randallstown MD

This example is one I came across during the attic inspection at a home energy audit.

I have been in a lot of attics and seen a lot of plywood in attics (for which I am very proud!).  I was not exactly sure on this one at first. 

I have seen small patches of discoloration due to poor bath exhaust ventilation or a disconnected duct, but I have not had a mold case.  

At first when I got up in the attic I could see the stains.  It was apparent quickly that there was an issue beyone what I usually see.

At first, I considered the possible interaction of the conditioned air in the living space below and the warm humid attic air.  A large opening or bypass?  That seamed unlikely based on what I had seen up to this point.  Then I turned around.

The poor duct was lying completely undressed and quite dirty.



Summer Moisture - 4 Things to Lookout For

Here is the recipe for mildew and mold growth on the plywood in warmer months:

1. Second HVAC system located in the attic.  All ductwork servicing upstairs is in the attic.  Ducts are leaky and improperly insulated allowing cold air to escape.

2. Second HVAC system improperly sized.  Too large for the space/square footage it is servicing.  The larger blower is pushing cooler air through leaky ducts and into the attic.

leaky ducts maryland attic

3. The thermostat is located in an area that was reported to be a "hot spot" on the floor.  This is due to the lack of HVAC system performance and exposure to the hot attic due to a lack of proper insulation methods and techniques.

4. Poor ventilation in the attic due to improper baffle placement and insulation covering the open areas.  

Incorrect baffle installation

A baffle is supposed to act as a channel for the open soffit  intake below.


covered soffit vent

The soffit opening is covered with insulation not allowing good flow.

proper attic ventilation


The Solution

Control the humidity and the mold growth disappears.

Here are the precise steps recommended to solve this issue for the long term.


1. Remove the insulation around the soffit/eave areas - full perimeter

2. Reinstall baffles at the eaves to promote the best possible organic attic ventilation. 

3. Re-position the insulation to not cover the openings.

4. Seal the duct seams and tightly insulate the supply plenum (main AC duct) to eliminate leakage (lost energy) into the attic.



Written by Eric Gans
I have over 1000 energy audits under my belt in Maryland.  I like to take my personal experiences with each of my audit customers and try to get the things that concern them out into the world so others can make good home improvement decisions - in the right order - according to their needs.

Do you have something going on around the house? 
Not sure if an energy audit would be able to solve it for you? 

Ask Me!
I'll let you know if it is something that is included in the service.

I would like to hear your comments about this article.  Feel free to post something below...

Put new roof on house 4 years ago.Went in attic black staining on plywood in attic .But not the whole attic just the front of house back of house has nothing on plywood.Was wondering why it has staining front of roof none on back part ? Thanks
Posted by William riches on 12/08/2020 - 12:46 PM
Hi William, What direction does the front of the home face? Eric
Posted by Eric Gans on 12/09/2020 - 05:12 PM

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