Mold In Attic

By Robert Robillard on Home Maintenance, Insulation

Underside of Roof is Wet – Frost In Attic

Moldy, wet roof surfaces in attic is not a good thing. There are a myriad of reasons for moisture buildup in the attic.
Normal thinking would tell you that having enough airflow or ventilation would solve this problem. That is not the case for every home, as some homes are leakier than others. Some homes do not have proper air sealing on the floor of the attic. Many homes have HVAC equipment in the attic which leaks. (All HVAC units leak by design) .

Air Leaks Into the Attic  – Most Likely Cause

The most common is air leaks allowing moisture to travel up into the attic space and condense on the cold surface of the roof deck. This is a very common problem in climate zone five here in New England.  We see two to three cases a week in our travels, were this is extremely evident.

In the winter air leaks show up as frost on the underside of the roof. Frost eventually melts and leaves plywood, framing and insulation wet, eventually mold grows.

Frost gets into the attic from air leaks, or attic bypasses.  The best way to prevent frost from accumulating in an attic is to seal off attic air leaks.  Once you seal the air leaks, the attic will be frost free. The problem is that unless you remove ALL the insulation, it’s almost impossible to find all the air leaks.

Note – It can be very difficult to find every air leak without completely removing the attic insulation. Learn How to Seal Air leaks here.

Prevention  NOT Ventilation

Typically it requires mold remediation and dehumidification to dry the area. An application of a closed cell spray foam can prevent the moisture from getting to the roof deck and condensing. Most folks would believe that adding ventilation would solve this problem ( which in some cases can help) but that is not always the case.
Closed cell spray foam is the only solution that we have found for this particular problem. There is a lot involved prior to installing closed cell insulation, such as:
  • Removal of the existing insulation
  • Mold remediation and/or cleanup
  • Dehumidification
  • Ensure prover venting of bath fans and dryers vents

Bath Fan Ventilation

There are a lot of other contributing factors that can cause this, such as bath fans not being connected properly. Those same bath fans being vented in the soffit and not properly ejecting the steam outside.

Encapsulating the Attic

Closed cell spray foam if installed on the roof deck, and side walls is a one stop repair that will solve this issue for almost all attics. Once the underside of the roof is encapsulated with foam the attic becomes a conditioned space. Read more on benefits of Encapsulating an Attic with spray foam.
About the Author:
Frank Bood owns Insulation Done Right, a sprayfoam insulation company servicing homeowners and contractors.
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About the author

Robert Robillard

Robert Robillard

Carpenter / Remodeler / Editor / Writer / Video Talent

Robert Robillard is a remodeler, general contractor, and principal of a carpentry and renovation business located in Concord, Massachusetts. He also writes the "Ask the Carpenter" advice column in the Boston Globe, and serves as the Editor of Tool Box Buzz and founding editor of A Concord Carpenter . Rob is in charge of our Tool and Product Review - Tool and Product Review - Video Channel, , where we post all of our tool reviews and video tutorials. Rob enjoys using his knowledge and experience to help and educate building professionals as well as DIYers on best practices in the remodeling industry. The Concord Carpenter's motto: "Well done is better than well said!" : Read more about Rob <a href=""

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