Mold In Attic

By Robert Robillard on Home Maintenance, Insulation

The Underside of Roof is Wet – Frost In Attic

Moldy, wet roof surfaces in the attic is not a good thing. There is 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, where 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 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 is 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 conditioned space. Read more on the benefits of Encapsulating an Attic with spray foam.
About the Author:
Frank Bood owns Insulation Done Right, a spray foam insulation company servicing homeowners and contractors.
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About the author

Robert Robillard

Carpenter / Remodeler / Editor

Rob Robillard is “The Concord Carpenter” Rob is a builder, general contractor, carpenter, woodworker, and editor of Concord Carpenter and ToolBoxBuzz As a General Contractor and carpenter, Rob owns and operates Concord Carpenter LLC. A full-service remodeling and construction company. Rob is a recognized leader in home building best practices and a source for how-to information for building professionals. On this website, Rob covers all aspects of home construction, building science, home improvement, woodworking, remodeling, and some of the best product and tool reviews. Rob is in charge of our Tool and Product Review series - Concord Carpenter Videos where we post all of our tool reviews and video tutorials. Rob approaches remodeling and building construction with a pragmatic and problem-solving approach. He enjoys using his knowledge and experience to help and educate building professionals as well as DIYers on best practices in the construction and remodeling industry. He's a strong advocate for "raising the bar" in the construction trades and promoting the trades to youth. #BeAMentor #Green2Great Craftsmanship, quality, and pride guide his journey on this channel The Concord Carpenter's motto: "Well done is better than well said!" : Read more about Rob

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