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  • Writer's pictureMorgan Beall

A Big Problem in Florida - Web Trusses and the Outdoor Environment

When constructing a home it is really important to seal interior ceiling cavity spaces from exterior ceiling/roof cavity spaces. Web trusses or ceiling trusses running from above a living space to an exterior outdoor living space will leave a gap if not solid sealed. If you do not air seal between these areas before finishing your interior construction it will result in mold growth and other damages related to severe moisture/humidity damage. What happens is outdoor air or heated air from the roof cavity or ceiling cavity over the outdoor living area will mix with conditioned air and cold surfaces of the ducts over the conditioned interior living space resulting is what is called a temperature inversion.

Humidity within this ceiling cavity above the living space reached 70% RH with dew points exceeding 70 degrees F which can result in precipitation. Basically this means most surfaces in the ceiling cavity will accumulate water resulting in significant mold growth (i.e. ducts, wood framing, plywood and drywall).


This construction mistake seems to be an reoccurring trend in SW Florida building. Hire a building science expert or structural engineer to help prevent this very expensive and potential risk to health and safety in your new home. This simple construction mistake has cost our clients a significant amount of money and stress.

See an example of this in a short video by the best building science channels on youtube. https://youtube.com/shorts/uMTvzNnT4_8?si=04UcwI7Qzoerq-HH


Below is an example of this section of ceiling cavity between the outdoor ceiling cavity and the indoor ceiling cavity. Plastic was put up at this threshold to help control the interior environment during remediation efforts. This is where the insulated wall building envelop should have been located.

Solution? Hard seal with plywood or exterior board the gap between the interior and exterior ceiling cavities and insulate the hard sealed surface properly with traditional methods with proper R-rated insulation or closed-cell spray foam insulation. It is also important to seal all utility penetrations that may run through this threshold (i.e., electrical lines, plumbing lines, gas lines and communication lines).


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