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Mold/My A/C and blown in insulation


After having an air conditioning unit put in our attic and the ducts cut in our ceiling we had blown in insulation put in the attic for energy conservation.  The installer of the A/C unit said that it is now important for us to not only run the fan on the A/C every few weeks in the winter but informed us not to close the vents in the ceiling for fear of mold building up in the unit.  Do you agree with this?  We have a ridge vent in our roof and soffit vents too.  I do not understand this.  We had insulation put in to save on heat but now we have constant cold air blowing down on us from the A/C vents and intake.  Since we have baseboard forced hot water heat, I feel like they are working against one another all winter long. Do you have any information about this situation for me?  Thank you!


You don't say where you live, but I'm assuming somewhere in the northern tier of the U.S.  This is important because how an attic duct system behaves is related to climate.

That aside, here's what can happen:  Warm air (which holds more moisture than cold air) can rise up into the attic ducts in the winter, where it may be rapidly cooled and then deposit moisture on the interior surfaces of the colder ducts.  This is less of a problem if the ducts are well insulated, but most flex duct and fiberglass duct insulation only have an R-value of 5 to 10.  So if your attic is well ventilated (which it should be, for other reasons) and you live in an area with really cold winters, then it is likely that you will get condensation in the ducts, which, under the right conditions, can promote mold growth.

These attic systems work well in southern climates where the ducts are normally never cold enough to cause widespread condensation.  

One solution is to fully seal the supply and return registers in the winter, so moist air can't enter.  However, this is technically difficult to do.

When you run the unit in the winter, you are subjecting the air to the colder ducts and thus cooling it.  You are indeed fighting your baseboard heating units.

There is no great solution, short of installing heat with the  A/C in the attic so the system runs comfortably year-round.  If it was my house, I would over-insulate the ducts and close them off, including the return side.  Then I would periodically inspect the duct interiors in a couple of places with a strong flashlight, looking for condensation or mold.  If it does not occur, then the measures worked.

By the way, because you have hot water baseboard, adding a heat exchanger to the attic system may not be so difficult.  But careful planning would need to take place to avoid ANY possibility of freezing the water-filled lines.

Best of luck, I wish I had a more clear cut solution.

Steve Major


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Steve Major (Principal -- Lakeland Environmental)


I can answer questions on indoor mold (fungal) infestations, indoor air quality, and indoor moisture problems. This includes mold testing, investigation, and remediation. PLEASE indicate your state or region, so I can provide the best possible answer.


I have extensive experience in the investigation, testing, and remediation of indoor fungal (mold) infestations, including the design and oversight of remediation projects, hands-on cleaning and removal, and safe work practices. I developed a successful mold remediator training and certification program, and have trained many mold workers and remediation supervisors in proper techniques. I have a strong knowledge of the ways in which moisture and airlow patterns in buildings can affect fungal growth and air quality.

BS Cornell University. IAQA Certified Mold Remediator. 40-hour HAZWOPER certification. NYS Department of Health Certified Training Director.

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