Uninsulated fan ducts in an attic (C) Daniel FriedmanBath Exhaust Fan Duct Insulation
Why & how should we insulate the exhaust duct on a bathroom exhaust fan system?

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Bath Exhaust Fan Duct Insulation:

How should we insulate the exhaust duct on a bathroom exhaust fan system to prevent condensation and leaks into the building or wall?

This article series explains how to install bathroom exhaust fans or vents, the vent ducting, the vent termination at the wall, soffit or roof, vent fan wiring, bath vent duct insulation, bath vent lengths, clearances, routing, and we answer just about any other bathroom ventilation design or installation question you may have.

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Bathroom Vent Fan Ductwork Insulation

Insulate the bathroom exhaust vent fan ducts: In un-conditioned space such as an attic, where otherwise your fan duct will be exposed to cold attic air in winter, use insulated solid metal ducting or insulated flex duct.

Uninsulated fan ducts in an attic (C) Daniel Friedman

We insulate the bath vent ductwork to minimize the condensation that will otherwise occur as warm moist bathroom exhaust air passes through cool ductwork in the building attic or roof cavity.

Our photo above shows a long routing of bath vent fan ducts across an attic floor to an exit at the building soffit or eaves and the condensation of moisture in such vent lines.

These ducts should be insulated (or replaced with pre-fab insulated ductwork or flexduct), and a better installation would have shortened that very long duct as much as possible, exiting to a soffit closer to the bathroom.

Metal bath exhaust fan duct installation in new construction (C) Daniel Friedman Bath exhaust fan in insulated ceiling after foam insulation installation (C) Daniel Friedman

Above we illustrate bath exhaust vent ducting before and after it is covered by a spray foam insulation job in a cathedral ceiling.

Watch out: we do not recommend purchasing vent ducts with insulation on the duct interior.

Although fiberglass-lined HVAC ducts are widely used in heating and air conditioning systems as well as in heat recovery ventilator systems, for a standard bath exhaust fan vent duct, the combination of permeable fiberglass or other insulating material and very high moisture levels is asking for mold or other trouble inside the vent line.


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Publisher - Daniel Friedman