Guide to HVAC Duct Sound Control in buildings
     

• Principles of heating system or HVAC sound transmission in buildings
  • How to make a building quiet: blocking heater or air conditioner or duct noises
  • Sound insulation designs for building mechanical systems
  • Sound control for building boilers, furnaces, ducts, pipes, and mechanical rooms
• Questions & Answers about sound or noise transmission through HVAC ductwork
• References

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Our page top photo shows a typical fiberglass lined HVAC duct interior. The gray debris stuck to the fiberglass is usually house dust, comprised chiefly of skin cells and fabric fiber. Sometimes more troublesome debris collects on interior HVAC duct insulation.

Our building sound control articles begin at SOUND CONTROL in buildings. Other noise and sound diagnosis and control articles are found at NOISE / SOUND DIAGNOSIS & CURE.

As stated in Best Practices Guide to Residential Construction: Use fiberglass ductboard or fiberglass duct liners to quiet the noises of fans and moving air. Avoid sharing a common duct between two rooms that need sound privacy.

Elaborating on some duct insulation considerations: HVAC experts advise us that conventional practice is to insulate the interior of metal ductwork in order to minimize transmission of HVAC equipment sounds throughout a building.

Some fiberglass duct liners are plastic coated and may be able to be cleaned using gentle procedures.

But most common is the use of un-faced fiberglass duct interior insulation, typically treated with a surface resin binder to help reduce movement of fiberglass particles into the air stream.

Our fiberglass lined duct insulation photo (left) shows clean new metal ductwork with a pink fiberglass mat sound and temperature insulation installed on the duct interior. Our page top photo shows that building dust and debris quickly adheres to fiberglass interior duct insulation.

However our work on indoor environmental and air quality topics suggests that from an indoor air quality maintenance view, we prefer to see insulation on the exterior of metal ducts.

That approach permits the ducts to be cleaned, and it reduces the chances of mold growth in the ductwork.

See DUCT SYSTEMS and see Other Places to Look for Hidden Mold for more about places to look for duct system defects and hidden mold on HVAC systems.

Mold in Fiberglass Insulation illustrates problem mold growth in fiberglass insulation including in duct systems.

Construction of HVAC ducts from foil-faced insulating board (photo above left) combines sound and temperature insulation with aluminum foil to product ductwork that is quiet and cleanable using gentle methods.

Other steps to reduce HVAC system duct noise in buildings include:

• Proper HVAC duct mounting and routing to avoid tight contact with floor framing without use of sound isolating mounts
• Selection of the proper air handler blower size and speed
• Location of the air handler unit away from occupied space; if constructing a sound-isolated framed-in enclosure for an air handler for heating systems that burn fuel (LP gas, natural gas, oil, wood) be sure that your soundproofing does not prevent provision of safe and adequate combustion air.
  
  An outdoor combustion air supply can permit near total isolation of HVAC equipment from the rest of the building.

Our photo (left) shows a furnace mounted in a mobile home closet. Owners, in an attempt to reduce furnace noise in the adjacent living space, closed off the return air inlet by installing a solid door. Heating output was substantially reduced and heating costs increased by this bad practice.

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