How to Insulate Interior Soffits and Dropped Ceilings


Key Considerations:

1.Interior soffits and dropped ceilings are often used to cover heating ducts, water lines, drains, wiring, and framing.  It is often difficult to make the air and thermal barrier continuous inside these spaces because of all things that are packed inside of them. 

2.Interior soffits often have recessed lights in them that can loose heat either by radiation or air leakage into thesoffit space and eventually outdoors.

3.Taped and mudded sheetrock is an effective air barrier in most homes.  In order tosave time and money sheetrock is often left out, or left unfinished, aboveinterior soffits and dropped ceilings so the air barrier is compromised.

4.The very nature of an interior soffit or dropped ceiling is to cover something unsightly. Many times air barrier andinsulation details are left unfinished because the flaws will be covered up. 

Possible Downsides:

  • Poor energy efficiency
  • Air infiltration
  • Possible condensation and mold inside the hidden space
  • Ice dams (if the soffit is below a roof area)
  • Cold floors (if the soffit is below a second floor)
  • Pipe freeze ups
  • Low efficiency from your heating system because the delivery system is trapped in a cold space.

Recommendations:

The exterior faces above the soffit or dropped ceiling must be treated the same as any other exterior wall or roof- make the air and thermal barrier continuous. Make sure your insulation has the full thickness it should have for an exterior wall and that it is continuous.  In a sheet rocked building install and finish sheetrock with at least 1 coat of mud (this is called fire taping) on all exterior surfaces before building the interior soffit.  If you can’t install sheetrock use foam to make your insulation layer airtight. All mechanical penetrations through the air barrier should be foam sealed before the soffit is installed. These points are especially true if you are going to have recessed lights or water lines inside the ceiling or soffit. Radiant and convection heat loss from recessed lights or hot water lines are fine as long as the heat can only be "lost" into other parts of the house and not to the outside.

Pretty typical interior soffit above kitchen cabinets. 


This is what the soffit looks like with an infared camera.  Darker colors are colder.The soffit appears cool except for the wood block that is behind the seams between the soffit panels. You will also note an air leak in dark black under the range hood.  

The cold air inside the soffit space makes its way into the house at a gap formed between the last cabinet and the wall.  The black streakwashing down the wall indicates this air pathway. These images were taken while I had the house depressurized by a blower door.  Under normal conditions warm air most likely exits through these pathways rather than cold air being drawn in through them.  Depressurization is a technique used to help a thermographer “see” air leaks that would otherwise be invisible to the infrared camera. 

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