Insulate Attic Ductwork


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Old 04-05-21, 11:43 AM
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Insulate Attic Ductwork

I have non-insulated metal ductwork in my unfinished attic that is exposed to the temps up there. It spans the home and I was wondering about using unfaced polystyrene rigid foam insulation boards and sealing the seams with the metal tape everyone recommends (not named duct tape). Is there drawback to doing this that I don't see? Is it allowed? If not is there a rigid foam product I can use for this purpose? It seems like it would be a ton of savings vs trying to wrap the fiberglass batting around it or spray foam.
 
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Old 04-05-21, 02:59 PM
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Make sure that it is uninsulated. Many attic ducts use insulated liners. The metal exterior reflects the radiant heat. You can also improve efficiency by sealing leaks at the joints and trunks. Foil tape and insulation mastic both work well as sealants.
 
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Old 06-02-21, 05:21 PM
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Cool air in ducts looses BTUs to higher temperature attic air. Duct exterior, even shinny aluminum has minimal benefit.

Duct Insulation reduces that loss, whether it is placed inside or outside of duct. Adding duct insulation wouldl significantly lower AC heat load and reduce power costs.

Home ceiling heat load is significant in AC comfort and costs.

Also, many attic ventilation exhaust fan thermostats do not activate until over 100F to 120F degrees needlessly adding to heat load.

DH found simple, easy way to lower attic temperature and reduce AC operating costs. Uses ordinary thermostat to activate attic ventilation fans above 85F.

Here 85F is highest summer night time outside temperature so stop fans when sleeping.With that setup on hottest summer day attic air is only 7F to 10F above outside air temperature, minimizing AC load.
 

Last edited by doughess; 06-02-21 at 08:02 PM.
  #4  
Old 06-11-21, 10:28 AM
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In an attic:

* avoid anything that can mold

* avoid anything that can blow loose insulation around (ie, into electrical boxes)

* use properly sealed electrical boxes

* meet minimum R value

* it cannot allow bugs into the dwelling

* venting bathrooms is different: be careful where / how it vents (moisture control, bugs, clogging, etc)
 

Last edited by argile_tile; 06-11-21 at 10:29 AM. Reason: add
  #5  
Old 06-12-21, 07:34 AM
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Three questions about prior post “In an attic:”

?1. *avoid anything that can mold"
Like floor, walls, roof, insulation, AC ducts, etc.?

?2. * avoid anything that can blow loose insulation around (ie, into electrical boxes)"
No fans, open vents, etc.?

?3. * use properly sealed electrical boxes"
Who says what are properly sealed?
 

Last edited by doughess; 06-12-21 at 07:56 AM.
 

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