Insulating around heating ducts running between floor joists
I’m in the process of renovating an older bungalow home in climate zone 7a. It currently has an unfinished basement but I am in the beginning process of retrofitting this. It is a heated basement and plan to finish it into a living space.
I’ve been taping the seams and joints of all ductwork with foil tape and then planning to insulate the ducts. ALL of the heating ducts run under the main floor between the floor joists.
My question is, seeing as some heat loss from these ducts goes up through the floor into the main living space instead of into an attic, by contrast, is it necessary to completely insulate the ducts around their entire circumference, or should I just insulate under them and enclose the joist space with a reflective foil?
Thanks
Todd
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Replies
Todd,
Use your judgment. Is your basement now warmer than the rest of the house? Cooler? Have you finished your air sealing work and insulation work yet?
If the basement walls have been air sealed and insulated, you don't want to insulate the ducts if the basement is cool. If the basement is too warm, insulate the ducts.
-- Martin Holladay
If the goal is just to isolate the ducts to keep them from parasitically heating/overheating the basement, just insulating the underside is fine. Forget the foil- it needs an air gap on both sides to have a reasonable thermal benefit. If there's room for a 3.5" batt with a paper facer to snug up to the duct (fiber side to the duct, paper downward toward the ceiling gypsum) that's "good enough."
Foil tape works well if the ducts are shiny and clean, but for older & dirtier ducts duct-mastic is more reliable.
Thanks for the insight, Martin and Dana. I only just started my research on ductwork but what you've said makes sense and will greatly reduce the research I would have done!
Cheers
Todd