Ducts in a blanket?

[MinMichigan]

I have a furnace and associated duct work in a cement block crawlspace with a sand floor covered by plastic sheeting. The original installer said not to insulate the heat ducts. I understand the rational of heat rising. But with Michigan winters of -0 sometimes, crawlspace ave temp of 42 degrees I'm curious if other folks have a different opinion. I'm wondering if the insulation would reduce that initial cold air purging of the heat ducts. thoughts??

thanks, Marcia

 

[dkpd1581]

Is this duct work for Heat ONLY or is for both AC/Cooling AND Heating??

What type of duct are we talking about?? (Metal round or rectangular, fiber board duct, spiral insulated, flex - if flex what is the listed R value).

Answer these questions and we can give you a correct answer.  The organization that is the authority on HVAC sizing, unit selection, and installation is ACCA and is located in the Manual J and Manual D for duct sizing and installation.

[MinMichigan]

The square plenium (feel free to correct my terminology)  runs out from the 45,000 btu furnace for about  for about 5 feet (guessing right now but could get down there to measure if needed). The 8 inch round galvanized pipes leave the plenium to farious registers. The plenium is perpendicular to the floor joist with most of the other running up in between the floor joists. It would be easy to span the joists with fiberglass.  I have a 4 ft. wide roll of fiberglass with the foil backing. 

Also, the cement block walls have 1 inch Styrofoam. Maybe it would be better to add to that rather than wrapping the duct work?

thanks, Marcia

[dkpd1581]

You can insulate the duct and it will offer you some savings in heat lost during the heating cycle; however, regardless of how much insulation you install, the metal duct will eventually get cold during the off cycle. 

Compare this you your refrigerator.  If you never opened the door, the compressor would still start several times in a day and remove heat from the box.  The best insulation never really stops the transfer of heat, it only slows it down.  As long as there is any appreciable difference in heat (ie. energy) between two points, there will always be a transfer/movement of heat/energy.  The greater the difference between the two points, the greater the degree of movement.

You will notice some improvements by insulating.  First make sure ALL seams and joints are well sealed using mastic and/or silver tape.  This will reduce air infil/exfiltration.  The real question will be at what point will the cost outlay justify the cost/comfort savings.  That's an analysis that you will have to conduct.