Three of the vents in my family room are connected to the main HVAC trunk via 25-foot runs of round, uninsulated 7-inch sheet metal duct pipes. These ducts run along the ceiling of my basement, which is part living space, part storage space, though I rarely visit the basement. In winter, these ducts radiate a lot of heat into the unused basement. I’d like to minimize this heat loss and direct the heat to where it’s really needed, upstairs. Between the two levels is hardwood flooring and some carpeting. I reside in the Mid-Atlantic. My furnace is based on natural gas.
I would like to wrap the ducts with insulation. My choice is Reflectix R-21 attic wall unfaced foil/bubble wrap insulation (sold in long sheets at Lowes) or traditional R-13 fiberglass batting insulation. The fiberglass insulation would be tightly wrapped around the round ducts and taped into place, in 25-inch-wide sections. The foil insulation is sold in one big roll, meaning I would cut 25-foot lengths and wrap that around the length of the duct, without joints.
I vaguely recall that foil insulation should NOT come into contact with HVAC ducts. Instead, there should be a narrow air space between the two, maybe 0.5 inches. Is that correct? How can one maintain the needed gap along a 25-run of round pipe? Using furring strips? Reflective foil insulation was hugely popular 10+ years ago based on some eye-rolling claims. The R-21 vs R-13 properties of the two insulations make the foil seem attractive, but I doubt it really has 50% higher insulative properties vs. the much thicker fiberglass batting.
Conventional wisdom says: Leave basement HVAC ducts UNinsulated if there is living space above them.
I understand the heat in the basement warms my floors above it. But a LOT of that basement heat is wasted heating up boxes in storage. I want to use that heat, not lose it. Finally this: I will probably be moving within the next 3-4 years. I’m not sure I will recoup the cost of insulating these ducts.
The foil acts as a radiant barrier. It works by reflecting heat away as well as not radiating heat. If you want to keep the heat in the vents, the foil will actually work for that. It’s not obvious, but shiny things aren’t great at radiating heat away. You can demonstrate this yourself something hot like a baked potato. If you put your hand near a hot potato you can feel the heat radiating from the potato into your hand. But if you wrap the potato in foil, you can’t feel that heat as easily. The shiny aluminum doesn’t radiate heat away very well. So if you want to keep the heat in your ducts, you can actually do that by wrapping it in the foil insulation. The benefit is not so much from the insulating properties of the foil. It’s from the shiny aluminum that won’t radiate heat away.
Actually, you could try an experiment with the ducts and some aluminum foil. Assuming you can feel the heat from the duct when your hand is near (but not touching), cover part of the duct with foil. Put your hand near the uncovered and covered parts of the duct. You should feel a difference in heat. If you have an infrared thermometer, it should register a temperature difference between the two sections.
But one thing about foil is that it’s a great conductor of heat. If you touch the foil, it will be hot. The air molecules which touch the foil will get hot and rise (just like they do with the plain duct). If the hot air can easily rise up and spread out, the hot ducts will end up creating a convection cycle where hot air is rising, spreading out, and pulling cold air up to the ducts. If the ducts are recessed into rafters, then that can prevent the cycle. The hot air rises into the semi-enclosed space of the rafters and gets stuck there. But if these are ducts that are against a flat ceiling, fiberglass would probably work better. It would stay cooler and not heat up the surrounding air as much so a convection cycle wouldn’t form.
Part of the insulating qualities of the Reflectix arise when you stretch it along the bottom edge of the floor joist and the joist cavity becomes insulating space. It does work well. When I did the ceiling of my crawl space I did it in two stages and as you walked across the hardwood floors above it you could easily tell where the Reflectix ended. I suppose a hot duct might compromise the Reflectix.
The Reflectix would a lot easier to put on. I have wrapped ducts in fiberglass and it is a pain in the ass.
Here is a diagram showing the recommended installation for ducts. They use a narrow strip of the Reflectix to space the product off the surface of the duct. The air space is required to get the full insulation value of R-6.
Looking at their website, yes their crawl space product is explicitly intended for sealing off the air gaps between floor joists on a crawl space. As the floor heats the air under it, ambient breezes and winds can carry that warm air away. Anything you can do to keep that warm air from escaping will reduce heat loss from the floor to the crawl space. Fiberglass insulation isn’t really required here, since the sealed-off space won’t develop a buoyancy-driven circulating convection cell: the air at the top gets warm, and then it stays there. So yes, it seems entirely plausible to me that Reflectix could achieve an an R-21 insulating effect simply by establishing and protecting a quiescent air gap 12" tall between the subfloor and the ambient winds, even with just a thin layer of foil-coated bubble-cell material.
The picture is less clear for wrapping heating ducts with the “attic wall” product specified by the OP. In fact, the claimed R-value ranges from 3 to 21 depending on the installation, likely because there are many situations where using the product won’t prevent buoyancy-driven convection. Wrapping a heating duct strikes me as one such situation. If the desire is to prevent wasting heat on an unoccupied basement, then ISTM thick fiberglass insulation is going to far outperform a thin layer of bubble wrap. Note that the fiberglass will also virtually eliminate radiant losses just by virtue of keeping the outer surface of the insulation very close to ambient temperature (by preventing conduction).
That flexible insulated duct is not meant for heating runs. Thats for bathroom fans. Its an actual duct, and the duct part is a tough but not heat rated plastic film.
There are similar products meant specifically for heat runs that are fiberglass insulation with foil backing. It comes both in rolls for wrapping around existing ducts and tubes to use when installing the ducts. You want the rolls.
Well I’m not going to look up the building code. But in Alberta you cannot use insulated flexible duct for heat runs. Like I say, there is specific product, both sleeves and wrap for insulating metal duct.
Maybe it is different in your area, but I doubt it.
two ways to lose heat - radiation and convection. (and a third, actual air leaks).
Fix the air leaks, as mentioned, with foil tape on all joints where you can. (I’ve used that flexible grey “duct tape” once upon a time, the handyman’s fix-all. Over time it dries and peels on heating ducts.)
Is the heat from radiation, or from the air around the hot metal pipes geting heated? Do you feel a radiant heat from the pipe, or is it simply the air is generally hot? Is the basement floor warm, or is it cold near the floor and nice and hot closer to the ceiling? (Or do you have air circulation happening in the basement to stir it up?)
My guess it is, the major heat loss is convection. For that, air gap insulation is needed - the fiberglas with a foil outside coat, wrapped around the pipe. AFAIK that should stop radiation too.
My suggestion was to use the insulated flex duct over the existing hard pipe – 8" dia flex over the OP’s 7" dia rigid – not to replace the hard pipe with insulated flex duct.
Cheap, easy, and effective way to insulate the existing run, once it’s definitely sealed properly.
Yeah I got that. But there is a specific product without the extraneous flexible duct part made for the purpose of insulating metal duct. It will literally be on the next shelf over and will be cheaper. This thing won’t even fit over a 6" duct without pulling out the flexible duct part.
There’s one more aspect of my recommendation (the 8" insulated, flexible duct over your hard pipe) that may or may not work well for your application.
It just occurred to me.
Mine was a crawl space. The length of duct that I wanted to insulate had a ‘free end’ where I could access the duct to slide the flex over it. If your run doesn’t have an accessible end, then it may not be the best option for you anyway.
Without an accessible end, you’d have to break the run somewhere – pretty simple to do if the joints aren’t well sealed; less simple (and, arguably, less worth it) if the joints are well sealed.
With an accessible end, either the product I describe, or duct sleeve would work.
Without an accessible end, then – yeah: I’d go with a duct wrap type of product.
And – yeah: foil tape. And don’t be stingy with it. A few wraps around at every seam.
[7" duct sleeve looks like it may be an oddball size and offer limited options. Going to 8" would be my choice in that case]
I just watched this video. IMHO, this is a really good tutorial that involves both scenarios that I described:
you have access to a ‘free’ and and
you do not have access to a free end
They use duct sleeve and duct wrap, and show you how to install both.
They also show slowing down a furnace fan speed. This has some risk as a DIY project, because it’s important to keep the temperature rise within the manufacturer’s specified range. Altering the fan speed will quite likely affect this temp rise.