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fritzycat1 09-27-2009 05:03 PM

AC Flex Duct - Suspend above attic insulation or lay it on top?
Could someone tell me why my flexible attic ducts are suspended off the floor instead of resting on top of the blown-in insulation?

I have been recently reducing leaks in my AC ductwork by re-taping the flex duct connections and/or applying mastic. Most of the flex duct is suspended above the attic 'floor' at a height of about 3 feet. The ducts are suspended using metal straps every 5-7 feet - making for some unusual kinks and bends.

I was wondering if it might be more efficient for me to remove the metal straps and lay the ductwork on the attic floor - on top of the blown-in insulation. What would be the disadvantage of doing so?

Here are the advantages I see:
  1. I could route the duct work more efficiently - fewer kinks and bends.
  2. The bottom of the flex duct would be sitting on the blown-in insulation of the attic - perhaps covering about 25% of the total surface area of the ducts. This could improve the insulative properties of the ducts.
  3. I can step over the duct work when moving about the attic - makes it easier to do other things up there.
  4. The air near the floor is slightly cooler than it is 3 feet up - also improving efficiency?
Can anyone tell me why I shouldn't remove the straps and move the ductwork to the top of the insulation?


tinmanrob 09-27-2009 07:18 PM

I don't work with flex ducts in attics, but, everything you say makes sense to me.
I'd go for it.

sktn77a 09-27-2009 10:18 PM

I think it's to prevent condensation from the ducts getting to the insulation. I don't like it - unless you have top quality flexduct (read "expensive") you get kinks, no matter how close the straps are. In LA, you shouldn't have too much problems with condensation on the ducts, so I would go for it too.

beenthere 09-28-2009 04:36 AM

The straps should be 4 to 6" wide to prevent those kinks.

Laying on the insulation. Can cause the bottom of the flex duct to sweat during cooling mode.

Doesn't happen often, but it does happen.

walt1122 02-09-2010 09:03 PM

I don't know if this is wrong or not but... I put the flex ducts (R-8) on the insulation and then covered them with more insulation. R-40 below across the whole ceiling and R-30 above covering the ducts. Works fine for me haven't noticed and problems with condensate and heat loss not a problem.


fritzycat1 02-09-2010 10:23 PM

Thanks Walt.

I haven't re-routed my ducts yet. Been caught up in a few other projects. I've also been considering replacing the ductwork entirely since my flex duct is only R-4.7.

It is good to hear that it is working out for you.

Are you using Fiberglass batt insulation - with R-40 below, and some additional batts of R-30 on top of the ducts?

walt1122 02-10-2010 09:23 AM

Hi fritzycat1, yes we used batts both above and below. Cellulose insulation or foam were not an option at the modular home facility when we built in 2000. Had some exposure to the cellulose on this home a 3 car garage with an apartment above we are building here in Tennessee. I love the blown-in but unfortunately they wanted way, way too much to put in between the first story ceiling and the second story floor so we had to use batts. but here too I put up R-11 (unfaced) first against the floor and then ran the ducts (R-6 HVAC guy put thm up instead of R-8 and I didn't catch it in time, nobody here believes in insulation caue "it doesn't get that cold down here" but they are wrong it is 33F outside and snowing Plus it gets hot here in summer so insulation is even more important to keep the heat out) Then followed that up with R-19 before putting up the rock. If I had the money I would have used blown-in and filled every cavity with insulation.. But they wanted more than 5 times the price of batts the way I described to put up blown-in. Can't imagine what they would want for foam. Guess the technology is new? so they price it high cause they don't want to do it. But they did a great job on the walls with the blow-in. so don't know what the issue is. The netting to hold in the cellulose is the only added expense and some stapling

hope all this helps.


Big N8 02-10-2010 01:08 PM

I have an idea. This coming from a guy that has never used flex duct and in fact has never run duct work through the attic ever ( the Minnesota cold would eat the efficiency up for dinner). But one thing I have done is run a lot of fresh air exchange runs across the basement ceiling. What I have done in the past to make the pipe follow the joist is tape it to a length of pvc pipe then tie the pipe to the joist. That way you tape around both the 5" plus pipe and a 1/2 to 3/4 in pvc. Just a thought.

jimchi 08-17-2011 08:17 PM

This topic has been dormant for awhile, but I need some advice on the same question.

Because of a combination of UV ray damage, age (22 years), and rats (I live in a rural area, so rats are inherent) all of my flexible ducting needs to be replaced in a fairly large home. One HVAC contractor says it should be suspended above the insulation as it won't be as susceptible to rats and with almost no loss of effectiveness, while another contractor says just to lay it on top of the existing fiberglass insulation (with no additional added over the top) as that will provide some insulation from heat and cold in the attic.

The "suspending" contractor says my attics are large enough that there will be no tight bends- just wide arcs. He also wants about 10% more for the job due to the additional labor of suspending.

Due to the size of the attics and the current economy, I'm not in a place where I can afford to have additional insulation added.

Which one would serve me best under these cirumstances?

Thanks for any advice.

sktn77a 08-17-2011 08:44 PM

Suspend it. but make sure it's good quality flexduct, pulled good and tight (so it doesn't sag and minimizes resistance to airflow) and suspended with 2-3" nylon tape every 3 feet.

How did it get "UV damage"?

biggles 08-17-2011 08:53 PM

the least rises bend and kinks the truer the air delievery will be down to the space...including thereturn duct if it need that leak fixing pays off...did you check the bottom of the supply duct where it connects to the air handler..if it to is on the attic beams usually duct taped and dries out...:wink:...might consider a secondary pan under that furnace if you don't have one...and pipe it outside seperate from the units condensate piping:whistling2:

jimchi 08-17-2011 09:08 PM

Thanks guys. As this is about a $5000 job, I'm open to any more recommendations. I want it to be right. I have zero knowledge about HVAC so I need as much advice as I can get to decide between two estimates I've received. There's $500 difference between the two quotes (suspending it being the higher). Maximum height in my attics is about 4', narrowing down to less than 2'. I's a 4/12 pitch with most of the vents on the perimeters of the attics and the returns in the highest parts.

Master of Cold 08-17-2011 09:14 PM

Your local codes may mandate that the ducts be suspended. Condensation being the main issue. Flex duct is the only thing around here. I don't like the suspension method because the weight of the ductwork causes compression of the metal coil inside the duct, reducing the diameter. On the other hand, the transition from laying on the rafters and the angle of going up,over and down into the boot (box that holds the duct and the ceiling register) can add a lot of resistance to the air flow.

jimchi 08-17-2011 10:19 PM

Local codes allow for it either way. Only thing local code requires is a building permit and a pressure test after the job is completed.

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