# Thread: Duct wrap stretch out formula doesn't make sense

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## Duct wrap stretch out formula doesn't make sense

I'm preparing to place duct wrap on a 6" round metal duct. Just as much out of curiosity as for practical purposes, I want to know why the stretch out table only considers the duct shape and perimeter plus an extra amount, rather than multiplying by a factor based on the perimeter (for round ducts). My only guess for this mathematical strangeness is that the table is "dumbed down" for contractors by considering an average or max duct size, and that addition is easier compared with multiplication. To illustrate why I'm puzzled, consider a really small diameter duct vs. a large one. The small duct would end up with a big amount of insulation overlap and the large duct would have a little overlap. Doesn't make any sense. More info here:

http://www.naima.org/insulation-reso....php?fileid=30

2. ok so lets make it really simple take your tape measure and wrap it around the duct then take that measurement and i always add four in. now make sure that you cut that four inches off but leave the foil paper intact so that when you wrap it around the duct and pull it tight the four in. over laps and then you staple the seam make sure that you have the correct stapler its the kind that the staple spreads outward instead of inward than you take string tape and the little plastic squegie and tape and seal the seal for a good air tight seal and thats the prper way to insulate round duct. this aint rocket science its hvac work hope this helps

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martin&sons that makes sense, thanks. What is strange is that the "official" instructions would have me waste 13" (17" recommended according to their table minus your 4" estimate), about a foot per cut section of waste with lots of extra overlap. Lots of wasted material per official instructions.

4. no problem the way i said will save you time and material good luck and have fun lol

5. Originally Posted by ScorpionLeather
I'm preparing to place duct wrap on a 6" round metal duct. Just as much out of curiosity as for practical purposes, I want to know why the stretch out table only considers the duct shape and perimeter plus an extra amount, rather than multiplying by a factor based on the perimeter (for round ducts). My only guess for this mathematical strangeness is that the table is "dumbed down" for contractors by considering an average or max duct size, and that addition is easier compared with multiplication. To illustrate why I'm puzzled, consider a really small diameter duct vs. a large one. The small duct would end up with a big amount of insulation overlap and the large duct would have a little overlap. Doesn't make any sense. More info here:

http://www.naima.org/insulation-reso....php?fileid=30
all you have to do is multiply the diameter times pie, 3.14 and add a couple inches for overlap, but cut the insulation off the backing that couple extra inches for taping purposes. For example: a 4 ft. section of 7 in. hard pipe, 7 x 3.14 = 22 so cut a piece of insulation, which is usually 4 ft. in width 24 inches.

6. Don't pull the insulation tight, it reduces the R-value.

Leave it nice and fluffy. Much of the insulating value comes from all of the air trapped in the insulation.

I went to a duct fair years ago. Certainteed was there. They said 2" duct wrap that was installed at 2 1/4 inch thickness was R-6. Compressed to 1/2" (Nice and tight) it was only about R-2.

That is what some of that add on is for, to keep the insulation fluffy so that it really insulates well.

7. why is there always someone who has to nuke things just do it the way i was saying except mr. oniel is correct you dont want it to be pulled to tight but you also dont want it loose the thicker the insulation the more the r value so you dont want to reduce the thickness but you want it snug because if it has any air in between the insulation and the duct condensation will form and then you know what the r value of wet insulation is ZERO!!! SO ITS BETTER TO HAVE IT A LITTLE TIGHT ER THAN LOOSER

8. Its just duct wrap dude

9. i know its only duct wrap but with my company there are no exceptions for crappy work i mean what are you going to say next its only pulling a vacumn or its only sub cooling and superheat i mean thats were it all starts its always the minor stuff and then they just dont care period if my employees ever said that to me "its just duct wrap dude" i would fire thier a\$\$ on the spot. now true you dont have to make this stuff into rocket science but i only ecept the best and thats the way my company does it PERIOD!!!

10. you know what first of all I was just telling him the way that I do it. I have been visiting this forum for about 5 days now and it just seems like a place for hvac guys to give other hvac guys a bunch of mouth behind a computer, reminds me of my girlfriend and her facebook junk. I'm just an apprentice and hoping I could learn somethings here. Not into dogging other people's work just worried about my own. But go ahead and wrap your duct like Martin says just better hope you make enough \$ to invest in some tape measures if you gonna keep wrapping them around pipe.

11. The thicker the duct insulation the longer the "ADD" has to be for the overlap to keep the stated R-Value.

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Originally Posted by martin&sonsa/c
if my employees ever said that to me "its just duct wrap dude" i would fire thier a\$\$ on the spot.
I agree, when a contractor comes in my house and says something like this, I instantly decide I'm going to keep a close eye on them, plus they will never get my repeat business.

The intent of the manufacturers calculation table seems to be precisely what the pros here are pointing out - to make it fluffy to keep the R value. But too bad the table math doesn't make any sense (because the extra length is the same regardless of circumference), so we go back to rule of thumb.

13. hey miller wasnt trying to bust your chops to bad im also new to this site and all i hear is a bunch of know it alls that dont take pride in there work or that try to make it into a science or something you will understand once you get further into this field and one thing that will take you real far is pride in every detail of your work. set yourself apart from the rest and i gurantee you will succeed where others fail and one thing i see alot of is it seems like alot of contractors have forgotten what it is really all about and that the customers because with out them we are nothing i guess its easy to get blinded by money but for me its about doing an honest good quality job its about making people happy with thier hvac system or atleast it is to me. but you are right its a bunch of drama on here lol. i would love to talk to all these people in person bet there would be alot less running at the mouth

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