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View Full Version : Duct wrap stretch out formula doesn't make sense

ScorpionLeather
12-30-2010, 05:42 PM
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-resources/visit.php?fileid=30

martin&sonsa/c
12-30-2010, 06:15 PM
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

ScorpionLeather
12-30-2010, 06:23 PM
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.

martin&sonsa/c
12-30-2010, 06:32 PM
no problem the way i said will save you time and material good luck and have fun lol

millertime77
12-30-2010, 06:46 PM
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-resources/visit.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.

Kevin O'Neill
12-30-2010, 06:49 PM
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.

martin&sonsa/c
12-30-2010, 07:25 PM
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

millertime77
12-30-2010, 07:32 PM
Its just duct wrap dude

martin&sonsa/c
12-30-2010, 07:49 PM
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!!!

millertime77
12-30-2010, 07:57 PM
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.

energy star
12-30-2010, 08:05 PM
The thicker the duct insulation the longer the "ADD" has to be for the overlap to keep the stated R-Value.

ScorpionLeather
12-30-2010, 08:07 PM
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.

martin&sonsa/c
12-30-2010, 08:34 PM
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

millertime77
12-30-2010, 09:17 PM
Man I have re-cut so much duct wrap trying to cut by multiplying by pie, you really do have to add like 4 or 5 inches to that and I used to try and cut just right to make it nice and tight and thought I was doing something good, but it does decrease the r-value. Ounce my boss told me that I looked at the way the insulation looked on the pipe and it made perfect since. Definitely not trying to make any enemies here, it takes a bigger person to come back and say what you said and I respect that. I see the techs on here that are obviously really smart, but sometimes don't use much common courtesy, which is pretty elementary. No hard feelings. This site is really a cool place to share work experiences and to learn if you aren't so bull headed like we all tend to be sometimes, me included.

ScorpionLeather
12-30-2010, 09:44 PM
Turns out that with 3" thick insulation, 4" is not enough, needs more like around 10" of extra. The circumference of the duct is just the inner portion but the outer insulation foil travels a longer distance.

Someday there should be a table that shows the extra needed for round ducts, based on thickness of insulation and the diameter that results in the ideal compression for the R value: just enough to make it tight but not too tight. Such a table would be easy to publish for the manufacturers but I think they just want people to waste material in order to sell more of it.

Certainly one would get the hang of this in the field and be able to measure it right with eyes closed, but I'm sure there are a lot of apprentices squeezing it too tight using the rule of thumb measures.

energy star
12-30-2010, 09:52 PM
Turns out that with 3" thick insulation, 4" is not enough, needs more like around 10" of extra. The circumference of the duct is just the inner portion but the outer insulation foil travels a longer distance.

Why not tell us what diameter duct you are insulating and the thickness of the insulation you want to use.

Are you talking about the same 6" diameter duct with three inch thick insulation wrapped around it?

6 x 3.14 = 18.84 That would be the perimeter of the duct. Now if you use 3'' insulation, you would average 2.25 thickness of that insulation around the duct to achieve the stated R-Value. So if you were to measure around that pipe 2.25" out (bigger) all the way around you would need to add 17" to the piece of insulation to allow it to cover the pipe and allow for the paper over lap.

millertime77
12-30-2010, 09:57 PM
Why not tell us what diameter duct you are insulating and the thickness of the insulation you want to use.

Are you talking about the same 6" diameter duct with three inch thick insulation wrapped around it?

A pretty much fool proof way is to lay your roll of wrap on the floor and roll some out then go ahead and tape the end of the wrap to the pipe and roll it til it goes all the way around and then make your cut.

millertime77
12-30-2010, 09:59 PM
but dont tape real close to the ends of the pipe so you can peel some back to fit it together with other pipe.

martin&sonsa/c
12-30-2010, 10:04 PM
6 in. round duct should be about 23 in and that will give you a 3 inch overlap

martin&sonsa/c
12-30-2010, 10:06 PM
hey miller i think he is trying to wrap duct that is already installed because your absolutly right the best way is to lay the duct down and roll it up and then add a couple inches for overlap

millertime77
12-30-2010, 10:13 PM
thats always fun, bassackwards

martin&sonsa/c
12-30-2010, 10:23 PM
are you kidding duct wrap is just a blast either way NOT!!! the day someone comes out with insulating material that is fiberglass free they will have a customer for life with me lol.

millertime77
12-30-2010, 10:31 PM
are you kidding duct wrap is just a blast either way NOT!!! the day someone comes out with insulating material that is fiberglass free they will have a customer for life with me lol.

You know I have rolled some insulation in a friends attic for him,don't know what it was but white and fluffy, and it didnt bother me at all. Why the same stuff isnt used for duct is beyond me, yeah it will eat you up 140 degrees in an attic and then the roofers want to use 4 inch nails

martin&sonsa/c
12-30-2010, 10:36 PM
AMEN TO THAT!!! someday someone will come out with duct wrap and duct board that is fiberglass free and make a million. i know ill buy it

Southern Mech
12-30-2010, 10:38 PM
have we figured out how to wrap a piece of duct yet?

ScorpionLeather
12-30-2010, 11:02 PM
Are you talking about the same 6" diameter duct with three inch thick insulation wrapped around it?

6 x 3.14 = 18.84 That would be the perimeter of the duct. Now if you use 3'' insulation, you would average 2.25 thickness of that insulation around the duct to achieve the stated R-Value. So if you were to measure around that pipe 2.25" out (bigger) all the way around you would need to add 17" to the piece of insulation to allow it to cover the pipe and allow for the paper over lap.

Ok the simple generic formula I came up with is: pi x (D + recommended width for R-value) + paper overlap

So in my case: 3.14 x (6 + 2.25) + 2 = approx 28 inches including paper skirt

28" - duct radius of 19" = adding 9 inches (not 17)

17 appears to be a rule of thumb number the manufacturers threw out there because they're assuming the biggest duct diameter possible. This ends up wasting material for smaller ducts if everything is pre-cut before going into the attic before realizing the cuts are too big.

Hope someone finds the formula useful (p.s. let us know if you spot errors in it!)

Southern Mech
12-30-2010, 11:08 PM
rocket science and duct insulation doesn't mix, design yes insulating no.

Whats wrong with taking said pipe, wrap duct around snuggly and properly mark where the meet, add 4'' and cut, multiply this by how many of same diam ducts, redo for next size.

ScorpionLeather
12-30-2010, 11:19 PM
Whats wrong with taking said pipe, wrap duct around snuggly and properly mark where the meet, add 4'' and cut, multiply this by how many of same diam ducts, redo for next size.

Installed too snuggly, the R value will rapidly drop as the little air pockets decrease inside the fiberglass (air is the insulator, not the fibers). The manufacturer publishes a recommended compressed width which makes it snug without dropping the R value too much.

With your method it probably comes close to the manufacturer's recommended width but probably only experienced installers will know what is "just right" for the snugness. Since I'm a newbie, I need something simpler like the rocket science to tell me how much (I want it tight but don't want to over-do the tightness).

p.s. I'm talking about the foil-backed insulation like Microlite XG, not the plain fiber wrap that is just spun around the pipe in a spiral.

Southern Mech
12-30-2010, 11:32 PM
Installed too snuggly, the R value will rapidly drop as the little air pockets decrease inside the fiberglass (air is the insulator, not the fibers). The manufacturer publishes a recommended compressed width which makes it snug without dropping the R value too much.

With your method it probably comes close to the manufacturer's recommended width but probably only experienced installers will know what is "just right" for the snugness. Since I'm a newbie, I need something simpler like the rocket science to tell me how much.

as long as it doesnt conmpress you don't drop R-value, as sson as it compresses a 1/4 '' r-value drops, as in any insulation situation. if it's 2'' insulation the cross section better be 2'' when done.

it doesn't get any more simpler than what I described. But if you insist

energy star
12-31-2010, 12:05 AM
2.25 is added.two times. Not once

ScorpionLeather
12-31-2010, 12:52 AM
2.25 is added.two times. Not once

Good catch! Now, it lines up:

For a 6" duct with 3" deep insulation

pi x (D + (2 x recommended width for R value)) + paper overlap

3.14 x (6 + (2 x 2.25)) + 2 = approx 35 inches including paper skirt

energy star
12-31-2010, 12:59 AM
Yeah I can't get it to work out exact either, but we sure did try.

seatonheating
12-31-2010, 01:03 AM
Are you guys really cutting away insulation and overlapping the flap??

Why not just make sure it overlaps by an inch or two and then tape it up?? The good Nashua tape I use sticks to anything. I've gone to jobs I've done 8 years ago and the stuff is still holding strong without any problems.

Staples? That's for ductboard junk.

hvacrmedic
12-31-2010, 01:33 AM
First I'll give you the correct formula.

The perimeter of round duct is given by pi times the duct diameter. In the chart they're calling the perimeter of the duct "P", and

P = pi * duct diameter.

The outer diameter of the installed insulation will be 2 times the installed insulation thickness plus the duct diameter. Here's a little ASCII graphical illustration of that

(-(------)-)

The length of the outer perimeter (pi*outer diameter), which is formed by the insulation backing, will be equal to the length of the insulation backing when the insulation is stretched out.

So the streched length of the insulation required to wrap perfectly around the duct will be given by

pi*(diam + 2W), where W is the installed thickness of the insulation.

Assuming a 3 inch stapling flap, the total cut length needs to be

pi*(diam + 2W) + 3, which BTW rearranges algebrically to

P + pi*2*W + 3

Now that we have the correct formula let's generate some entries for our table.

For round duct, with the "installed" insulation thicknesses listed on the left, we get a table like this (replacing 2*W in the formula with the actual values):

installed thickness-------------------------------streched length
3/4 ------------------ P + pi*1.5 + 3 =________ P + 8
1 1/8 --------------- -P + pi*2.25 + 3 =_______ P + 10
1 1/2 ---------------- P + pi*3 + 3 = _________ P + 12
1 7/8 ---------------- P + pi*3.75 + 3 =_______ P + 15
2 1/4 ---------------- P + pi*4.5 + 3 =_______ P + 17
2 5/8 --------------- P + pi*5.25 + 3 =_______ P + 19
3 -------------------- P + pi*6 + 3 = _________ P + 22

The numbers on the far right are rounded off to the nearest whole number.

hvacrmedic
12-31-2010, 02:38 AM
Good catch! Now, it lines up:

For a 6" duct with 3" deep insulation

pi x (D + (2 x recommended width for R value)) + paper overlap

3.14 x (6 + (2 x 2.25)) + 2 = approx 35 inches including paper skirt

Or, if you want an easy to remember formula that you can use in the field, you can use this:

P + 5(nominal insulation thickness) + overlap.

For different nominal (uninstalled) thicknesses and a 2" overlap this gives

1" -- P + 5 + 2 = P + 7
1 1/2" -- P + 7.5 + 2 = P + 9.5
2" -- P + 10 + 2 = P + 12
2 1/2" -- P + 12.5 + 2 = P + 14.5
3" -- P + 15 + 2 = P + 17
3 1/2" -- P + 17.5 + 2 = P + 19.5
4" -- P + 20 + 2 = P + 22

Given the exact correlation of the results of this and the table in your link, this is apparently the actual formula that they used to make that table.

ScorpionLeather
12-31-2010, 02:47 AM
Or, if you want an easy to remember formula that you can use in the field, you can use this:

P + 5(nominal insulation thickness) + overlap.

Excellent - this seems easy enough to remember. One of those formulas all HVAC pros might want to memorize.. then just pre-cut the foil-backed insulation rather than dragging it up to the attic in a big roll.

beenthere
12-31-2010, 05:51 AM
i would love to talk to all these people in person bet there would be alot less running at the mouth

There will probably be another HVAC TALK convention this year. Attend, and you can meet and talk with some of the members in person.

energy star
12-31-2010, 07:01 AM
Or, if you want an easy to remember formula that you can use in the field, you can use this:

P + 5(nominal insulation thickness) + overlap.

For different nominal (uninstalled) thicknesses and a 2" overlap this gives

1" -- P + 5 + 2 = P + 7
1 1/2" -- P + 7.5 + 2 = P + 9.5
2" -- P + 10 + 2 = P + 12
2 1/2" -- P + 12.5 + 2 = P + 14.5
3" -- P + 15 + 2 = P + 17
3 1/2" -- P + 17.5 + 2 = P + 19.5
4" -- P + 20 + 2 = P + 22

Given the exact correlation of the results of this and the table in your link, this is apparently the actual formula that they used to make that table.

Nope, mine is correct.

Gunnery Sergeant Hartman
12-31-2010, 07:20 AM
Some folks put way too much thought into everything...

There is this thread, another thread where the HO is measuring power consumption in 12 hour intervals, ect...

9hats1van
12-31-2010, 09:40 AM
Anyone ever use Reflectix? It costs more but looks awesome and no insulation fibers to make you itch! R- values are pretty well maintained if installed properly. Because this insulating material is so thin it's easy to work with especially in tight spaces and where the ductwork is already installed. I use regular insulation when the ducts haven't been installed yet and can roll the fittings on the insulation. This makes quick work in wrapping any fitting.

martin&sonsa/c
12-31-2010, 01:57 PM
hey beenthere when is the hvac talk convention and where if its not to far than i look foward to it.

beenthere
12-31-2010, 02:01 PM
I think indy, in sept.

martin&sonsa/c
12-31-2010, 02:09 PM
do they ever have one in tx. ok. la. indy is is a bit of a run.

beenthere
12-31-2010, 02:16 PM
Its in a different location every year.

martin&sonsa/c
12-31-2010, 02:21 PM
ok thanks for the info

smstooge
12-31-2010, 06:21 PM
duct wrap that is fiberglass free is already out and has been out for years