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Brian G

Bubble Wrap for Ductwork

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Yesterday a family friend asked me if bubble wrap applied to ductwork could create a condensation problem, and if there were any necessary steps to avoid the issue. Her's was apparently put on right over the old, existing duct insulation.

I can't say I'm up on this one; never seen any. Anybody got the hard intel on this stuff?

Brian G.

Bubble Rap - What Toddler Gangsters Listen to in the Bathtub [:-paperba

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Brian--

Admittedly, I have no "hard intel" on the issue. I've never heard of anyone doing this, after all. But I can't see how there would be a condensation issue if the bubble wrap -- of any type -- is applied over a functional layer of insulation as you indicate.

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I'll have to ask about foil / no foil thing.

It was put on during an overhaul to a new HVAC system, by a local contractor named....Scott Patterson. Seriously. [:-slaphap

Brian G.

Branching Out Scotty? [;)]

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In order for that stuff to work, the manufacturers say that there has to be an air gap between it and the surface that you're trying to insulate or you get conductive/convective heat loss. Reflectix recommends cutting long strips of their material and taping them to the surface about an inch apart and then applying the wrap over that and then taping all seams.

ONE TEAM - ONE FIGHT!!!

Mike

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If there was no problem with condensation with the existing insulation, I can't see how adding anything more would cause condensation.

We all know why condensation happens at the surface of a cold bare pipe. But insulation doesn't change the temperature of the surface of the pipe. In fact, if anything it makes it colder. So what insulation does is keep the air and water vapor away from the surface of the pipe for enough distance that the temperature can rise above the dew point. So if there was no condensation before, there really can't be any condensation if you add more insulation.

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I can't back it up with references, but I did read somewhere that piling more wrapping around any duct increases its heat loss by increasing its surface of circumference. So that it is sort of like (to quote either Guy Clarke or Jerry Jeff) it is like peeing in the wind, or bettin' on a losin' friend.

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I can't back it up with references, but I did read somewhere that piling more wrapping around any duct increases its heat loss by increasing its surface of circumference.

Adding more insulation to the ducts or a pipe or the water heater won't increase the heat loss. Not in any residential setting, although it can increase the heat loss in some situations.

There are two basic types of heat flow that we need to consider. Conductive and convective. (Radiation plays no part in this discussion.) Conductive heat transfer will always be reduced by adding insulation. But convective heat transfer can actually increase, because you're increasing the area available for heat transfer.

Convective heat transfer from a pipe can be greatly affected by the orientation of the pipe (vertical, horizontal). But as a simple approximation the critical radius of insulation (beyond which you're actually hurting rather than helping) is = k/h, where k is the thermal conductivity of the insulation and h is the convective heat transfer coefficient. Notice that as h goes to zero, the critical radius goes to infinity. Meaning that if conductive heat transfer dominates then you can just keep piling on insulation. This is the condition we encounter in a home inspection with ducts, pipes, and water heaters -- particularly if the outer shell of the water heater is at the same temperature as the home's environment (always the case, right?).

To get convective heat transfer you need the surface of the pipe/duct to be dramatically different from the surrounding medium (not likely, especially after you've added the first layer of insulation), or you need a fluid moving over the surface of the duct/pipe (again not likely in a residential setting). Otherwise conductive heat transfer dominates.

Here's a good link:

http://www.cdeep.iitb.ac.in/nptel/Mecha ... 2.6.4.html

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I can't back it up with references, but I did read somewhere that piling more wrapping around any duct increases its heat loss by increasing its surface of circumference. So that it is sort of like (to quote either Guy Clarke or Jerry Jeff) it is like peeing in the wind, or bettin' on a losin' friend.

This has come up a few times as an almost folklore.

In the original discussion from Pete Engel, Mathematician, Real Rocket Scientist, and Boy Genius of New Jersey, it was posited in theoretical conditions, and within some unremembered construct, that there could be, mathematically, greater heat loss from the water heater with thicker insulation. Or, something like that.

It ain't true. Pete isn't pushing the idea; he was just riffing with Newton and thermodynamic principles.

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Originally posted by SNations

If there was no problem with condensation with the existing insulation, I can't see how adding anything more would cause condensation.

My guess is that the "condensation due to bubble wrap" theory is based on the vapor barrier aspect of the bubble wrap, rather than on the insulating value. I've seen the original insulation on that particular ductwork during my carpentry years, and it was some sort of fiberous stuff with a heavy paper wrap. It probably breathed somewhat, though I can't say that type of insulation is common around here, or how anyone else would have known that. Locally, most of it is fiberglass with a foil/paper wrapping.

I thought the idea of an impermeable vapor barrier on ductwork creating condensation was at least passingly plausible, but it might be total B.S. in actual practice.

Brian G.

Knowledge Permeable, To Both the Useful and the Useless [:-boggled

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To me, the zero perm property of bubble wrap is it's dominant characteristic when considering it's appropriateness as duct insulation. It's insulation qualities is mostly lost by the convection currents that can arise within the single cell bubbles. It's a lousy choice. Stick to packaging media.

Marc

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I appreciate the elevation of this discussion from the folklore level where I dragged it to the borderline nerdly.

So the one house I saw with condensate on every single duct in the crawl space that was so voluminous that the foiled wrap literally dripped and sagged from the weight of its water was mostly due to way too much ambient moisture, as I suspected and as I helped explain when I found the condensate drain from the crawl/located fan/coil buried just outside the foundation wall?

Marc, please help me grasp the idea that there could be convective action inside those little bubble wrap cells.

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Bubble wrap isn't insulation, it's a radiant barrier. The published R value is based on testing that bears little resemblance to the recommended uses. The only way to get anything approaching the advertised R is to provide significant air space between the 'insulation' and the object being insulated.

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I appreciate the elevation of this discussion from the folklore level where I dragged it to the borderline nerdly.

So the one house I saw with condensate on every single duct in the crawl space that was so voluminous that the foiled wrap literally dripped and sagged from the weight of its water was mostly due to way too much ambient moisture, as I suspected and as I helped explain when I found the condensate drain from the crawl/located fan/coil buried just outside the foundation wall?

Marc, please help me grasp the idea that there could be convective action inside those little bubble wrap cells.

It's a gas filled plastic cell and wherever you have a temperature differential across them, convection currents develop within it. Think of a room as a cell. If one wall is cold and the opposite wall is hot, convection currents develop. With bubble wrap, it's the same idea, just on a smaller scale.

Marc

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Convective currents don't function on that small a scale, if they did insulated glass packs wouldn't work. In reality the airspace can be as large as 3/4" before there is enough volume to jump start convective currents. There needs to be enough space for the air to move and enough surface area to create an effective heat exchanger, there's neither in those little bubbles.

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