I’m in the process of having a complete hvac system installed in a house being remodeled. A geothermal heat pump was installed in the attic and flex duct was used. I know, two things you shouldn’t do but there was no room in the crawl space and the flex was a compromise I felt I had to make. The heat pump will be in an insulated, weather-stripped room and the ductwork will be covered by a couple feet of cellulose.
This evening, I was moving one of the register boots (for esthetic reasons) and discovered that the inner liner of the flex duct was taped to the register boot with plain old gray duct tape. All the other (visible) connections are taped with foil tape as it should be.
Anticipating my conversation with the contractor, I’d like your expert opinions. Is duct tape acceptable on the inner liner-to-boot connection, where it will be covered with the outer wrap and fastened with nylon ties? Or am I justified in insisting on the foil tape?
I know flex duct is frowned upon by almost everyone here, but I thought I could live with the compromise. Seeing old fashioned duct tape used by a professional contractor is disappointing.
Do I have a leg to stand on?
Replies
well i'm no hvac guy,but on my flex there is no tape,just a big zip tie..................so i'm in worse shape than you.
YOU ONLY NEED TWO TOOLS IN LIFE - WD-40 AND DUCT TAPE. IF IT DOESN'T
MOVE AND SHOULD, USE THE WD-40. IF IT SHOULDN'T MOVE AND DOES, USE THE
DUCT TAPE.
Our HVAC code requires foil tape and a 1/2" wide TyWrap on the inner sleeve and a TyWrap on the outer jacket. Don't forget to seal all metal to metal joints with mastic or mastic tape.
DO NOT, repeat DO NOT cover your duct with insulation! It will condense because you are trapping warm moist air next to the duct and lowering it to the Dew Point. Creates an amazing amount of water. Let's just say I learned the hard way !!
And by the way, Duct tape is acceptable for almost everything but ductwork!
DO NOT, repeat DO NOT cover your duct with insulation! It will condense because you are trapping warm moist air next to the duct and lowering it to the Dew Point. Creates an amazing amount of water. Let's just say I learned the hard way !!
Interesting.
Did this happen in summer or winter? I assume summer. But I'm having a hard time understanding how this would happen. What is the source of the warm, moist air? If it's from the outside air, which in your NC climate would naturally be warm and moist in the summer, wouldn't there always be warm, moist air in any ventilated attic? And would it not still be in contact with the cooler duct?
Can you clarify? Thanks.
Hot humid summer. Hot air has the capacity to hold a much greater volume of water as compared to cooler air. As this air is cooled the moisture condenses out(ice tea glass as an example). When you insulate over the duct work the hot moist air seeps through the insulation, cools and condenses on the duct, then drips onto your nice finished ceiling! Crawl spaces in this area, if not sealed with vapor retarder will have a drip line under each duct run. The cooler air in the crawl space is so close to Dew Point that it nearly rains off the ducts. Huge problem in this area of the country. Before code would allow a sealed crawl space we use to put in fake (non-functioning) foundation vents, vapor retarder up to within 3" of the sill plate and a dehumidifier plumbed to the exterior. Now we do the same thing, we just don't have to put in the fake vents.
I understand cool surfaces condensing moisture from warm, humid air, as in your example of the glass of ice water forming condensation.
What I don't get is your warning about wrapping the duct with insulation as making the problem worse.
Taking your cold glass as an example, would not condensation be reduced if you were to wrap a towel around the glass?
Or, an example from construction practice: roof drains are typically wrapped with FG insulation as a deterrent to condensation forming when cold water, as from melting snow, runs inside the pipe, the outside of which is exposed to the warm, humid air of the building's interior.
Flex duct wrapped with insulation is fine as this is how all ductwork is to be treated. But the missing element in your reply is the vapor retarder. If you were to add additional insulation around your duct with a vapor retarder that would be fine. But what happens is that the hot moist attic air will reach the duct thru the loose -fill insulation and will cool sufficiently to bring it to the dew point, at which point it will condense. Attic air in the 95-100+ degree range can be near total saturation. Because of this, a small change in temp can cause condensation.An exposed duct has enough air movement around it that it will prevent the air around the duct from cooling enough to condense. I hope this helps.Edit to add:If you wrap a towel around a cold adult beverage and it stays cold long enough, the towel will become totally saturated.
Edited 5/8/2009 8:26 pm ET by WHILEWEREATIT
Thanks.
Just one more question: Is this true for juvenile beverages? :)
I don't think it's a real problem with sippy cups<g>.
"Attic air in the 95-100+ degree range can be near total saturation. Because of this, a small change in temp can cause condensation.An exposed duct has enough air movement around it that it will prevent the air around the duct from cooling enough to condense."Assuming the attic space is vented, the absolute humidity of the air inside it can't get higher than that of the outside air, unless there is considerable movement of moisture into the attic from inside the house due to human activity. One might argue that moisture under the roof shingles could be driven into the attic from solar heating, but a well-vented attic space will vent the additional moisture easily.I guess everyone here is on the same page, though, when it comes to duct heat losses in an attic. Leaving off the insulation may avoid condensation, but at the cost of great heat gain into the cool A/C air. The correct thing to do, of course, is to insulate to keep the heat out of the duct, and cover that with a vapor barrier to keep the humidity on the hot side of the insulation.
Besides, leaving off insulation WON'T avoid condensation. There just won't be any insulation to trap it, so it will drip off onto whatever surface there is below.
The modern conservative is engaged in one of man's oldest exercises in moral philosophy; that is, the search for a superior moral justification for selfishness. -John Kenneth Galbraith
I suspect you are correct for at least most of the time, for humid air outside of an uninsulated duct. There are three resistances to heat transfer to the conditioned air inside the duct. There is the thin film of stagnant air next to the metal, the metal duct wall, and finally the air film inside the duct.The metal resistance can be ignored, since the metal is very conductive and thin. The resistance of the inside air film will be lower than that of the outside film, due to the turbulence of the air flow. I would expect the air inside the duct to be cooled to, what? Upper 50s? The attic air [edit: dew point] could reach low to mid 70s worst case in the northeast, or low 80s for southern US. Unless the hot attic air were moving fast around the duct, to keep the metal above the dew point, I imagine some condensation should be expected.I guess it would take a very, very hot attic and only moderate humidity to keep the metal above the dew point.
Edited 5/11/2009 7:39 am ET by DickRussell
Apparently you and dan haven't read the original post. No one has suggested not insulating the duct, this is a given. The Op originally stated that he was going to apply loose fill insulation over the duct work. This is a definite no no. Also as the temperature in the attic is substantially higher than ambient, its RH will be considerably higher.
> Also as the temperature in the attic is substantially higher than ambient, its RH will be considerably higher.
Wrong! Exactly the opposite is true.
The modern conservative is engaged in one of man's oldest exercises in moral philosophy; that is, the search for a superior moral justification for selfishness. -John Kenneth Galbraith
Good catch. In an attempt to keep this simple, I confused myself!
Let's just take this example: With an attic temperature of 100 degree F and 20% RH, lowering the air temp. to 50 degrees would give you 100% RH (total saturation) and a dew point of 50 degrees. This is the point I was trying to make in reference to the OP's original statement. If you cover an a/c duct with loose fill insulation, you will likely lower the air temp around the exterior of the duct insulation vapor retarder to the dew point. Most of todays a/c units evaporator coils operate at 40 degrees or less and most system will operate with a 20 degree temp. differential or more. This post has caused me to use parts of my memory that i haven't visited in some time<G>.
You are assuming a considerable amount of air movement through the loose fill insulation. The OP is going to use cellulose, which if properly installed will reduce the ducts' exposure to the warm, moist air moving through the attic. His specific statement was a couple of feet of cellulose IIRCC.
I could agree with your observations and thinking if the duct were covered with BIBs (f/g) of only an few inches or even cells of an the same shallow depth, but a foot of cellulose would need a pretty strong wind to drive the heat/humidity down to the condensing surface.
I have AHUs sitting in roof top rooms with out insulated supply ducts. We pull in a minimum of 10% outside air at all times, and the return air plenums for the building are the AHU rooms themselves. According to what you are saying my supply air plenums ought to be raining in those rooms. It doesn't happen here and we have a few months of 90+ degree weather with 80 to 90% RH. My chilled water set point is 45 degrees and my discharge air temp is 52 to 54 degrees. How come my uninsulated or minimally insulated supply air plenums don't sweat?
You are in a very different part of the country than many people and while you may be exact for your region and the conditions you describe, be careful with your advice.
Other parts of the country, we rarely see crawl spaces, they must have vapor barrier and 3 1/2" of concrete and insulating the duct works ducky. A lot of people are moving to conditioned crawls, (as Joe L talks about in recent article) but in vented crawls, insul duct can work aok.
It's not the duct insulation thats the problem, it's the additional insulation (loose fill) without vapor retarder that is the problem. See my previous post. It isn't just a regional problem, almost any attic gets hot enough at some time to create this problem. Duct insulation with vapor retarder is a must and as far as know is required by code in any cooling application.Edit to add: The original post was not concerning crawl spaces, I was just using that as an example.
Edited 5/8/2009 9:13 pm ET by WHILEWEREATIT
But if your duct insulation has a vapor barrier like it is supposed to and it is sealed the way it is supposed to ... you wouldn't have condensation. Insulating is right to do ... as long as you do it right. If you don't insulate it, you will 1) still condense moisture out of that air and 2) increase your energy use.
Insulate it, but do it right!!
Please read the original post, my response wasn't about whether or not to insulate the duct.
The number you need to remember and see is: UL181.
UL181 is the standard that was developed in recent years for flex duct products.
It covers the duct itself as well as all the components that are used to connect it and seal it. So, there are a few fabric duct tapes that meet UL181. If they do meet the standard, they will be printed their entire length with the UL181 number. The manufacturer pays a lot to have his product tested to prove its compliance with the std, so he is proud of that. Imitators are not allowed to print their product with the std. even if they claim that it's made just the same.
If your contractor used fabric duct tape that meets the std, then you don't have the backing of the Code to make him change it. And presumably, you don't need to change it since the product was tested in accordance with the requirements of the std.
When the std was first written, no fabric duct tapes met it--only foil tapes and mastics.
IMO, fabric tapes should be avoided in favor or foil tapes or mastic. Mastic is very effective, but messy to use.
Also, in regard to your doubts about using flex duct: It's like many newer products, it's OK if it's sized and installed right. But it seldom is, even though every box of flex duct comes with a page of clear instructions on its proper installation. But you always find that page still in the bottom of the box, or in the trash. Like I always say: If these guys wanted to read for their living, they'd have been lawyers instead.
In process right now of changing some of the ductwork in a customer's house from 6" round metal to 6" round flex.
Purpose is twofold, 1) reduce noise and 2) reduce heat / cooling loss as the ducts travel through a vented (not conditioned) crawl space.
Ranch house, roughly 2/3 of the house has a full basement which has been improved. Remaining 1/3 is over crawlspace w/ concrete floor. Crawlspace "ceiling" is insulated with the insulation which has VB on top (next to floor above) and a vapor permeable paper like covering on the bottom - yes it is installed with the correct side up!
Majority of work is being done in the crawlspace.
HO bought the insulated flex duct from Lowes, unsure of brand. The manufacturer's directions specifically call for taping the joints with duct tape - Inner plastic tube thingy to any metal fittings (also secured with screw type band clamp) and then outer plastic tube to the metal fittings.
We didn't have any cloth duct tape so we used the foil tape.
Will have to check on the box for any specs regarding the duct tape -- but I don't remember any......
Jim
I'm sorry ... that isn't a good thing to do ... replace round rigid w/ flex. Insulate the rigid if that is what you need to do. Flex just reduces your air flow, so you pay a big price for the swap, even though you gain insulation. That is the worst reason I've heard to swap to flex. Sorry to be so blunt. If not you, others need to understand this.
>>Purpose is twofold, 1) reduce noise and 2) reduce heat / cooling loss as the ducts travel through a vented (not conditioned) crawl space.<<
HO idea / experiment and it is performing very well in attaining goal #1. All rigid metal ducting is laying in the crawlspace, ready to reinstall if needed.
Jim Never underestimate the value of a sharp pencil or good light.
Conventional "duct tape" (which really is just mispronounced "duck tape") should only be used for temporary fastening, for a very good reason: The adhesive dries out and loses its adhesion after 2-5 years.
I can't tell from your description whether this may have been a temporary situation or not.
Actually I believe that the true name is duck tape. It was originally made from a duck cloth material.And used for temporary repairs. After called 100 mile an hour tape with used on slow speed airplanes. But that number was used raised when they started using it on higher speed planes.Then after the war people used on HVAC ducts and over time it because duct tape..
William the Geezer, the sequel to Billy the Kid - Shoe
That's what I said. Never was intended for ducts.
The modern conservative is engaged in one of man's oldest exercises in moral philosophy; that is, the search for a superior moral justification for selfishness. -John Kenneth Galbraith
I bought a roll today - on the wrapper, it says...DUCK TAPE! So that's 2 votes for the duck...
There is a brand of duct tape that calls itself "Duck Tape", in a sort of reverse mispronunciation.
The modern conservative is engaged in one of man's oldest exercises in moral philosophy; that is, the search for a superior moral justification for selfishness. -John Kenneth Galbraith
Yes, that's the one. I have been doing a lot of taping lately, dust barriers for a major renovation in my occupied house, & I've become something of a connoisseur. Duck is a good one - nice & sticky, & tears cleanly with the teeth.
"Speed tape" is still used in the military, even on fighters. Don't ask me how I know.<g>
Do you know how they made "speed tape"?Write up a mil-spec.Take duck/duct tape and make it with the mil-spec.Charge 3 times the price.BTW, I did some software for controlling blenders and extruders for plastic molding equipment.Years later I get a call from the company that manufactured the electronics that used the software that a duct tape manufacture was using the system to mix and dispense the adhesive on the duct tape.I don't remember the details, but either one of the bins ran out or one of the augers broke and they ran a couple of miles of tape before anyone noticed error message on the display panel..
William the Geezer, the sequel to Billy the Kid - Shoe
Seen enough dried out duck tape to know it shouldn't be used for anything that involves HVAC. As someone else mentioned if it has UL listing that is a whole different story.
I've used flex-duct with good results. It all depends on the way it's installed. I've seen some pretty unacceptable applications that definitely don't do much for the products reputation.
In my brothers house the contractor snaked the duct through open web trusses and did some pretty acute bends that probably really kills air flow.
Flex will never be as good as rigid duct but with careful attention paid to detail you can greatly improve the product's performance. Use metal fittings at any bends, I screw the zip- ties to the fittings, stretch and pull the inner liner tight between fittings to smooth out the walls as much as possible, drape and tape and zip- tie the outer insulation over the fittings.
I personally would never use cloth duct tape on anything- UL rating or not.
Aluminum foil tape costs more than duct tape, so worst case is your contractor used foil where it is seen, cloth where it is not. Best case is that was the last connection, and they ran out. Easy way to find out is to look.
The Mylar flex tape costs less than aluminum, but that is made to repair the torn outer flex jackets.
Some use zip ties and run a screw through the tie. Nylon becomes brittle with age, especially in an attic. That screw point could be the place where the tie breaks off. If the flex is pulled tight, then it comes off the connection. Some guys make a couple passes with the duct tape, then screw through that... better than screwing through the zip tie, but back to relying on duct tape.
Zip ties or tape can't be made tight enough to stop air leaks over time. Some building specs require a stainless steel large hose clamp (for lack of a better description). The stainless clamp won't become loose, pop off or disintergrate like nylon or duct tape.
If the metal to flex connection is sealed with mastic (brush some on the inside of the inner liner before making the connection), there is little possibility of that connection coming loose if anything happens to the zip tie or duct tape. No air leaks, either.
My opinion is that the old timey grey cloth (i.e. duct) tape is as appropriate as the flex duct. Expedient and inexpensive. Adequate usually, but scorned nonetheless. Any monkey that ever got paid to install anything related to an HVAC system counts as a "professional" contractor, so that means little.
Any monkey that ever got paid to install anything related to an HVAC system counts as a "professional" contractor, so that means little.
Roar