Condensation from ductwork


  #1  
Old 08-20-02, 09:25 AM
Dave1363
Visiting Guest
Posts: n/a
Question Condensation from ductwork

I own a 2nd fl. condo. Apparently there are water stains showing up on the ceiling. It appears that on really hot humid days the A/C ductwork in the condo attic is sweating and dripping into the ceiling. Is there anyway to wrap the ductwork to keep it from sweating? If so what is the best material? The ductwork is round if that has any bearing.
 
  #2  
Old 08-20-02, 05:17 PM
hvac4u's Avatar
Member
Join Date: Aug 2001
Location: NW atlanta
Posts: 3,145
Received 0 Votes on 0 Posts
foil faced

duct wrap is what you are looking for. it comes in a roll 4 foot by about 75 or 100, generally priced 65 to 80 bucks. special staple guns are used that spread the ends of the staple to hold it together. tape is then applied. if the duct is bare metal, which is what i am assuming, apply mastic duct sealer to the joints. if this duct is already insulsted and still sweating, you may have a humidity issue in the attic. also, the larger of the 2 copper lines going to the outdoor unit must be throughly insulated, or it will sweat and leave marks on the sheetrock as well.
 
  #3  
Old 08-20-02, 05:20 PM
bigjohn
Visiting Guest
Posts: n/a
The existing ducting is probably insulated and the insulation is no longer effective. It's probably saturated with water and has to be removed and replaced with new. This is not an easy job. What follows is some posts from another forum by myself and others in response to a question by a homeowner about insulating metal duct. I copied & pasted it so it looks a little disjointed. It was too much to retype. For round duct, you can skip the pins. This type of job isn't beyond a handy DIY, but would surely want to be defered to cooler weather-



I don't know if HD or Lowes sells what you need. Perhaps someone else does. You want to buy DUCT WRAP with an FSK backing at least 2" thick, not just any old insulation. [actually, for a hot attic you might want to go with 2/1/2"] You also want tape and mastic that is UL181 listed. Don't use that cheap a$$ grey duct tape. You will probably have to go an HVAC supply house to get what you need. I don't know where you are in FLA, but there is a Johnstone Supply in most areas. There are several keys to a good job:

1. Before you reinsulate, ensure that there are no leaks in the duct itself. Paint all the seams of the ductwork with mastic. Caulk them if need be.

2. If your duct is rectangular/square you want to install pins on the bottom of the duct first. On a duct that isn't real wide, one pin at the middle every foot or so is plenty. The pins have a 2" square base. One type has an adhesive backing and another has a perforated base thru which you drive self tapping screws to secure the pins to the duct. I prefer the screw on kind. Paint the pin base with mastic. When you put on the insulation, have the seam on top. Push the insulation on the bottom so that the pins penetrate and then push on the pin covers. The idea behind the pins is that they will support the weight of the insulation on the bottom of the duct. When you push on the pin covers, dont "crush" the insulation down. The more you compress the insulation, the more you decrease its insulating value. The insulation manufacturers figure about a 25% installation compression into their R-value tables. When you bring the seams together, fold them together and down and then staple them to the insulation and then tape. You know how you fold aluminum foil over the top of a Thanksgiving turkey? Kinda like that. At the edges, overlap the insulation, fold together, staple together, and tape. As you encounter hanging supports, takeoffs, etc, you'll need to cut and fit and be sure to overlap, staple and tape.

3. All together? Now, staple 2" wide mesh over every seam. Mastic all the seams. [don't be bashful with the mastic. It takes a lot to fill in the mesh. Don't forget to mastic over the pin covers.

The staples want to be the OUTWARD CLINCH type. You'll also have to buy a staple gun. The tape is pressure sensitive. You press a little hard plastic squegee to press the tape down after you put in place. If you have any flex duct runouts, replace them with new flex. Be sure the boots are insulated and the registers are secure. Sometimes on the boots, I'll build a form around them with thick cardboard and encapsulate them in expanding foam and then paint the hardened foam with mastic. Depends on how hard they are to insulate. In the literature section of http://www.certainteed.com/ you'll find a couple specification sheets on ductwrap. When you're doing the install, keep thinking that the idea is to prevent the water vapor in the air surrounding the ducting from coming in contact with the cold duct. Leave a little opening and the sweating will ruin the job over time. Even a well done job has a life cycle. This will be a sweaty, ictchy, dirty job which why you'll have a hard time finding someone to do it. I don't believe it's beyond the realm of an thinking DIY.







I hope I haven't given the impression that i like doing this type of work because I don't. I've just learned the hard way that you can't leave any spots for moisture to sneak its way in. I think that FSK is an acronym for Foil Scrim Kraft. It refers to the type of backing. If you look this stuff up in a catalog, you'll see that you can get it with FSK or Vinyl backing. I'm not 100% sure, but I believe that that FSK is less permeable to the tranmission of moisture than the Vinyl. It's kind of a moot point because I've only ever seen the FSK stuff at the supply houses. The spec sheet at http://www.certainteed.com is interesting. It contains a graph that shows what thickness to use versus RH and ambient temperature. I've found that using quality products is the real secret to this type of job. That, and the mesh over the seams that you apply the mastic to. I didn't think up the mesh idea myself, someone showed it to me. Also, I don't remember what it's called, but there is a type of pressure sensitive tape that has strings of fiberglass imbedded in it in kinda like a diamond pattern. I have to use scissors to cut it. It's much better than the regular foil tape and sticks to anything.



You are right, the FSK is Foil-Skrim-Kraft. It's the materials that are used to make the facing you recommend. The tape you discuss is FSK tape -- same stuff that the facing is, only in tape form.

Reason for the mastic system for seal:
Creates a more complete, better vapor seal than FSK tape because it conforms to irregular surfaces much easier. One key point is to make sure that the mastic used to seal the insulation is a VAPOR BARRIER mastic. Not all mastics are created equal. Usually, to get a true vapor barrier mastic you have to find an industrial/mechanical insulation supplier. If vapor barrier mastic is not used, you run the risk of moisture entering the insulation envelope if the temperature difference and humidity gets high.

The mesh with the mastic is an insulation industry standard practice that has been used umpteen years in mastic systems. It acts like reinforcing mesh in concrete -- strenghtens the system and keeps it from tearing easily.

One way to eliminate some labor from the job, is that when you apply the pins, buy glue that is made for the pins or use silicone caulk/adhesive. Spread this on the base plate of the pins. Glue the pins to the duct, then you won't have to go back to seal the holes from the screws. This is every bit as durable as screws in all those pins.

Vapor permeance on vinyl vs. FSK is the same. Vinyl is not as strong. Vinyl faced duct wrap costs more than FSK anyway and as stated availability is not great for vinyl.



Great
explanation. The only thing different that I do, is I use an iron on the tape. It really makes it grip well. And don't ask me if the setting is permanent press or linen. I think someone makes an iron for that, but I think it was origninally meant for the ductboard tape. I don't use ductboard.
__________________
 
  #4  
Old 08-20-02, 05:31 PM
hvac4u's Avatar
Member
Join Date: Aug 2001
Location: NW atlanta
Posts: 3,145
Received 0 Votes on 0 Posts
bigjohn

is right, although if the insulation is soaked and needs to be replaced, you must find out whty, or it may happen again
 
  #5  
Old 08-21-02, 06:17 AM
Dave1363
Visiting Guest
Posts: n/a
There is no insulation on the ductwork. Since the ductwork is in the attic of the the condo, should I assume that the condo association should be footing the bill. Does common code require that these ducts be insulated?

Thanks for the info.
 
  #6  
Old 08-21-02, 08:00 AM
MZW
Visiting Guest
Posts: n/a
type of duct work

I also have an issue with dripping ducts although mine are fully insulated.

I live in Houston and have had 4 different companines recommend replacing my existing "hard pipe" ducts with flex duct since flex duct is plastic lined and will not sweat. However, I had another company say to absolutely not use flex duct since and that it will decrease my airflow by 40%. In addition any dust or mold that may exist in my system tends to build up faster on flex duct and that flex duct cannot be cleaned.

I also am confused on what to do.
 
  #7  
Old 08-21-02, 09:14 AM
bigjohn
Visiting Guest
Posts: n/a
To MZW- Well sir, I'll solve the problem for you right now. The guy that's telling you to NOT USE flex duct is telling you the truth. The resistance to air flow with flex duct is much higher than round pipe. A round pipe duct system is about the best you can have. [assuming it's designed right] It sounds like you need to have the ductwork reinsulated. Now, Houston is hot and muggy and you need a lot of thickness in the duct wrap and a vapor proof job. When the hot weather is over, the contractors will be looking for work and that's the time to have your ducts redone or you can tackle the job yourself.

To Dave 1363- I find it hard to believe that someone would install ducts in an attic on a cooling system and not insulate them. Did your condo orirginally have cooling or was it added later? You would need to check with your local building dept. to find out what the code is in your area. I'm not a lawyer but, as for the bill to fix the problem- I imagine that if you have a central system where the condo association provides heating/cooling with the assoc. fee, then they would pick up the tab. If you have individual heat/cool systems for each condo, then I'm inclined to think it's on you.
 
  #8  
Old 08-22-02, 06:20 AM
Dave1363
Visiting Guest
Posts: n/a
The condo's A/C is orginal with the building. I guess I will have to foot the bill, even though I agree that they should have insulated any ductwork in the attic.
 
  #9  
Old 08-22-02, 08:46 PM
MZW
Visiting Guest
Posts: n/a
thanks!

Thanks for the info on the duct work. I would do it myself if I knew how. Although it probably isnt' that hard to do, I suspect it takes a lot of skill to do "right". Not sure how I would make sure that the insulation was air tight even though your instructions above were very good. Sounds like it takes a little (or a lot) of experience to do it right.

I saw a posted message somewhere else recommending Central City Air. I called them but unfortunately they are very expensive. On a different subject, they do not install your typical a/c system. They custom make each system for each home (i.e. they might use a lennox coil with amana heat strips and put them into a generic "box"). Very interesting.
 

Last edited by MZW; 08-22-02 at 09:50 PM.
 

Thread Tools
Search this Thread
 
Ask a Question
Question Title:
Description:
Your question will be posted in: