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Duct Insulation

2114 Views 9 Replies 6 Participants Last post by  HomeSealed
First post! Huge fan of the site, and finally decided to register and join in on the fun. I am a first time home-buyer and budding DIY'er in search of direction and advice.

Not sure the cheesy intro was entirely necessary, but it seemed proper as I've been silently stalking the forums for over a year now. :)

Anyway, to the point. I have a 1900's ranch style home. On the large scale, my plan is to make the home more energy efficient. Insulating walls, attic space, etc. I am starting with the unconditioned crawl space.

I believe I have a fairly simple heating system, my trunk line runs the length of the house, with all branch lines running to appropriate room vents. With the exception of my return lines, all the duct work is in the crawl space and I feel I am losing efficiency to leaks, and lack of insulation.

As of right now all branch lines are 5-6" metal (unknown exact). My plan is to seal any leaks, and joints first. I have identified a major leakage issue on many of the lines where they make the connection to the trunk. I have enclosed an image of this. Please reference the shoddy duct tape job someone deemed acceptable.

1. What is the recommended method to sealing these areas and leaks? Should I keep the connections as is, or is there some type of fitting or collar that would be better to connect to the trunk.

I spoke with a gentleman at a local plumbing store. He advised I should replace all the branch lines with pre-insulated flex. I have mixed feelings about this as I know if not installed correctly, the flex ducting can create an air loss due to friction. This and the the system can be more susceptible to damage rodents and animals. Branch lines seem fine, and it seems it would be easier to insulate them rather than replace the lines.

2. What is recommended practice for this? Home depot carries a fiber glass insulation for this, but is only rated r-6. Is there a fiberglass roll in r-8? Where can I find this? Are the foil insulation rolls claiming r-8 worth a damn?

Thanks very much for any ideas/suggestions!


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Someone used duct tape which is wrong, it should have been foil tape instead.
For the rest it may be best to use mastic.

It also looks like someone tryed to do a DIY job on there it connects to the trunk line.
is the branch line hanging on the screws? i.e. all the weight is pulling on the screws
I don't believe it is "hanging" by any means. There are screws randomly placed around the whole circumference of the connection. The screws with the alternating in-out tabs seem to be holding it in place okay. I think the biggest concern is just the seal. This connection is one of the better ones under there. There are more obvious leakage issues, and even worse duct tape patch jobs on a good amount of them.
Ironic the one thing duct tape can't be used for is ducts. I would use the mastic on the larger gaps and then foil tape on all seams even if they don't seem to leaking now. And remove all existing duct tape. I am not an insulation guy and you don't say where you are located, but would the difference between R6 and R8 be that much.
I am in Colorado Springs, CO. South of Denver.

I'm not really sure if I NEED the r-8 over the r-6...will the difference be pretty nominal??

As far as mastic goes, does it go on with a brush?
Brush works good. I have been sealing all my duct work and have an old tooth brush I am using to get at the tight spots. The foil and bubble duct wrap is a good choice for the same reason, you can wiggle it into tight spots. My concern is more for condensation during AC operation than heat loss.
No AC right now, so no worries there. I did find this on Amazon:

Anyone have any experience with such a product?
Looks like a good product and will do what you want but I really don't know if that's a good price or not. Between that and sealing your air leaks you should really notice a difference.
Foil tape or mastic are great. Mastic is probably best since its a crawlspace so you probably don;t care if it looks sloppy.... Then treat the crawlspace as a whole (seal and insulate properly). There is a lot of good info on this over in the Insulation section.
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