To replace old leaky ducts with new flex ducts or to seal the old with mastic?

A number of home energy pros I'm in discussion with don't think there is much use in keeping old ducts and prefer to replace them with new insulated flex ducts.

Can anyone give me a more definitive economic argument, findings or a study? Are there any studies on the effectiveness on one way or the other. I'd love to see which method is more effective at controlling air and making money crawling under houses.


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Is the existing ductwork properly size based on Manual J/Manual D? Also, where is the location of the ductwork, the duct material (sheet metal, fiberglass ductboard, etc.) and the R-value of the existing duct insulation? I attached an article by Iain Walker that may be helpful.

If it's UNDER the house I'd keep the metal stuff. Critters LOVE flex !!

In a humid climate in a crawlspace if there is a tear in the outer plastic then water condenses in the duct  insulation so then it might be better to replace with new flex duct. Is there an easy way to reinsulate existing duct and make the outer plastic totally air sealed to keep out humidity?

I am going to read the LBL report soon, but my sense for this is that you should keep the existing ductwork (assuming it is sheet metal) rather than replace it with insulated flexduct. If the ducts are accessible (and I do understand that this is somewhat subjective), then seal them up like a champ with mastic and\or butyl foil tape. After this has been completed, try FSK-faced duct wrap with an R-value of R-6. Tape all seams and joints in the duct wrap insulation, and you should have a happy customer.

This is what I have been doing for older homes with forced air systems, and I almost always get the comment that "I was never really comfortable on the 2nd floor, but now I am!" 


I think it must be a case by case evaluation. In most cases I think you will find it cheaper and more effective to simply replace the existing duct work..

As far as the economic argument is concerned simply get it bid both ways replace or repair. Explain to the bidding contractors that you will test and achieve x results or they will not get paid. being here in California I would suggest the title 24 6 percent on new duct systems as a minimum.

If you are doing it yourself it will take a few jobs before you can bid it really well. 

As far as making money under crawlspace, that is tough work environment.The improvements in crawlspaces can have a huge positive impact on indoor air quality and durability of the structure. There is most defiantly a sense of accomplishment when improving those spaces. You improve an area that few will ever see and for the most part only the weatherization and building performance geeks will really appreciate it.  The work is tedious and uncomfortable and therefore at the end of the day expensive.

Amen to that Glen.

I am one of those geeks that relishes in the chance to get dirty in a crawl space. While you're in those crawl spaces I recommend beefing up the insulation in the floor in addition to sealing and insulating the ducts or replacing the ducts (should that be the better route).

If any of you guys (in the northeast) ever have a problem finding capable folks to handle a crawl space then I may be able to help.


How do you keep the brown recluse and black widow spiders from biting you in the crawl space?

Take a big shot of "hope for the best" and go to it. 

I also set up a series of shop lights before I really go at it. I take a quick look into the crawl in order to determine whether this day may be my last or not. 

Nevertheless, it really just comes down to getting in there and keeping your mind on the prize. The more you let your mind wander into the realm of this spider, that insect, etc. the more you will not be able to deal.

Just my humble take on this.


I am creeped out when crawling around-I keep something in hand to knock down the webs, etc. But the fear of those two is largely overblown. For example, BRS does not use a web, per se; they build more of a cocoon to sleep in & lay eggs(pictured on the site). And they sleep in it during the day.

The BWS is very easy to identify, so I am not so concerned about that one-the Recluse to me, looks like a "normal" spider.

Both critters are rarely deadly. My guess is that I-95 is more dangerous on any given day.



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