HVAC Ducts Reinsulating vs. Replacing

December 13, 2016

duct_sealing1If you have read any of my last articles on HVAC ducts you recognize that, like the HVAC units, the duct network is a system as well. In the past I was an advocate of reinsulating the existing ducts over replacing them because of the cost. However, I am changing my tune as I continue to see more and more duct systems that were either so poorly installed from the get go or they have been patched, damaged or added to (incorrectly)  they weren’t worth saving.

On my most recent project, I found the return air plenum was sucking return air right off the dirt floor in the crawl space. Some people might say “it just smells like an old house”; trust me, even an old house shouldn’t smell musty if the system is in good condition.


For the most part, properly installed ridged system can be superior, but if not, they stink. One of the biggest problems with ridged duct systems are: They are ridged! Just like the biggest problem with flexible ducts are; they are flexible!  As with any house construction there are lots of obstructions that have to be dealt with. The flexible ducts are great for these applications as they can be easily routed around structural beams. However, all turns and bends should be gradual and smooth. No kinks or sharp turns.

One of the most disappointing things I see in residential construction is what I call “the lack of trade respect”. Think of it like fighting siblings, the older brother is always going to pick on the younger brother, just cause he can.  Time and time again, I find shoddy work that is caused by a sibling trade.  In a  recent remodel project, we found that the HVAC contractor purposely removed several supporting beams because: they were in his way. In turn, it was left that way and didn’t show up until the roof was replaced some 40 years later. The roofers couldn’t re-deck the roof because of 6 split rafters, in other words, the roof was caving in. Admittedly, the workers are long gone but the negative effect of their work lives own.  The rafters had to be rebuilt and purlins had to be replaced. Though our investigation, it was obvious that the HVAC tech removed them so he could have a straight path for his ridged duct.  Negatively impact the integrity of the house was trumped by his laziness or willingness to cut corners. As with most attic or crawl space work, there is no one checking their work.


New Duct system installed using flexible ducting

Its not uncommon to find a 50 year old house that has had new HVAC units installed multiple times but the ducting is still 50 years old. Granted, when an HVAC unit fails to work, the problem is the units not the ducting. As the homeowner it is easy to just have the units replaced and leave the ducting as is since the new ducting can be as expensive as the unit itself. Unfortunately you can easily be giving up some of the  efficiency gained by replacing the units. To learn more about “Improving the Efficiency of Your Duct System“, I have attached a link to a paper written for the US Department of Energy by Brookhaven National Laboratory. After reading the article and you realize it is time to replace the ducting, look to schedule the work in off-peak months. You will probably get a little better deal and a lot better job.

Re Insulating HVAC Ducts

December 1, 2009

The US EPA reports leaking ducts reduce overall HVAC efficiency by 20% accounting for a loss of over $140 per year due to these leaks.

 As mentioned in earlier articles, HVAC ductwork has not received the attention it deserves. Take a look at  my HVAC Duct article to evaluate and inspect your existing duct work. If you have determined that it is in need of attention you have three choices, 1) do nothing, 2) have them replaced or 3) re-insulate them.

  Duct Replacement: Be prepared, HVAC contractors typically want to sell new systems.  Here is the SW part of the US, their big season is the summer, having this done during the cooler months may be a better choice. Duct replacement is not as profitable as system replacements, but keeping their technicians busy can be better than not working them at all.

Considerations when discussing this issue with a HVAC professional:

  1. Flex ducting is a widely used proven product but does not have the long-term performance rating found with a typical sheet metal product. But is widely used in the residential market and most professionals like to use it because it is easy to install and not as labor intensive as rigid duct work. This product  can easily be mis-installed by creating kinks and  sharp bends that can reduce the product performance. Read this flexible duct inspection method before you meet with the contractor to better understand the product and how it is applied.
  2. After an evaluation, most HVAC specialists will want to replace the ducting in lieu of  repairing or re-insulating. Why? It’s quicker, cheaper and it becomes a known value. This is not a bad thing, these folks are trying to make a living and attacking the problem by replacing everything may be an economical solution for both of you.
  3. If you allow a professional to replace the ducts, make sure to ask some of these questions. Will all the joints and edges be sealed with (paint on) mastic? What is the R value of the new duct? What is the life expectancy of the duct product? What is the product warranty? What preventative measures are followed to avoid kinking (of the ducts). Will the system be tested for leaks after completion?
  4. If the professional didnt mention it,  also consider having all the output and return registers (this is the box in the wall/ceiling where the vent cover is attached) replaced or re-insulated. Most new registers are sealed and insulated to insure a tight fit. Also, ask them to use spray foam insulation between the register protrusion and the sheet rock, this will further seal the interior  from the attic space. Most HVAC professionals do not address register penetration into the interior space.
  5. If you have any rooms that never seem to adequately heat or cool, make sure and mention this to the professional too as he may have to resize or reroute the new duct work to better balance the system.

Duct Re-insulation (for rigid ductwork): This is a great DIY project if you consider yourself cheap labor and you dont mind working in the attic. Consider this a mult-weekend project. Before you start, ask yourself these questions. 

  1. Does your system work reasonably well? If you have any rooms or areas in the house that never properly heat or cool, consider adding an additional output duct as this would be the time to address it. Consider using a professional to perform this work as you may need to rebuild some of the system to retain the system balance.
  2. Is your attic adequately insulated? If not, plan to do it, but after all you other attic work is complete.

 The process if fairly straight forward and here is an outline of the necessary work.

  1. Strip the existing insulation material
  2. Ensure all joints are secure and snapped and screwed together
  3. Use duct mastic to seal ALL seams and joints
  4. Use HVAC tape to complete any seals not treatable with mastic
  5. Use expanding foam insulation to fill any cracks between the duct registers and the sheet rock
  6. Use duct wrap to re-cover all the exposed duct, seal with duct tape and mastic

Material Required for the Job

  1. Duct wrap rated at an R value based on your region (see table below)
  2. 1/4,1/2, and 3/4 self tapping sheet metal screws
  3. HVAC Duct Mastic
  4.  Take adequate precautions while working in the attic. Avoid stepping directly on the sheet rock ceiling and wear protective clothing and dust masks.
  5. If a professional indicates the duct are under/oversized, you may ask them to provide  the Man L or Man J duct analysis supporting their position. (This is an engineering schedule that is used to properly size ducting.)