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Do Cardboard Ducts Belong in a LEED Certified Home?

Hvac Duct Return Cardboard Unsealed Air Flow Leakage Hole

hvac duct return cardboard unsealed air flow leakage holeRecently I came across a LEED certified house that had cardboard return ducts. They were all inside the building envelope, but it seemed clear to me that they could lead to problems. Even if the home builder or HVAC contractor comes back and seals all the big holes, these things will leak. A lot.

Recently I came across a LEED certified house that had cardboard return ducts. They were all inside the building envelope, but it seemed clear to me that they could lead to problems. Even if the home builder or HVAC contractor comes back and seals all the big holes, these things will leak. A lot.

In talking with the HERS rater who does the inspections and testing on these houses, I found out that he can never do a total duct leakage test because he can’t pressurize the system. Basically, they’re turning most of their return duct system into panned joist return ducts, which I’ve suggested before is possibly the worst duct idea ever.

In this house, the cardboard panned joist return ducts are all inside the building envelope, but that doesn’t mean there won’t be any problems. Here’s my thinking:

  • This house is in a cold climate.
  • These cardboard ducts are in the basement with two stories above.
  • The stack effect (i.e., warm air rising) is stronger in cold climates and taller houses.
  • The stack effect creates negative pressures at the bottom of the house and positive at the top.
  • Return duct leakage in the basement creates more negative pressure at the bottom of the house.
  • Pressure imbalances can cause more air leakage and lead to air distribution problems, affecting comfort, efficiency, healthfulness, and durability.

hvac duct return cardboard unsealed air flow

The argument that the rater hears from the builder and HVAC contractor there is that the ducts are inside the building envelope so it doesn’t matter, and it costs too much to use real ducts. I’ve heard from people in other places that cardboard ducts used to be popular but are starting to go away. Except not in this location.

What do you think? Is this acceptable for a LEED certified home? Is it acceptable for any home? Is it OK if the house has really low air leakage?

This Post Has 18 Comments

  1. Nice metal trunk lines &amp
    Nice metal trunk lines & air intake/exhaust. What did they save? … a few hundred bucks? And now the distribution system is subject to the various interzonal pressures … penny wise / pound foolish (IMHO).

  2. It looks like part of the
    It looks like part of the ducts are already metal, would it really cost THAT much more to finish the job with metal? I just love the big holes around the joist braces in the 2nd photo…

  3. This is the problem with
    This is the problem with programs that assume ducts inside conditioned space do not need to be tested, or to meet any type of standards. Does teh cardboard meet the required R-value for the area. Depending on he local code this may be R-6? I always recommend that ALL ducts systems be testeed, even thos ein conditioned spaces, and would not personally qualify either (a) cardboard ducts, (b) panneed returns, (c) building cavities as ducts, or (d) leaky/untested ducts.  
    I assume this home might have combustion appliances as well, and bsements are subject to infiltrating soil gses, along with whatever is stored in basements. Do you want these blowing throughout the home and into your baby’s face?  
    I think not. 
    A rater, especially LEED, should advise against such practices, and either disqualify, or walk away from a job like this.  
    It’s a question of ballance. To earn a few dollars, or possibly asist to mak people sick. Hummm I suggest run, don’t walk!

  4. Yet another reason why LEED
    Yet another reason why LEED should be the benchmark for residential green building by which all other guidelines are to be measured. 
    OK – that was a joke!  
    On a serious note, if the house has a bike rack in the front yard, a show rack by the entry door, and a grill/mat inset into the deck or porch floor at the front door, who cares if the ducts are cardboard and leak? I mean, come on, they got all 8 of their LEED points for using materials made withinthe 500 mile radius so does it really matter? 
    The money they spent for the LEED certification would have been better spent on an HVAC contractor who cared about installing the best system!

  5. Maybe the support beam will
    Maybe the support beam will act like a chilled beam and soak up radiated heat and transfer it to the return air – yeah that will offset the lack of insulation attached to the cardboard.

  6. I work in SE PA and NJ. In
    I work in SE PA and NJ. In NJ you must have real ducting and seal with mastic. I find few problems with testing ductwork in NJ (except design and sizing in non program houses). In PA panned joists and wall cavities are still legal and poorly sealed. Even with PA’s revising state code (1/1/10) to include 2009 IEEC with ducting standards, little has changed. As reported in a recent Home Energy article: local building inspection departments reponse to the new code fall into three catagories. Some require third party inspections, some do plan review but not the field inspections required to verify the Perscriptive Path, but a large number just totally ignore the new code requirements. 
    At present the PA state code committee is holding meetings to accept comments about the possibility of not including much of the increased energy efficiency components of the proposed 2012 code. There is even a push by some builder groups to roll back some of the sections of the current 2009 code because “it costs too much to meet these requirements, we cannot sell a home with these extra costs” 
    I really want to see the required testing and other components of the 2012 code put in place. It serves the people the code was designed to serve – the home buyers.  
    Allison, the Georgia requirement for third party duct testing is yet another thing to be added to the list of things I miss about living in the Atlanta metro area, where BBQ is a noun not a verb.  

  7. RE: Code Enforcement (or Non-
    RE: Code Enforcement (or Non-) 
    The builder, subs, and Rater (even for LEED) are required to meet, or god-forbid, exceed code minimums. This requirement is not subject to local code officials lack of enforcement. 
    Just because no “authority” inspects, or issues a CO in spite of the building clearly not meeting code, does not absolve the rater or verification consultant from their responsibility. 
    PA, and many other areas across the country, has never enforced their codes. I first conducted inspections in the early 80’s and found almost zero compliance, even were local municipalities required inspections. It is the rater’s professional responsibility to convince the owner that quality and building science matter, and to stand by their guns to fail bad, and especially dangerous, practices. 
    Just my opinion.

  8. This is the first time I have
    This is the first time I have ever heard of cardboard ducts. My first impression is strongly negative, but that is the opposite of an open minded, informed opinion. You could do a great service to us readers by telling us how they perform. 
    Do they leak? I think the article mentioned that they do. Do they HAVE TO leak because of the material used, or is this an issue of quality, of craftsmanship? After all metal ducts have the same problem. 
    What longevity can be relied upon with this material? Is this material treated so it is in any way moisture proof, and does this really matter like I assume it does? 
    Apparently these ducts are uninsulated, is that going to lead to any bad consequences being in conditioned space as it is? Can someone quantify the temperature loss over a long duct run and and discuss those consequences? 
    I have seen people become stubborn and unthinking about the superiority of certain materials, e.g. “metal ducts are best and flex is garbage”. Some of those are based on not thinking fully about the consequences of each choice, e.g. chronically designing flex duct systems too small and then complaining about the results. 
    It is easy to ask questions and difficult to answer many of them. You will notice I harp on “what are the consequences of doing X vs. Y?”. I find that if you cannot discuss the answer, you may be right but have not made your case. Thank you.

  9. I don’t understand how this
    I don’t understand how this is a problem with LEED for Homes. This home should never have been certified. Its more a problem of a dishonest/ignorant rater who let this through.

  10. Indeed. Our program
    Indeed. Our program (Northwest ENERGY STAR) specifically disallows the use of building cavities as ducts in our v3 HVAC checklists and while we do not require testing for duct systems inside conditioned space, we do require all seams and connections to be sealed with mastic. The national program’s v3 checklists don’t specifically disallow joist panning, but do stipulate that the leakage to exterior test target must be met, which would disqualify this system since the test can’t be completed. I would also suggest that the material being used is not the problem so much as its poor installation.

  11. This is trash. Good call on
    This is trash. Good call on the potential impact on stack pressure. Also, it will likely introduce construction dust into the return system. However, I beg to differ with a couple of comments arguing that interior return ducts should be insulated.

  12. Sorry to be coming in so late
    Sorry to be coming in so late to the discussion, and I don’t have time to address each of you, so let me answer the main questions above. 
    Geoff H.: I’m not sure about this location, but most codes allow you to use building cavities for return ducts and don’t require insulation on ducts inside the envelope, although it’s a good idea to insulate supply ducts to prevent condensation during the cooling season. I’m with you, though, and think we should do better, especially in a LEED-certified home. The furnace was sealed combustion, and the water heater was solar/electric. 
    Jeremy T.: LEED for Homes doesn’t have anything in their requirements that forbids the use of cardboard for panning joists for return air. They allow building cavities for returns (I believe) and don’t require a total leakage test. Also, I can attest that the rater on this project is definitely not dishonest or ignorant. 
    Thomas A.: Disallowing building cavities to be used as ducts is definitely the way to go. Yes, raters can enforce higher standards than programs require, but they stand to lose builders to raters who are seen as being easier. I don’t completely agree that this is only an installation problem, though. Since the basement will be unfinished, a homeowner or contractor coming in later might have no idea why that cardboard is there and decide to remove it because it’s in the way of something. They might think it’s just left over from the construction process and the builder forgot to remove it, whereas at least with sheet metal, I think they’d assume it was there for a reason. 
    David B.: Yep! I thought you might weigh in on this one. I wrote about the insulation in this comment before I saw your comment about it.

  13. throw on a louvered door and,
    throw on a louvered door and, BAM! heated basement….

  14. I will admit – I am the first
    I will admit – I am the first person to not get too excited about sealing off ductwork that is within the thermal envelope. But this is particularly disgusting. 
    At least if they were tight fitting, but with no mastic I could be OK with it. 
    And, yes, I am OK with building cavities as ducts, as long as they are within the thermal envelope and the combustion appliances are sealed or power vented combustion. We see that a lot in our area and they have very low (or no) leakage to the outside. 
    But these are a particularly bad example.

  15. This is the typical “You
    This is the typical “You got to be kidding me” stuff we run into all the time (of course this is a bit extreme). Unfortunately, I think this may also speak to that raters ability to explain to the builder why this is a bad idea. Educating builders is just as or even more important than educating homeowners. Either that or the builder is just your typical hard headed builder thats going to do it his way regardless. Either way, it’s not good. I am shocked at the number of homes I have been in that have one form of certification or another and in my opinion, should have never received certification in the first place.

  16. The LEED for Homes Rating
    The LEED for Homes Rating System originally prohibited the use of framing cavities as ducts. That was changed a few years ago, and we don’t understand why. 
    Although we don’t like to see the practice in new construction where it often causes problems in the duct leakage test, panned ducts and the use of framing cavities have been a disaster in rehabs. The framing in older buildings often has many bypasses which allow air to flow just about anywhere it pleases. 
    And even if all the ducts are inside the building envelope, a system with panned-in areas, like shown in the LEED project above, will not pass the total leakage test required under E-S Version 3.

  17. What is a “LEED”
    What is a “LEED” certified home? And what does it mean to use the term “using the building cavities as ducts”.We were considering using the cardboard ductwork (the home is in no.west Michigan..a basement over a manufactured home. But, now wonder about that! Thanks

  18. Sandy:
    Sandy: LEED stands for Leadership in Energy and Environmental Design. It’s a green building program created and administered by the US Green Building Council (USGBC)
    Using building cavities as ducts means using the spaces between floor joists or between wall studs to move air. Building codes don’t allow you to do that for supply air (the air coming out of the HVAC system after being heated or cooled) but some do allow it for return air.  
    You’re better off not using cardboard ducts for the reasons listed above. If you need help with the design of your HVAC system, just let us know

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