Hello all,

I am a BPI BA, HERS rater and QC inspector. I cant find a ton of information on this topic and I wanted to see what others had to say. Assuming all other qualifying testing parameters are met; If there is a drop ceiling in a commercial office, and it is assembled correctly with no gaps or missing tiles and the drop ceiling is insulated with batts and the U factor of the roof assembly is greater than the U factor of the drop ceiling assembly, can duct testing be done? I have heard from other sources, that I'm unsure of, that say duct testing is not feasible if the ducts are above a drop ceiling. Also, what are your opinions on whether or not the space between a drop ceiling and the roof is conditioned or unconditioned?

Thanks for your input


Tags: Commercial, duct, testing

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You can test anything. The question you seem to be asking is 'should I test the ducts under a drop ceiling?' Well, is it conditioned space or not? Does it matter? What do you want to find out? Is the delivered air sufficient? Test the registers. Is there enough flow through the air handler? Test the air handler. What is the external static pressure? What does the filter look like?

What do the ducts look like? Are they rigid metal or insulated flex? What would you do to test the ducts, use a duct blaster? Or a smoke generator?

And what do you hope to learn? Are you interested in leakage? Flow rate? Delivery efficiency?

So many questions. Yes, you can test the ducts. Why do you want to?


The IECC code states that the area between a drop ceiling and roof is considered as unconditioned.

I don't know why it would be unfeasible to test the ducts, and I'm not sure of the parameters of why they would be tested. Is someone saying that they need to be tested, and if so, under what guidelines?

Is this for a LEED or ES project?

I test Commercial just like home - I turn on the blower and turn on AC with out compresses running in winter but above 60' turn on compresses to get a wet coil.   I test presser and size on pipe/trunk that is with out bends or holes.   Above "t"bar is unconditioned  and does not add R.  unless there is an air path stopper.   Just did a church with T bar with 2'x2' with 40 tons of AC.  We wrapped above 2X10 - above the duct work and the duct work.   dropped  the load 20 tons  

I am assuming you are talking about doing a duct blaster test on this system.  Of the commercial buildings I've looked at over the past few years I would not do any pressure testing unless there was a rebate or commissioning requirement for it.  Yes the ducts are in unconditioned space, but if the drop ceiling is just acoustical tiles, or there are a fair number of troffer style fluorescent light fixtures, the ceiling is not likely a great pressure boundary - especially at test pressure of 25 or 50 pascals.  I would not expect a leakage to outside duct test to tell you anything useful.  A total leakage test may be feasible if all ducts are in unconditioned space, but you have to watch out for any outside air system, either a return side fresh air duct or just a damper or economizer if the HVAC is a roof top unit.

I run into this all the time.  You CAN test those ducts.  If you want a good quality installation - you should test those ducts and they should meet the leakage specified..  A typical installation leaks 30% of the air thru the ducts.  If the installer knows it won't be tested I would expect at least that much leakage.  Why is it important?  Someone has paid to condition and move the air in those ducts and now they are throwing away a significant portion of what they have paid for.  And they will continue to lose that year after year for the life of the building.

Commercial duct leak tests requirements are a joke.  SMACNA test standard is to test the trunk ducts BEFORE the taps are installed.  The taps (spin in or dovetail) are major sources of leaks.  Why would you test what typically doesn't leak and then create all of the leaks?  If you want to use the SMACNA duct leak test, install ALL of the ducts including the flex duct to the diffusers and test at the lowest pressure rating (probably 1/2 IWC for the flex runout).  I will even drop the test pressure to 1/4 IWC or even 1/10 IWC (25 Pa).  Most of the time it will still fail. 

I strongly recommend the duct leak test be done BEFORE the insulation is installed.  That way the leaks can be found and fixed.  After the insulation is installed the leaks will never be fixed unless the insulation is removed. I say that based on experience from many, many projects.  Insulation will never fix duct leaks - only hide them so they can't be fixed.

Now back to what I think you were really asking. Per the 2012 IECC all ducts *or building cavities used for ducts" should be sealed to duct leak standards and tested to duct leak standards.  If they are using the above ceiling space for a return plenum then the entire plenum space is part of the return duct and should be tested as a duct.  How do you do that?  The same way you test other return ducts.  Mask the return grills and the return at the air handler.  Hook up your test fan and test it.  If it is especially hot or cold outdoors I put a Hobo logger in the return near the fan inlet so I can estimate the percentage of leakage from the outdoors.

Move info available by googling "Commercial Duct Leakage".  Here is one example - https://www.energycodes.gov/sites/default/files/documents/cn_commer...

I will get off of my soap box now...

Most sealing done on duct work is the insulation.   That's why the vapor barrier  is the air seal in most new duct work.   Most new work gets awarded on low price,  no standard - no test out - no low energy goals - no watts per CFM - no BTU's SF   unless you plan for low energy use.    Most HVAC supply houses do not have much duct seal  in stock -  a case or two  some times some non UL listed 181 tape.  

I do see some codes now are  saying to ducted return and sealed supply.  BUT local code will let builder pass sealing.  

"Most sealing done on ductwork is the insulation"?  How do you figure that?  Do you have any data to support that idea?

When the insulation gets larger on supply when the blower is on that's duct leakage.    If the insulation vapor barrier is draw in on return with blower on that's duct leakage.   

I agree with you so far.  But that just means that the vapor barrier is less leaky than the duct.  (If the vapor barrier was more leaky than the duct, the vapor barrier wouldn't move.) 

But my experience and test data says that the vapor barrier is never sealed as tight as the duct is required to be sealed.  I have never seen a single case where the duct leak test failed and the contractor made it pass by sealing the vapor barrier.  I have seen it tried repeatedly.  But I have never seen it work.  Do you have a case where the duct leak test went from fail to pass by sealing the vapor barrier?


I don't know where you're located, but I've attached an excerpt from the CA Energy Code about non-residential duct testing.  At least it should give you an idea some of the conditions/exceptions in California.


Steve Mann


If above the ceiling tiles the wall is a finished assembly and there is insulation on or under the roof assembly I would call it conditioned space. Which are has a greater r value or lower u value would not sway my opinion one way or another.

I have tested drop ceiling systems in commercial buildings. Not sure what is meant by not feasible. It is not easy however especially if the system has returns in every room. 


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