I am just wondering if a lot of people are feeling the way I do on the subject of testing Duct Leakage To Outside on duct systems that are fully within the envelope.
And the way I feel is - it's completely pointless, any reading you get is just an inaccuracy of the testing method, not the actual leakage to outside, which is 0.
For some reason some major programs (I don't want to point fingers, but those of you who are involved in it will understand) require testers to do this test if the house is above 3 ACH50.
I am not an energy rater ... I am an Architect...
but I can think of situations where leaky ductwork could cause problems...
Even if all ductwork is within the Air Control layer...
It depends on where the enclosure leaks and where the ducts leak.
Oh, certainly, John. What I am bringing up here is not the fact that duct leaks don't matter withing the envelope, they do.
I am just pointing out some unreasonable requirements of some programs to do redundant testing. The test that needs to be done in this situation is Total Leakage test.
In general there are two types of Duct Leakage tests - Total and To Outside. To outside is not applicable in this situation.
Like I say I am not a rater
thanks for the explanation
Total Duct Leakage is important. Leakage to outside is important. They do have a relationship and testing the relationship is correct is part of the complete process. These tests will give you some results so you can compare the 'test out' results.
Just as important is to understand the limits of your testing and the problems with the specific home to be able to effectively seal the ducts.
Would you say then that there IS Leakage to outside on the ducts that are withing the envelope?
In my experience if there is shell leakage of concern to the auditor, and the total duct leakage is measurable, then their will be measurable leakage to the outside. Sometimes sealing the shell will help, sometimes it will not. It depends on where and how the duct leakage to the outside is.
Cost effectiveness of sealing the ducts is a different issue.
Think about this; a 2-story house on slab, all ducts are within the ceiling/floor assembly and interior wall cavities, all within the envelope. The leaks in the duct system leak into the cavities which, in turn, leak to the outside. WaLaa! Duct leakage to the outside. If there truly is NO leakage to the outside you'll see it when you test, but you must test to find that fact. I've had a few that had that situation, but more that had leakage to outside. We're being paid to do the work to the standard. It's not a race, take the time and enjoy your findings. Life's short, enjoy your work.
Since most homes use wall and floor cavities for returns they is a good proability of the home having air leakage to the exterior. Older homes had the returns running to an exterior wall. Take off the vent cover and you will see the rim joist. Any air leaks to the exterior will be under a negative pressure. With ballon construction you my see a return in an exterior wall running from the baseboard level to the basement in an exterior wall, this could be a 2nd floor return no less. There may be little blocking to prevent pulling from above too.When the return is in an interior wall the air may be pulled from the attic past the seams in the top plate.
When a supply is run up a wall any leaks will pressurize the wall cavity and leak to the attic via the seam at the top plate. If the 2nd floor has supplies running in the floor air leaks will be communicated to the exterior at the band joist.
I have seen all of theses leaks to some extent.
I spoke with Colin, the president of Retrotec, to get his opinion on the matter. Here is what he had to say:
"On the surface of the issue, it may appear to make no sense to do a duct leakage test to outdoors if the ducts are completely within the envelope. However, ducts are not themselves on the "surface" but are usually hidden in wall cavities which always connect to outdoors to some degree. Any duct leakage will have a component that is back into the house and another which is to outdoors and you cannot tell just by looking at the registers when the ducts are covered up.
If you are talking about the house at rough in and all the ducts are exposed and are visibly within the enclosure, that would be a different story and a case could be made for NOT using that method to test them but that only applies if ALL ducts are clearly visible.
Some of the leakiest duct systems are those in two story buildings where interstitial spaces between the floors are used for returns. These spaces connect to outdoors through the joist cavities and into the wall cavities. Leaky returns in those houses will depressurize a large part of the exterior walls, filling them with outdoor air creating huge energy losses and dropping moisture into the wall cavities in most cases that will lead to mold problems down the road.
If the ducts truly are not leaking to outdoors, the duct tester will be on the lowest possible range and should be displaying "TOO LOW" or "--------" indicating the flow signal if too low to yield a meaningful result."
Hope this helps to bring some clarity to the situation. Let me know if you have any questions.
I agree John. Leaky ductwork causes comfort issues. Comfort issues cause occupants to mess with thermostat settings. Messing with thermostat settings causes energy bill to go up, pretty quickly from what I'm seeing.
Jack, How would you define "within the envelope"? The only circumstance I can think that exists is a ranch with unfinished basement and spray foamed rim. Anything with more than one story have duct work in? I would argue the duct work may not really be fully inside, as I wouldn't assume tightness of duct inside walls, or of the walls the duct work is in.
The "why" of the question might be more important than the question. With limited time in a home, we all want to spend our time where it will provide the greatest benefit. I suspect you are attempting to quantify value of how you spend your time? When testing justifies the effort?
So I'd agree, if you are doing duct testing, do duct testing. You never know what you'll uncover.
EDIT: I see Stan, Robert and Silvie have conveyed this thought in similar fashion.