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Question DetailsAsked on 12/5/2015

Question on Duct Work. Is Flex Duct ok to use in a crawl space.

I have a house with flex duct work in the crawl. Purchased as a rental property and had HVAC installed (well almost) I fired the initial installer and I'm getting estimates for someone to finish the job.
(The house originally had baseboard heat).

The installer put Flex Duct in the crawl and it is hanging somewhat and I was told it should not hang at allt. It is secured at the boot/vents that open into the house and to the air handler. however I was told that there will be heat loss due to it somehwhat hanging in spots. I was also advised to replace with metal duct work.... but at a big expense.

Is the Flex ok is there a way to maker sure it has good air flow without waste. The flex duct is insulated.

thank you


I've had different opinions on it. Some say it is ok, others would advise that metal is the best way to go.

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1 Answer


I would not expect much "heat loss" due to sagging - the difference would be small for ducts tucked up against the floor versus exposed unless you crawlspace is pretty cold.

He may have meant the sagging causes air obstruction due to the bends in the sag, so the airflow out the registers would be reduced. Certainly, ducts should be supported and is not hard to do - there are hangers designed for that purpose, and also some contractors (like liking how narrow and sharp they can be) use foam slip-on pipe insulation over hanger wire or plumbers tape to suspend them. (Plumber's tape is not a "tape" - it is a flat galvanized or copper metal strip about 3/4 to 1 inch wide with holes every inch or so designed for suspending or tying up pipe). If used on flexduct needs to be padded to prevent cutting into the duct over time, and contact width should be not less than 1.5 inches -

This type tends to cut into the insulation and even cut through the ducting over time if narrow (less than about 1-1/2 inch) - but padding like above can solve that -

This type is fine for metal ducting but cuts into flexduct (shows "plumbers tape" as vertical part of hanger) -

This type is better because it is wide enough and flexible but more expensive in some cases -

Certainly, flex duct, especially if the corrugated inside type rather than smooth inside type (which is by far more common and cheaper than the latter), results in more air friction losses along the way - so while it is as airtight as rigid ducting or even more, generally speaking it is going to result in noticeable reduction in airflow at the registers. Also, if it was not stretch out during installation (and the inside corrugated type) even a 30% reduction in installed length for a specific piece can quadruple the airflow friction - so even if it is suitably sized and allowable in your application, if sagging a lot it may need to be provided with more hangers AND possibly stretch out more - easy to do unless access is bad. For a given heating condition, the greater flexduct airflow resistance will mean a longer time to heat up to the desired temp, and a longer runtime on the furnace during firing cycles, and if the furnace is undersized for the unit could result in excessive furnace utilization which can result in premature failure.

Bottom line - you need to find out if the ACCA Manual D (Duct/Distribution) calculation for the new HVAC unit assumed that length and diameter and type of ducting or not - installer should have run Manual J, S, and D (total heating/cooling demand, HVAC unit sizing, and distribution ducting) calcs and be able to provide them to you. If it was sized for that size and type of duct, and of course assuming its use is legal for that location and application in your area (yes in some, not in others, and for some areas or applications has to be specifically fire rated), then just having it stretched out and tied up according to manufacturer specs (typically about every 2-3 feet, not more than 4' support spacing by code) should do it.

However, if it was sized based on rigid ducts and this flexduct is too small for the design load, then it could cause an issue or not depending on HOW undersized, and of course on how remote the served areas are from the rest of the house. If a fairly compact house poor ducting in one or two places is commonly overcome by air mixing with the rest of a house, although it can result in rooms that are chronically hot or cold, but adjusting registers or dampers can commonly overcome that. If a stretched-out house like a U shaped ranch or patio-surrounding hacienda or serving an independent unit like an addition or in-law apartment or is only service to upstairs or attic rooms, that area could end up starved for HVAC if the duct was not adequately sized for flexduct to that area.

Here is a link to a very good synopsis on flexduct installation and rules - I have NOT checked that this conforms to current code but is very easy to read and looks like a pretty good synopsis and is from a known ventilation industry group -

A Heating and A/C contractor who, when the flexduct issue is explained to him, says that a Manual D calculation will be necessary to determine if suitable would be your best bet for this. First, I would call your local HVAC building inspector (usually at county or city building department) and ask if flexduct is even allowed in your area - if not, you have a claim against the first installer (or his bonding company) for installing defective materials. From there, knowing if adequate in capacity and if legal, that will tell you where you go from here.

BTW - if legal but undersized, you may well find that installing a parallel second flexduct to provide the required airflow capacity may be the cheapest solution, especially if access is tight or a lot of corners are required. Normally, if that is required and legal, I would recommend NOT tying both into one register - I would run to new registers if feasible, located on other sides of the serviced rooms, to provide more even airflow to the served areas - also avoids buying expensive dual-duct register adapters or wyes.

Answered 3 years ago by LCD

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