Flex Duct vs Sheet Metal

How do flex and sheet metal ducts differ?

Flex duct is a wire-coil inner core wrapped in insulation and an outer jacket, while sheet metal duct is rigid galvanized steel, round or rectangular, that is insulated on the outside. Flex goes in fast and bends around trusses. Sheet metal holds its shape and has a smooth inside, but every seam and fitting is a joint that has to be sealed.

Neither material is good or bad on its own. LBNL's duct-sealing slides contrast basement rectangular metal, where "Many joints = many leaks" ran about 30% per side of the fan, with attic round metal or flex, where fewer joints meant about 15% per side. The same slides note that for insulated ducts outside the conditioned space, about 90% of the energy lost is not recovered. In a Valley attic, where you put the duct matters as much as what it is made of.

There's no primary head-to-head study behind the claims you'll hear about friction or lifespan, and Valley Home Ducts didn't find one. What the standards do give is a clear set of rules for installing flex well, and those decide most of the difference.

Why does compressed flex lose airflow?

Compressed flex loses airflow because a squashed inner core folds into ridges that drag on the air, which raises the pressure the blower has to overcome. The Air Duct Council's slide summary, linked from PNNL's library entry, shows pressure drop at about 2 times normal with 15% compression, 4 times at 30% and 6 to 8 times at 45%. Its instruction is blunt: "Ducts should be fully extended before and during installation."

Sag and pinching do the same thing. PNNL lists the results of poorly supported flex as "minimal air flow to a terminal, excessive noise, or fan burnout due to increased static pressure." ENERGY STAR's duct sealing page names "tangled or kinked flexible ducts" among its signs of a duct problem.

Looping is a related habit. PNNL's page on coiled or looped flex says "Some contractors intentionally loop the flexible duct as a way to decrease airflow to a particular room," and cites ENERGY STAR guidance that balancing dampers or proper duct sizing "shall be used instead." The balancing page covers dampers.

What are the flex duct installation rules?

The core rules are full extension, supports at most 4 feet apart, limited sag, wide straps and gentle bends. ADC's 5th edition, as summarized by PNNL, sets them out:

• Supports: straps at least 1-1/2 inches wide at intervals of no more than 4 feet, with sag of no more than 1/2 inch per foot of support spacing.
• Straps: PNNL says "Straps should not compress the inner core or constrict air flow."
• Bends: "Keep bends greater than or equal to one (1) duct diameter," in ADC's wording.
• Length: stretched fully, with no extra coiled in the attic.
• Joints: tapes or mastics listed to UL 181B, marked 181B-FX for tape or 181B-M for mastic.

The Air Duct Council has since produced a 6th edition of the standard; the figures here are from the 5th, the version PNNL summarizes. In dry climates like the Valley, PNNL's support page notes that flex may rest on ceiling joists and be covered with blown insulation, an approach its buried-duct guidance pairs with R-8 duct insulation and very tight ducts.

Where does sheet metal make sense?

Sheet metal earns its place where shape and durability matter: plenums, trunks, long straight runs and spots where a duct could be stepped on or crushed. A rigid trunk can't sag between straps, and its inside stays smooth.

The trade-off is joints. Each seam, collar and elbow needs sealing, and the DOE Building America measure guideline says many home energy experts "consider mastic superior to tapes in most situations." It identifies UL 181A as the listing for rigid duct closures. Cloth duct tape doesn't belong on either material; the mastic vs foil tape page explains why.

Older Valley houses sometimes have metal ducts with old wrap or tape. EPA notes that houses of the 1930s–1970s era are the likeliest to carry asbestos in duct wrap, tape or cement (transite) ducts. Don't cut, scrape or seal over suspect material; EPA says to leave it alone and have an accredited asbestos professional take samples. If you're unsure, call a licensed technician.

What should a mixed flex and sheet metal system look like?

One common arrangement pairs a rigid plenum and trunk with short, straight flex branches to the registers, all sealed at every connection. Look for flex pulled taut, strapped at least every 4 feet, with no loops and no bends tighter than the duct's own width. Look for metal joints coated in mastic, not wrapped in cloth tape.

Every new attic duct also needs insulation. Stockton, Modesto and Sacramento sit in climate zone 12 and Fresno in zone 13, and Title 24 asks for R-8 on new attic ducts in both; the duct insulation page has the detail. Longer replacements also trigger leakage testing, covered on the replacing ductwork page.

A supply house's duct pipe listings show round metal pipe by diameter, and its duct fittings listings show the collars, elbows and boots that join metal and flex. Seeing them helps you read a bid; choosing them is the contractor's job.

Flex and metal figures

• 4 ftmaximum flex support spacingAir Duct Council via PNNL
• 15%compression at which flex pressure drop about doublesAir Duct Council via PNNL
• 30%leakage per side seen in basement rectangular metal ductsLBNL
• R-8new attic duct insulation in Stockton through FresnoTitle 24 (Energy Code Ace)

Before any attic look, go early in the morning in a cooler month, step only on joists or a board laid across them, watch for roofing nails overhead, wear gloves, long sleeves, safety glasses and a dust mask, and tell someone you're up there. The attic safety page covers heat illness signs.

Questions from the field

Keep reading

• D1How Manual D sizes each duct
• C4Sealing attic ducts safely
• A3Ducts in conditioned space
• DAll duct design pages