Duct Insulation Wrap Area Calculator

Frequently asked questions

How do I find the duct perimeter for a rectangular duct?

Perimeter = 2 × (width + height) of the duct's cross-section. For a round duct, use perimeter = π × diameter instead.

Why use a 10% waste allowance for duct wrap?

Blanket-style duct insulation needs overlap at longitudinal and circumferential seams, plus extra material to wrap around elbows, transitions, and fittings — 10% is a typical allowance to cover this.

Does this include insulation for duct fittings like elbows and transitions separately?

No — this calculates the base wrap area for the straight run length and perimeter you enter. Elbows, transitions, and other fittings typically consume extra material beyond the waste allowance and may be worth estimating separately for a complex duct layout.

Which parts of a duct system need wrapping, and what length should I enter?

Only the lengths that need it, not the whole system. Wrap goes on ductwork carrying heated or cooled air through space outside the insulated envelope — a loft or attic, a crawlspace, an unheated garage — and on duct carrying cold air through warm space, where the concern is condensation rather than heat loss. On a whole-house heat-recovery system (MVHR in the UK, an HRV or ERV in North America) with the unit inside the heated space, that includes the two ducts between the unit and the outside wall, the fresh-air intake and the exhaust, both of which run cold through the warm house; with the unit in a cold loft or attic, it is the supply and extract ducts running to the rooms that cross cold space. Supply and return (extract) ducts that stay entirely inside the heated space may be left bare where the code in force allows it, so measure those out of the length.

How is duct wrap sold and specified in the UK and North America?

In North America it is a glass-fibre blanket on a roll with a foil-scrim-kraft or similar facing, sold by nominal thickness and labelled with an installed R-value worked at a stated compressed thickness. In the UK, ductwork insulation is a foil-faced glass or stone mineral wool roll, or a lamella mat of fibre strips bonded to foil, sold by thickness in millimetres and specified by thickness and the material's thermal conductivity, the thickness read from tables that set it by the air temperature and by whether the aim is limiting heat loss or preventing condensation. Either way, divide this result by the area printed on the roll and round up. Pieces are cut from the roll to go round the duct with the roll's width running along it, so on a large duct check how many whole girths each roll length gives — the short end left over is waste.

What perimeter do I enter for flat oval duct, and what about flexible duct?

Flat oval duct has two flat sides joined by semicircular ends, so its perimeter is π × the minor axis plus twice the difference between the major and minor axes. Flexible duct sold already insulated, with its own outer jacket, is not wrapped at all and should be left out of the length entered here.

Can the same wrap go on ductwork that runs outside?

Not as it comes. A faced blanket is not weatherproof, and insulation that gets wet loses much of its insulating value, so external ductwork is insulated with a product rated for outdoor use and finished with a weatherproof cladding or jacket — sheet metal or a sealed membrane — with its joints lapped to shed water. That cladding is a separate item, measured round the outside of the insulation, a larger girth again than the one this result uses.