Duct Insulation Wrap: Why Loft Ducts Drip and How to Stop It

Six months after your kitchen extension signs off, a brown stain appears on the new ceiling around the cooker hood. A month later it's bigger. By the time you call someone out, water is beading around the hood filter every time you cook a roast. The builder is long gone. The fix means pulling down plasterboard, rewrapping the duct that should have been insulated the first time, and redecorating. The material cost to prevent this is roughly £60 to £100 on a typical kitchen extension. The cost to fix it after the fact runs into four figures.

What it is and what it's for

Duct insulation wrap is the lagging applied to kitchen extract ducting where it passes through unheated space, typically a loft void or the ceiling void above a flat roof. It does exactly what pipe lagging does on a hot water pipe: keeps the duct wall warmer than the dew point of the air inside it.

The problem it solves is simple physics. A cooker hood pulls warm, steam-laden air off the hob. That air is maybe 30 to 40 degrees and saturated with moisture from boiling pasta, frying onions, and simmering stock. If the duct carrying that air runs through a cold loft in winter, the duct wall temperature can sit around 2 to 5 degrees. The warm moist air touches the cold duct skin, condenses on contact, and turns into liquid water on the inside of the pipe.

That water has to go somewhere. If the duct slopes down toward the hood (which happens by accident more often than homeowners realise), the condensate runs back toward the kitchen, drips out of the filter, and stains the ceiling around the hood. If the duct sags or loops, the water pools in the dip, grows mould, and eventually either drips through a joint or degrades the pipe.

Insulation wrap breaks the cycle. A 25mm layer of foil-faced mineral wool raises the duct wall temperature enough that the air inside never hits its dew point, so water never forms in the first place.

The governing standard for England is Approved Document F, Volume 1 (Dwellings), 2021 edition. It applies to all extract ventilation in dwellings, not just mechanical ventilation with heat recovery. Wales has an equivalent ADF. Scotland uses Standard 3.14. Northern Ireland has separate Technical Booklets.

What the regulations require

Approved Document F is precise. Any extract duct passing through an unheated space must be insulated to the equivalent of at least 25mm of material with a thermal conductivity of 0.04 W/(m.K) or lower. In plain terms: 25mm of standard mineral wool or foam, or a thinner layer of a better-performing material.

Where the duct exits above roof level, either the section above the roof is also insulated, or a condensate trap is fitted just below roof level to catch any water that does form.

Flexible ducting is capped at a maximum of 1.5 metres per run. This catches most homeowners off guard. Builders routinely run 4, 6, even 10 metres of corrugated flexible duct through the loft because it's quick to install. That's non-compliant and it's also the single most common cause of condensate drip-back, because corrugated flex holds water in every ridge. Longer runs must use rigid duct.

LABC Warranty and Premier Guarantee, the two main warranty providers for new-build and structural alterations, go one step further. Their technical guidance specifies that insulation should be factory-applied, not site-applied. For warranty compliance on a new installation, you want factory-insulated rigid duct or factory-insulated flexible stubs, not a bare duct wrapped on site. In practice, site-wrapping is still acceptable for Building Regs compliance on retrofits where the duct is already in place, and it's what most DIY and smaller jobs actually use.

Which sections need insulation

Not every inch of ducting needs wrapping. The regulation is specific: insulate where the duct passes through an unheated space. In a typical UK house, that means:

• Always insulate: any run through a cold loft void, through an unheated garage ceiling, through the void above a flat roof, or externally above roof level where the termination is exposed.
• Never needed: duct runs entirely within the heated envelope. If the duct goes from a cooker hood sideways through a kitchen ceiling void and out through an external wall at the same floor level, the whole run is in warm space. No insulation required.
• Borderline cases: duct running through a soffit over-eaves before exiting. The soffit is effectively outside air temperature, so yes, insulate it. Duct running in a partially heated utility room ceiling void: lean toward insulating if the adjacent loft is cold.

The practical test: stand in the space the duct runs through on a January morning. If it feels cold, the duct needs insulating. If the space is warm, it doesn't. Regulations aside, physics only cares about the temperature differential.

This matters because wrapping a 5-metre loft run costs £60 to £100. Wrapping an entire unnecessary 15-metre run inside the heated envelope wastes material, takes longer to fit, and makes the pipe harder to clip and support. Don't over-insulate; insulate what actually needs it.

Product types: which approach to use

There are three ways to meet the spec. Each suits different situations.

Retroactive foil-faced mineral wool wrap

This is the standard solution for wrapping existing rigid ducting or long runs where factory-insulated duct isn't practical. The material is stone or glass mineral wool bonded to an aluminium foil facing. You cut lengths to size, wrap them tightly around the duct with the foil on the outside, and seal every seam with aluminium foil tape.

The main brands are Rockwool DuctWrap (stone wool, 0.034 W/mK, Class A1 non-combustible) and Isover Climcover (glass wool, 0.037 W/mK). Both comfortably beat the ADF threshold at 25mm thickness. Ducting Express sells a generic foil-faced wrap at 0.032 W/mK for lower cost per square metre on larger jobs.

Pricing sits around £57 to £67 including VAT for a Rockwool DuctWrap pack (2 rolls, 10m² coverage, enough for roughly 20 linear metres of 150mm round duct including overlaps). Isover Climcover is better value per square metre on larger rolls, around £100 for 21.6m². Generic wrap can be under £85 for 24m² if you're doing a full retrofit.

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Preformed EPS insulation shells

Domus sells rigid expanded polystyrene (EPS) shells that clip around its EasiPipe rigid round duct. Two half-shells snap together along a tongue-and-groove seam, no tape or adhesive required. Bends (45 and 90 degree) are available to match the duct system fittings.

Pricing runs £7 to £11 per 1-metre shell for 100mm duct, slightly more for 150mm. A 5-metre loft run in 150mm needs 5 straights plus whatever bends you have, so you're looking at £40 to £60 total. The big advantage is speed and a neat finish. The big disadvantage is that it only fits Domus EasiPipe duct. If you're running Manrose or Envirovent or a generic round rigid duct, the shells won't fit.

Use this approach when you're speccing a full Domus EasiPipe system from scratch and you want a consistent manufacturer-matched solution. Don't try to retrofit it to non-Domus duct.

Factory-insulated flexible duct

Flexible duct with insulation factory-applied during manufacturing. Two main products: Blauberg SonoFlex (25mm Rockwool wrap on aluminium flexible inner, thermal conductivity 0.04 W/mK, meets ADF exactly) at around £33 for 5m of 100mm. Manrose INSDUCT102 and INSDUCT127 (HIPS polystyrene insulation, rated to Class O fire resistance) at around £58 for 10m of 100mm or 125mm from Screwfix.

These products are correctly specified for short connecting stubs between the fan and rigid duct. They are not the right product for a full loft run. Approved Document F caps flexible duct at 1.5 metres per run, and in practice flexible duct sags, corrugates, and traps water even when insulated if it's pulled too tight or supported poorly.

The right use: a 1-metre piece of insulated flex from the cooker hood outlet up through the kitchen ceiling void, connecting to the rigid duct that then runs across the loft. Another short piece at the external termination end if needed. Everything in between should be rigid duct, either factory-insulated or retroactively wrapped.

The slope rule

Insulation alone is not enough. Any extract duct should have a continuous downward slope toward the exterior, so that any condensate that does form (and a small amount inevitably will over the life of the installation) drains out through the termination rather than back toward the fan.

The installed slope should be at least 1 degree, which is 17mm per metre of run. In practice, most installers aim for a more obvious slope of 2 to 3 degrees so it's visible to eye.

The failure mode when this is ignored is familiar from community forums: flexible duct sags between supports, creates a U-bend filled with water, and eventually the water either backs up into the fan or drips from a joint. Even correctly insulated duct will drip if it sags. The slope rule and the clip-every-600mm rule are non-negotiable.

For vertical runs through a pitched roof (the duct going straight up out of the loft to a roof terminal), fit a condensate trap at the base of the vertical section with a 10mm drain pipe to the eaves. This catches any water that forms on the vertical run before it can run down and reach the fan.

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How to fit retroactive wrap

Wrapping existing rigid duct is the commonest job and the one most DIY-level and smaller installations rely on. The sequence is straightforward but unforgiving of shortcuts.

Measure the duct perimeter. For 150mm round duct, that's approximately 470mm. Add 50mm for overlap at the longitudinal seam. Cut the wrap with a sharp utility knife on a scrap board, foil side down, mineral wool side up. Dust mask essential, the fibres irritate lungs.

Drape the wrap around the duct with the foil side outermost. Pull it tight so the mineral wool is snug against the duct wall with no air gap. Bring the long edges together and overlap by 50mm. Hold the overlap closed with your knee or a second pair of hands.

Seal the longitudinal seam with 75mm aluminium foil tape, pressed down hard along the entire length. Not ordinary duct tape, which dries out and falls off within 18 months. Proper aluminium foil tape (Class O fire rating, around £6.79 at Toolstation or Screwfix for a 45m roll) is what's specified.

Cut the next length, butt it against the first, and seal the circumferential joint with another strip of foil tape wrapping all the way around the pipe. No gaps. Every seam fully sealed. Gaps in the foil facing are gaps in the vapour barrier, and vapour getting to the mineral wool condenses inside the insulation, soaks it, and defeats the entire system.

For bends, cut wedge-shaped segments of wrap and piece them around the curve. Tape every join. For branches and Y-pieces, wrap the main run first, then cut slit patches for the branch spigots and tape them in. It's fiddly, which is one reason LABC Warranty prefers factory-insulated duct on new jobs.

Mineral wool vs Armaflex: when the rules change

This is where MVHR installers and cooker hood installers diverge, and it causes confusion when homeowners read forum posts aimed at the wrong system.

For a standard kitchen cooker hood or bathroom extract, the air moving through the duct is warm and moist. The duct wall is cold. Condensation forms on the inside of the duct. Mineral wool wrap on the outside of the duct keeps the duct wall warm enough that no condensation forms. The mineral wool itself stays dry because the warm duct is the warm side and the mineral wool sits against it. This is the normal case and mineral wool wrap is correct.

For a mechanical ventilation with heat recovery (MVHR) system, some of the ducts carry cold incoming air from outside. The duct wall becomes cold because of the air inside it, and the warm loft-adjacent air (even in a cold loft there's some thermal gain from below) can condense on the outside of the duct. If you wrap mineral wool around a cold-air duct, the condensation soaks into the mineral wool from the outside, waterlogs it, and destroys its insulation value.

Cold-air ducts need a closed-cell insulation that water can't soak into. Armaflex is the standard product. It's a synthetic rubber foam with no open pores, significantly more expensive than mineral wool, and it's what MVHR specialists fit on supply and intake ducts.

Cost and where to buy

Prices are consistent across the main UK merchants for the standard products.

Rockwool DuctWrap 25mm foil-faced, per 10m² pack (2 rolls): approximately £57 to £67 including VAT. Stocked by Buy Insulation Online, Insulation Store Online, and similar specialist insulation suppliers. Not routinely stocked at the big-shed DIY retailers, which is why many homeowners don't know the product exists.

Isover Climcover 25mm foil-faced, per 21.6m² roll: approximately £100 to £115 including VAT. Better value per square metre on larger jobs. Same ADF-compliant specification.

Domus EasiPipe insulation shells, per 1m: £7 to £11 each. Pack of 4 drops the per-unit price toward £6.60. Available from BES, i-sells, and Domus Ventilation direct.

Blauberg SonoFlex insulated flexible duct 100mm, 5m length: approximately £33 from Blauberg UK direct. Manrose INSDUCT102 10m at Screwfix: £57.99. L&B Fans 100mm insulated flex 10m: £25.75 ex VAT.

Aluminium foil tape 75mm x 45m: £6.79 at Toolstation (generic) or Screwfix (Diall). Both Class O fire rated. Do not substitute cheap silver duct tape from the pound shop; it's not the same product and it will fail within 18 months.

Alternatives

Burying the duct under loft insulation is sometimes suggested as a cheap alternative. It can work if the loft has thick enough insulation (200mm+ mineral wool at joist level) that the duct sits fully within the insulated layer, and if the duct is adequately supported so the insulation doesn't compress it. In practice, most loft insulation sits between the joists and the duct runs above, so this approach usually leaves the duct exposed to loft air. It's not a substitute for proper wrap.

Pre-insulated rigid duct (Domus EasiPipe Thermal or equivalent) is the cleanest solution for new installations. It's a rigid duct with insulation bonded on during manufacturing. More expensive than bare duct plus retrofit wrap, but faster to install, warranty-compliant, and looks neater. If you're doing a kitchen extension and speccing from scratch, this is worth quoting alongside wrap-on-site.

Thicker insulation (40mm or 50mm mineral wool wrap) exceeds the ADF minimum and provides a larger safety margin in very cold lofts or on thin-walled metal ducting where heat loss through the duct wall is higher. It costs roughly 30 to 50 percent more than 25mm. For most UK lofts, 25mm is fine. For an exposed loft in Scotland or a partially unheated extension void, 50mm is sensible insurance.

Common mistakes

Installer skips loft insulation entirely. The single most common failure. The builder runs flexible or rigid duct through the loft, connects both ends, closes up the ceiling, and moves on. There's no insulation and often no slope. Stains appear six to eighteen months later. If you're project-managing the extension, your job is to inspect the duct route before the kitchen ceiling plasterboard goes up. Check for insulation, slope, and support spacing.

Flexible duct used for the full loft run. Approved Document F caps flexible duct at 1.5m. Builders ignore this because flex is faster to install. Corrugated flex traps water in every ridge, sags between supports, and is the commonest cause of drip-back regardless of insulation. Rigid duct for anything over 1.5m, full stop.

Wrap seams taped with ordinary duct tape. The silver cloth "duct tape" from DIY shops is not aluminium foil tape. It dries out, the adhesive fails, and seams open up within a year. Use proper 75mm aluminium foil tape with Class O fire rating.

Duct wrapped with no slope, or sloping the wrong way. Insulation reduces condensation but doesn't eliminate it entirely. Any water that does form has to drain out, not back. A level duct (or worse, one sloping toward the fan) will dribble condensate back into the kitchen no matter how well it's wrapped.

Missing condensate trap on vertical roof runs. If the duct turns vertical and exits through a roof tile, the vertical section above the loft is exposed to cold air and external weather. Water forms on the vertical inside surface and runs down into the horizontal loft run. Without a trap at the base of the vertical, it keeps going all the way back to the fan.

Wrapping with mineral wool for an MVHR cold-supply duct. Wrong product for that application. Closed-cell Armaflex is the correct specification for MVHR supply and intake ducts because mineral wool waterlogs from outside condensation. Check what system you're actually insulating before buying wrap.

Where you'll need this

Duct insulation wrap is a first-fix and second-fix material on any project that pulls a cooker hood, bathroom extractor, or MVHR system through a loft or unheated void. These scenarios appear across extensions, loft conversions, and retrofit kitchen installations:

• Extractor and ventilation - specifying the cooker hood, duct route, and insulation requirement before first-fix so the builder knows what to install
• First-fix insulation - duct wrap fits alongside the wider insulation package and is installed at the same point in the schedule, before ceilings close up

Every extract system running through cold space across any project type needs this. The material cost is trivial relative to the total ventilation installation. The failure cost (ceiling stains, mould, fan damage, plasterboard replacement) is not.