Aluminium Foil Tape: The Cheapest Material That Will Make or Break Your Insulation

You spent hundreds of pounds on PIR boards for your loft conversion, paid a builder to fit them between the rafters, and skipped the foil tape because it looked like an upsell. Two winters later your heating bills are higher than a neighbour with worse insulation on paper. The boards are doing roughly 70-80% of what you paid for. Air convects through every untaped joint, gaps open where the timber has shrunk, and the reflective foil facing that should be acting as your vapour barrier is full of holes. A roll of foil tape costs less than a takeaway. Skipping it is the most expensive saving in the entire build.

What it is and what it's for

Aluminium foil tape is a self-adhesive tape with a thin aluminium foil face (typically 30 microns thick) bonded to a layer of pressure-sensitive adhesive on a paper or film release liner. It comes in rolls 50mm, 75mm, or 100mm wide, almost always 45 metres long. Sold at every UK builders merchant and DIY shed for a few pounds a roll.

Its job in a modern UK extension or loft conversion is specific and consequential. Foil-faced PIR insulation boards (Celotex GA4000, Kingspan TP10, Recticel Eurothane GP, etc.) are designed so that the foil facing acts as a vapour control layer, the warm-side membrane that stops moist indoor air diffusing into the cold parts of the construction where it would condense. The foil on each board does this perfectly. The problem is the joints between boards. Every joint is a hole in the vapour barrier. Every cut edge exposes the bare foam core. Every screw or pipe penetration is a leak. Foil tape closes those holes by bonding to the foil facing on each side of the joint, restoring continuity.

It does three things at once: vapour barrier, airtightness layer, and thermal closure. All three matter.

The vapour barrier function relates to Part L 2021 compliance and BS 5250 (the British Standard for moisture management in buildings). The airtightness function relates directly to the AD-L 2021 air permeability target of 8 m³/hr/m² @50Pa, which every new dwelling must now achieve through pressure testing. The thermal closure function is the headline number: untaped board joints reduce effective R-value by 20-30% according to Insulation Manufacturers Association (IMA) November 2024 guidance, because air convects through the gap and short-circuits the insulation.

Why aluminium foil, not duct tape

The single most expensive mistake on this whole topic is reaching for a roll of cheap silver cloth duct tape and assuming it's the same thing. It isn't. Three differences matter.

Vapour permeability. Aluminium foil is impermeable to water vapour. Vapour resistance is measured in MNs/g (mega-newton-seconds per gram). A 30-micron aluminium foil exceeds 500 MNs/g, which is the practical definition of "vapour-tight." Woven cloth duct tape is vapour-permeable. The fabric backing wicks moisture along the tape itself. As a vapour control layer it does nothing.

Adhesive performance over temperature. Foil tapes for insulation use acrylic adhesive rated from -40°C to +80°C, and the better-quality variants are rated to -10°C minimum application temperature for cold-weather work. The rubber-based adhesive on cheap duct tape softens above 30°C and goes brittle below 5°C. Above an upstairs ceiling in summer or below a draughty loft floor in winter, that adhesive fails within a season.

Long-term ageing. Acrylic adhesive cures over weeks and gets stronger over time. It does not yellow, dry out, or release residue. Rubber adhesive on duct tape oxidises, dries out, and the tape peels off the substrate, often in lumps that take the foil facing of the PIR with them. On a sealed cavity above a plasterboard ceiling you will never see this happen, but you will pay for it in heating bills.

The corner case where duct tape is acceptable: under a screed floor where polythene is laid over the top as a separate slip layer and vapour barrier, the tape under the polythene is acting only as a screed stop (a physical barrier to wet screed seeping through joints during the pour). It is not the vapour barrier in that build-up, the polythene above is. Even so, foil tape costs the same and works better, so there is no real reason to use cloth tape anywhere in an insulated build-up.

Adhesive types: acrylic vs rubber

Within the foil tape category there are two adhesive chemistries.

For UK insulation work, choose acrylic. Every reputable foil tape sold by Screwfix, Toolstation, Wickes, and Travis Perkins (Diall, Ultratape, Wickes own brand, 4Trade, Hippo, Everbuild) uses acrylic adhesive. Rubber-adhesive foil tapes exist for HVAC ductwork where the tape is on hot or cold metal in a controlled space, not in a wall or roof build-up that needs to last 50 years.

Width: why 75mm or 100mm beats 50mm

All three standard widths cost roughly the same per metre of joint covered, so price is rarely the deciding factor.

The reason 100mm is the trade default: when you're applying tape along a 2.4m board edge, especially overhead between rafters, it's surprisingly easy for 50mm tape to wander off-line and leave the joint partially uncovered. 100mm gives you 47.5mm of margin either side of the joint centreline, so even a wonky run still seals reliably. On long ceiling joints between two PIR boards, you can apply 100mm tape one-handed and trust it to span the gap.

Use 50mm for penetration patches around cables, pipes, downlight cans, and small cut edges. Use 75mm or 100mm for board-to-board joints. Buy one roll of each width if you want flexibility; buy two rolls of 100mm if you want to keep things simple.

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How much do you need

Foil tape coverage depends on the joint length, not the floor area directly. On a typical install with 1200 x 2400mm boards, every board contributes one long joint (2.4m) and one short joint (1.2m), but the joints are shared between adjacent boards, so total joint length is roughly 2.5 metres of tape per square metre of board area covered.

Worked example for a 20m² loft floor:

• Board area: 20m²
• Number of boards: 20 ÷ 2.88 (one 1200x2400 board) = 7 boards
• Total joint length: roughly 50 linear metres
• Allowance for cut edges around the perimeter: +20% = 60m
• Rolls of 100mm x 45m needed: 2 rolls

For a 30m² roof or wall area, plan on 3 rolls. For a small en-suite extension floor of 10m², a single roll covers it with margin. Always buy at least one extra roll. A bricklayer pausing at a critical moment to send someone to Screwfix for tape costs more in their wasted time than the entire tape budget.

For penetration patches (one cable, one pipe, one downlight) budget 200mm of 50mm tape per penetration. A loft with 8 downlights and 5 cable penetrations needs roughly 3 metres of patch tape, easily cut from offcuts of the main rolls.

How to apply it properly

The tape itself is simple. The surface preparation is where most installations fail.

Step 1: Prepare the surface. PIR boards arrive from the factory with a thin release film or a waxy residue on the foil facing. Adhesive will not bond reliably to either. Wipe the foil with a clean dry cloth before taping, or in stubborn cases use a cloth dampened with methylated spirits. Brush off cutting dust from any cut edges. If the board has surface condensation from cold-weather installation, wait for it to evaporate before taping.

Step 2: Centre the tape on the joint. Pull a length of tape off the roll, leave it loosely positioned over the joint with roughly equal coverage either side, then press the centre line down first. This stops the tape stretching off-line as you apply it.

Step 3: Apply firm pressure across the full width. Run a clean rag, a wallpaper seam roller, or even the side of a credit card along the entire length of the tape, pressing across the full width. Press the edges, not just the middle. This is the single biggest determinant of bond quality. Tape applied with thumb pressure only will lift at the edges within months.

Step 4: Inspect for fish-mouths. A "fish-mouth" is when the tape lifts at one edge, creating a small open mouth shape. Causes: stretched tape during application, dust under the tape, wrinkles at corners. Fix immediately by lifting the affected section and re-applying with more pressure, or patch over with a fresh strip.

Step 5: Tape every cut edge and every penetration. Cut edges expose the PIR foam core, which has lower vapour resistance than the foil facing. Run tape along every cut edge that faces a warm space. Around every cable, pipe, and fixing through the boards, apply a small patch of 50mm tape and dress it down around the protrusion.

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How it differs by application: floor vs wall vs roof

The same tape does different jobs in different parts of the build-up.

Floor (under screed). PIR boards laid on top of the slab DPM, joints taped, then a 500-gauge polythene slip layer laid over the top before the screed pour. The tape's primary job here is to stop wet screed (especially liquid screed) from seeping through joints between boards during the pour. The polythene above is the vapour barrier in this build-up. Tape must still be properly applied because once the screed is poured, fixing a missed joint means breaking out the screed.

Wall (between studs or as insulated plasterboard). PIR boards cut to fit between studs or fixed to the inside face of a masonry wall. The foil facing on the warm side acts as the vapour control layer. Tape every joint, every penetration (sockets, switches, cables), and every cut edge. This is the application where foil tape is doing its full vapour-control job and where missed joints cause condensation in the wall cavity.

Roof (warm roof construction). PIR boards above or below the rafters, often in two layers. The tape on the warm-side (lower) layer is acting as the vapour barrier; the tape on the cold-side (upper) layer is mainly for thermal closure and resistance to wind-washing. In a fully PIR-as-VCL warm roof, every joint matters. If a separate proprietary VCL membrane (DuPont AirGuard, Pro Clima INTELLO, Novia) is installed over the boards, taping is still good practice but no longer mission-critical.

The PIR-as-VCL question

A point worth understanding: foil-faced PIR's vapour resistance is around 100 MNs/g, which is below BS 5250's recommended 250 MNs/g minimum for a vapour control layer. Strictly, a separate VCL membrane is the textbook-correct approach.

In practice, however, properly-taped foil-faced PIR is widely accepted by building control officers across the UK as an adequate VCL in domestic warm roof and wall construction. The IMA's November 2024 guidance treats taped PIR as the airtightness and moisture-control layer. Building control rarely insists on a separate membrane when PIR joints are sealed correctly.

The pragmatic position: tape the PIR joints as if it is your VCL, because it is. If you want belt-and-braces for a particularly damp building (a kitchen with no extractor, a bathroom over a cold loft, a basement conversion), add a separate proprietary VCL membrane over the top. A budget polythene VCL roll is a few tens of pounds for a 50m² roll. Compared to the cost of fixing condensation damage years later, that's nothing.

Cost and where to buy

Aluminium foil tape is one of the cheapest materials you'll buy for the entire build. The price is consistent across UK retailers for budget products and rises sharply for cold-weather and specialist variants.

£7 – £12 covers the standard range across DIY chains and online insulation specialists, with width and brand the main variables. The 75mm x 45m roll is the most common specification width on building sites and sits in the middle of that range.

At the time of writing, Screwfix Diall and Toolstation Ultratape sell 50mm, 75mm, and 100mm rolls at near-identical pricing (Class O fire rating, 30 micron aluminium). Wickes own brand and Insulation Superstore Ultratape are within a pound or two of those rates for the same widths; Cut Price Insulation lists a 100mm x 40m roll at a similar price (note: 40m roll, not 45m).

Premium specialist tapes carry a clear price step up: NOVIA Cold Weather VCL Tape, A Proctor Reflectafoil, and matched-system tapes for Pro Clima or DuPont membranes typically run between roughly fifteen and twenty-five pounds per roll.

For most extension and loft conversion work, the Screwfix or Toolstation 75mm or 100mm tape is the right choice. Don't pay specialist-tape prices unless you're building to high airtightness standards (Passivhaus or near-Passivhaus) where matched tape compatibility with a specific VCL membrane matters.

Builders merchants (Travis Perkins, Jewson) carry 4Trade and Hippo brands at trade-account pricing, often slightly below DIY chain rates if your bricklayer or insulation contractor has an account. List pricing at merchants is rarely shown publicly online but ask for a price match against Screwfix.

Alternatives and adjacent products

Three products often come up in the same conversation as foil tape but solve slightly different problems.

Gapotape. A BBA-certified product combining foil tape with a strip of compressible foam, sized to match the perimeter gap between PIR boards and rafters or studs. Where plain foil tape only seals board-to-board joints, Gapotape addresses the perimeter gap that opens up as timber framing dries and shrinks after installation. £85 – £100 for a 100mm x 50m box from Buy Insulation Online: about ten times the per-metre price of standard foil tape. The case for Gapotape: in roof and wall installs between timber rafters or studs, the perimeter gap will grow over the first 12-24 months as the timber moves, and that gap dwarfs the joint between boards as a thermal weak point. Gapotape pre-compresses to seal the gap continuously. On a tight budget, the alternative is to cut PIR boards with a deliberate 10mm gap, fill with low-expansion foam, and tape with standard foil tape. Cheaper but more labour.

Low-expansion foam. PU expanding foam in a low-expansion formulation, used to fill larger gaps (10-30mm) at the perimeter of PIR boards before taping. Not a substitute for foil tape: foam fills, tape seals. The standard sequence is cut boards close, foam any gaps, allow to cure, trim flush, tape over. Low-expansion is critical because standard expanding foam pushes boards out of alignment as it cures.

Proprietary VCL tapes. DuPont Tyvek Tape, Pro Clima TESCON Vana, Siga Rissan and others are designed to bond to specific membrane systems, not to PIR foil facings directly. If you're installing a proprietary VCL membrane over the PIR, use the membrane manufacturer's tape for the membrane laps and penetrations. Use plain foil tape for the underlying PIR joints. Mixing tape brands within a system is the most common cause of long-term seal failure in airtight construction.

DPM jointing tape. Looks similar to foil tape, sold for sealing damp-proof membrane laps under floor slabs. Different role, different adhesive (often butyl-based for the wet-DPM context). Don't substitute one for the other: DPM tape on a dry PIR foil and foil tape on a damp DPM will both fail.

Where you'll need this

Foil tape appears at every stage where insulation is installed:

• Insulation - taping joints between PIR boards in floors, walls, and ceilings; sealing penetrations through the insulation; completing the warm-side vapour control layer
• First fix electrics - patching around cables, downlight cans, and ceiling rose penetrations through PIR-insulated ceilings to maintain airtightness
• Underfloor heating - sealing PIR floor board joints before the slip layer and screed pour, preventing screed water ingress between boards

These applications appear across every UK extension, loft conversion, and renovation that involves rigid insulation. The product itself is identical regardless of project type or region.

Common mistakes

Skipping the tape entirely to save twenty pounds. The single most expensive saving in any insulation install. Untaped joints cost you 20-30% of the R-value you paid for. On a typical multi-hundred-pound PIR board package that is roughly a fifth to a third of your thermal performance lost permanently, plus higher heating bills for the life of the building. The two-roll tape budget is non-negotiable.

Using cheap silver duct tape instead of aluminium foil tape. Cloth backing is vapour-permeable, rubber adhesive fails at temperature extremes, and the tape ages out within a couple of years. By the time you discover the failure, the wall is plasterboarded over.

Applying tape to dusty or waxy boards. PIR boards have a thin release residue on the foil from manufacturing. Wipe the foil clean with a dry cloth (or methylated spirits for stubborn residue) before taping. Tape applied over dust de-bonds within weeks.

Using 50mm tape on long board joints overhead. Easy to wander off-line and leave the joint partially uncovered. Use 75mm or 100mm for any joint longer than half a metre, especially overhead in a loft.

Stretching the tape during application. Stretched tape relaxes back over the following hours, lifting at the edges as fish-mouths form. Apply tape loosely to the joint, then press down with full-width pressure. Don't pull it tight as you apply.

Forgetting to tape cut edges. The factory foil facing wraps around board edges to a degree, but cut edges expose the bare PIR foam core. Bare foam has 5x lower vapour resistance than the foil facing. Run tape along every cut edge that faces a warm space.

Not patching penetrations. Every cable, pipe, and fixing that passes through the PIR is a point leak in the vapour barrier. Cut a small patch of 50mm tape, slit it to the centre, dress it around the penetration, and press down firmly. Two minutes per penetration. Skip this step and you've sealed the joints but left the holes.