Why does this stop working if it touches something?

Because it works by EMISSIVITY, and emissivity needs an air gap to act into. A radiant barrier does not resist heat the way insulation does — it has almost no R-value of its own. What it does is refuse to re-radiate: a low-emissivity foil facing an air space gives off very little of the heat it receives, so the heat never crosses the gap. Press that foil against the underside of a deck and there is no gap for radiation to cross, so conduction takes over and the product does essentially nothing. This is the single most common way a radiant barrier is installed and wasted. It also means dust matters: a foil that has gone grey in a loft has a far higher emissivity than the one that was fitted, which is why the face is normally installed downwards.

How much waste allowance should I use?

5% is a reasonable starting point for a simple roof shape. Roofs with many hips, valleys, dormers, or obstructions generate more cut-offs and overlaps, and may need a higher allowance.

Does this include overlap between foil rolls?

The waste allowance is intended to cover typical overlaps and cut waste for a standard installation. For products requiring a specific overlap width at seams, check the manufacturer's installation instructions and increase the allowance if needed.

Where is radiant barrier foil typically installed?

It is most commonly stapled to the underside of roof rafters or laid over the attic floor, with the reflective side facing an open air space, to reduce radiant heat transfer into the attic.

Materials & Quantities

Radiant Barrier Foil Coverage Calculator

Calculate the radiant barrier foil material needed to cover an attic roof deck, with waste allowance.

Answers as you typeNo submit button
Every formula citedNamed published standards
Calculated in your browserNo account, nothing uploaded

SettingsSettings for this calculationUS

Market

Imperial · sales tax

Who is doing the work?

Waste is set to 5% by hand. Pick a tier above to replace it, or keep your own figure.

Roof Deck Area

The total attic roof deck area to be covered with radiant barrier foil.

Measure the underside roof deck area (rafter bays) where the reflective foil will be installed, typically stapled to the underside of the rafters.

Waste Allowance (%)

Extra material to allow for cut waste and overlaps.

5% is typical for a simple gable roof; increase it for complex roof shapes with many hips, valleys, or obstructions.

Radiant barrier material needed

1,690 sq ft

High confidence

Then change the inputs to see how far the answer moves.

Show calculation logic

How this was calculated

Formula source(s)

Material needed = roof area × (1 + waste allowance), the standard coverage-with-waste method applied to reflective radiant barrier foil installation

Inputs used

Roof Deck Area: 1610 sq ft
Waste Allowance (%): 5

Final result1,690.5 sq ft

What this calculation does not cover

Foil is a vapour barrier as well as a reflector, and where it lands decides whether that matters. Laid over the loft floor on top of the insulation, an unperforated foil sits on the cold side of the ceiling and traps moisture rising out of the house in the insulation beneath it - perforated products exist precisely so the assembly can still dry, and choosing between them is a climate and construction decision rather than a coverage one. The same job goes wrong at the eaves: foil run tight into them blocks the soffit ventilation the roof space relies on, and the barrier's own benefit assumes that air is still moving.
It is a sheet of metal in a space full of wiring and fittings. Foil conducts, so it must be kept clear of live parts, junction boxes, old rubber-insulated or knob-and-tube wiring, flues, and the tops of recessed downlights, which carry their own clearance requirements - stapling a continuous conductive sheet across the rafters is not a neutral act. Where it is left exposed in an accessible roof space its surface-burning rating matters as well; this sum counts area and nothing else.

Estimated cost — your price

Price per sq ft ($)

This site holds no price list for this material — local prices vary too much to publish honestly. Enter your supplier's price and the result is costed with it.

Computed in your browser — nothing you enter is uploaded. Presented in US customary units and US trade terminology. Where a formula follows a published standard, that standard and its edition are cited beside it on this page; where none governs, the page says so. Local amendments override model codes — verify against the code in force where you build.

Sources checked 2026-09-06 · in the site-wide review of 2026-09-06 · v1.0.1

Regulatory standards & verification citations1

Material needed = roof area × (1 + waste allowance), the standard coverage-with-waste method applied to reflective radiant barrier foil installation

Cite this page

Your workspace

Most jobs need more than one number. Add the calculators you need next and they open right here, underneath this one — your figures stay on screen and nothing is lost to a page change.

Now that you have the number

These guides cover the work this quantity is for — the first ones run this calculator inside the section that raises the question.

Fitting a Radiant Barrier Under a Metal Roofuses this calculator
Foil lands in the gap between the steel gang and the sheeters, and the roll count, the lap allowance and the condensation risk are all settled before it does.

Called something else where you work? Sarking and roof underlay — the term in each market, how close the equivalence really is, and the standard that governs it.

How to calculate radiant barrier foil coverage in 3 steps

Roof Deck AreaThe total attic roof deck area to be covered with radiant barrier foil.
Waste Allowance (%)Extra material to allow for cut waste and overlaps.
Radiant barrier material neededThe tool computes the radiant barrier material needed from those figures and shows the formula, its sources, and a confidence rating alongside it.

Radiant barrier material needed by roof deck area

Page defaults, not your figures above.

Roof Deck Area	Radiant barrier material needed (sq ft)
1,000 sq ft	1,050
1,500 sq ft	1,575
2,000 sq ft	2,100
2,500 sq ft	2,625
3,000 sq ft	3,150

Frequently asked questions

Why does this stop working if it touches something?: Because it works by EMISSIVITY, and emissivity needs an air gap to act into. A radiant barrier does not resist heat the way insulation does — it has almost no R-value of its own. What it does is refuse to re-radiate: a low-emissivity foil facing an air space gives off very little of the heat it receives, so the heat never crosses the gap. Press that foil against the underside of a deck and there is no gap for radiation to cross, so conduction takes over and the product does essentially nothing. This is the single most common way a radiant barrier is installed and wasted. It also means dust matters: a foil that has gone grey in a loft has a far higher emissivity than the one that was fitted, which is why the face is normally installed downwards.
How much waste allowance should I use?: 5% is a reasonable starting point for a simple roof shape. Roofs with many hips, valleys, dormers, or obstructions generate more cut-offs and overlaps, and may need a higher allowance.
Does this include overlap between foil rolls?: The waste allowance is intended to cover typical overlaps and cut waste for a standard installation. For products requiring a specific overlap width at seams, check the manufacturer's installation instructions and increase the allowance if needed.
Where is radiant barrier foil typically installed?: It is most commonly stapled to the underside of roof rafters or laid over the attic floor, with the reflective side facing an open air space, to reduce radiant heat transfer into the attic.

Preliminary estimate, not certified engineering. This tool produces an indicative quantity calculation for planning purposes only — it is not a certified structural analysis, a guaranteed material takeoff, or a substitute for building department approval. Always verify measurements on-site and have a licensed contractor or structural engineer review any load-bearing, code-sensitive, or safety-critical work before purchasing materials or starting construction. Spotted an arithmetic or standards error? Report it to contact@craftquantities.com with your inputs — a confirmed fix gets a permanent check of its own, so the same mistake cannot come back.