Insulation: R-values and U-values, batts, boards, fill and airtightness

What the insulation calculators cover

Insulation quantities are of two kinds, a thickness and a count, and this page keeps them apart. The thickness pages work in resistance and transmittance: the R-value of a layered wall, the thickness a target U-value needs, the minimum a climate zone asks for, and what framing does to an effective R-value. The count pages turn an area into batts, bags, boards, kits, tape and fixings. The conversion pages move a rating between the inch-pound and metric systems, and the moisture and airtightness pages check what the insulation is part of.

Batts, blown fill and spray foam are counted from the product's own coverage, boards from the face area of one board with the layers and the waste, and the cavity fill page from the volume of a closed cavity or of drilled stud bays, with the bags and injection holes that follow. Pipes and ducts are counted in sleeves or wrap, and reflective barriers by the roll.

How the answers are worked out

Resistances in a layered assembly add, which is the whole of the R-value calculator, with an allowance for the air films included. Within one system R and U are reciprocals, and the conversion between systems rests on NIST SP 811, with ASTM C168 giving the terminology of thermal resistance. The target U-value page applies BS EN ISO 6946: the U-value is one over the sum of the inside surface resistance, the layers and the outside surface resistance, each layer's resistance being its thickness divided by its conductivity.

Framing bridges the insulation. The thermal bridging page uses the parallel path method, weighting the framing's resistance and the cavity's by their shares of the area, and the continuous insulation page applies it to Z-furring and steel studs. The junction pages are different in kind: the thermal bridge heat loss page sums linear bridges as psi times length and point bridges as chi times count, citing BS EN ISO 14683, BRE BR 497 and ASHRAE 90.1, and the surface temperature factor page compares fRsi with the critical factors of BRE IP 1/06 under BS EN ISO 13788.

The counts take the product's published figures as inputs. The batt page asks for the package coverage, since coverage varies with the R-value, the thickness and the brand; the blown-in page uses the bag's rated coverage at its installed density; spray foam uses a kit's coverage at a thickness; the foam board page uses the sheet size of the market. Cavity fill follows BBA certificates and manufacturers' data sheets for installed density, hole pattern and bag weight, which differ by product, so the page asks which product you have.

Units and documents by market

The market setting chooses the system the answers appear in: inch-pound units and R-values for the United States, metric units with RSI and U-values for the other three markets, each answer restated in the other system. The climate zone calculator tabulates two codes only, the 2021 IECC for the United States and the 2020 NBC for Canada. It says that the UK's Approved Document L sets U-values for each element and that Australia's NCC sets its own zone figures, neither tabulated, and that the U-value calculators work in those terms.

Allowance and whole units

Counts round up because a package, a bag, a board or a sleeve is bought whole. The batt and board pages take a waste allowance, the pipe page counts sleeves in the stock length of the market it is set to, and the foam board and rim joist pages use the sheet module of the market. The mineral wool board page counts a full-area order for each layer of a multi-layer build-up, because staggering the joints between layers means that neither layer can be made up from the other's offcuts.

The order the questions come in

The groups follow the order of the decisions. First the rating: the thickness or the U-value the assembly has to reach, from the climate zone page or a target. Then the material that gives it, counted as batts, fill, boards or foam. Then the fixings and thermal breaks that hold continuous insulation off the wall, and the baffles that keep a vent path clear above insulation. Then the air barrier, tape and sealant, and last the moisture check on the temperature at the barrier and at the inside surface. A figure in one step feeds the next: the thickness the target needs is the thickness the board count is taken at.

What the calculators leave to others

A thickness, a count and a check are not a specification. The climate zone page tabulates minimums and says the code in force governs. The dew point page is a simplified steady-state method and says condensation risk exists when the temperature at the barrier falls below the interior dew point. The airtightness page carries a flow from one test pressure to another only with the exponent from the test's own fit. The cavity fill page names the checks that come before drilling a wall, among them a site assessment under the BBA certificates and the NFPA 70 rule on concealed wiring.

Whole jobs, taken off

A job takeoff asks for the dimensions once and returns every line the job needs, each from its own calculator. These are the ones where insulation is a main line.

• Attic insulation upgrade

  How much more depth an attic needs to reach a target R-value, and the bags or batts that supply it, from the attic's length and width.

• External wall insulation (EWI, EIFS)

  A house insulated from outside: the scaffold, the bonded and anchored boards, the basecoat with its mesh and the render finish, taken from the walls' dimensions.

• Cavity wall insulation (retrofit fill)

  A closed wall filled from outside: the bags or bales for the wall's net face at the installed density, with the ground sheeted and the frames masked.

Read an answer backwards

When you start from what you hold and want to know what it will cover, these pages run the same calculators in reverse.

• How much wall will my foam insulation boards cover?
• How big a crawlspace will my vapor barrier rolls cover?
• How much attic will my bags of blown-in insulation cover?
• How much attic roof will my rolls of radiant barrier cover?
• How much wall will my mineral wool boards insulate?
• How much roof will my cover board sheets cover?
• How much attic will my packages of insulation batts cover?

Worked examples

A job worked through with its inputs and its working shown, so you can see the figures before you enter your own.

• Loft insulation top up

  A loft topped up to a target: the thickness the target U-value needs, the bypass sealing, the baffles, the pipe lagging and the batts, in the order they are worked.

• A semi's cavity walls, in bales and holes

  Blown cavity wall insulation for the gable end and rear wall of a semi-detached house

Reference tables

• Production rates by operation

  How long each operation takes one person, in person-hours per unit of work.

Guides

The practice behind the numbers: how the work is done, what goes wrong and how to check it.

• Insulating an Attic

  Attic heat loss ranked biggest hole first: air leakage, bare gaps, depth, the hatch, ducts, then the eave perimeter.

• Insulating a Wall Without Thermal Bridges

  Framing is the hole in a wall's insulation: what interrupts the thermal layer on a job site, and what restores it.

• Warm Deck or Cold Deck on a Flat Roof Over a Room

  Insulation above the deck or between the joists: on an extension or a dormer the answer is settled by how far the roof can rise.

• Insulating a Sloping Ceiling in a Room in the Roof

  A 100 mm rafter cannot carry 100 mm of board and a clear air path. How the depth gets split between, under and over, and what it costs.

• Insulating a Suspended Timber Ground Floor

  The void under a period floor is a drying machine. Insulate between the joists without switching it off, and the floor gets warm instead of rotten.

• Installing an EIFS Wall System

  An EIFS wall fails at its bond lines, not inside its materials, so build the laminate outward and protect every interface.

• Attaching Cladding Through Continuous Insulation

  Setting out a clip-and-rail grid over exterior insulation: three grids to reconcile, three fastener families to count, and one wall you cannot re-drill.

• Deciding Whether a Cavity Wall Should Be Filled

  The suitability survey takes twenty minutes and cannot be undone: what disqualifies a cavity wall from being filled, and what to do if it already was.

• Filling a Hollow Wall: ICF Cores and Grouted Block Cells

  A hollow wall gets ordered twice — forms and units by the face, concrete and grout by the void — and only the first number is on the elevation.

• Getting a Large Building Through Its Air Leakage Test

  The target divides by envelope area, not floor area, and on a big shed that one distinction is most of the result. Booking it, sealing it, and the day itself.

• Sealing and Conditioning a Crawlspace

  Encapsulating a crawlspace is dew-point work: every liner, foam board and dehumidifier decision either moves a condensing surface out of trouble or leaves it there.

• Draught-Proofing Doors and Windows

  A candle finds the gaps; the merchant needs lengths. Surveying an old house opening by opening, then turning the round into rolls, sweeps, film and tubes.

• Lagging Pipework

  Insulation on a pipe is asked to hold heat in, keep a static line above freezing and stop a cold main sweating, and the three want different builds.

• Boarding a Loft Without Crushing the Insulation

  Decking straight onto the joists buys a shelf and spends the insulation. The raise height, the leg grid and the eaves gap, worked in that order.

• Fitting a Radiant Barrier Under a Metal Roof

  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.

Terms

• R-value and U-value
• Batt, quilt and insulation roll
• Thermal bridge
• Airtightness
• Vapour control layer
• Pipe insulation
• Cavity wall / drainage cavity
• Sealant and caulk

How the numbers are worked out

Every calculator names the method it uses and the standard or guidance behind it. These are the methods the most of the calculators above rest on, and the published documents they cite.

Methods

• Thermal Resistance in Series

  Why R-values add but U-values do not, and why the assembly you build never performs like the assembly you specified. Applies to 12 of the calculators above.

• Rolls, Sheets and the Lap Between Them

  Why covering an area with rolls is a strip count rather than a division, why the lap is bought twice over, and why turning the strips the other way can change the order. Applies to 7 of the calculators above.

• Quantity, Waste and Rounding

  Why a material count is a ceiling function rather than a division, and why the waste factor — not the geometry — carries the risk. Applies to 6 of the calculators above.

• Unit Conversion, Exactness and Rounding

  Why most conversion factors carry no uncertainty at all, which ones hide a material assumption, and why converting a measurement can never make it more precise than it was. Applies to 6 of the calculators above.

• Airtightness, Infiltration and Moisture

  Why a blower-door result is not an air change rate, why a leak at the top of a house matters far more than the same leak halfway up, and why sealing without ventilating moves the problem rather than solving it. Applies to 4 of the calculators above.

• Coverage Rate and Spread

  How a datasheet figure becomes an order quantity, why it is quoted per coat and calculated for a smooth plane, and what a real substrate does to it. Applies to 2 of the calculators above.

Standards cited

• Approved Documents to the Building Regulations (England)

  England (Wales, Scotland and Northern Ireland have their own). Cited by 2 of the calculators above.

• National Electrical Code (NFPA 70)

  United States. Cited by 1 of the calculators above.