Insulation calculators and R-value targets

Which insulation tool should I use?

Start with the target, then count. The R-value by climate zone guide shows how to find your zone and the code level for each part of the house. With that number, the batts or rolls of insulation calculator turns the area of an attic, wall or floor into packages of batts or bags of loose fill. If you have not chosen a material yet, fiberglass vs foam vs mineral wool compares them by R-value per inch and by where each one suits.

The category is small on purpose. One calculator covers every common product because the method is the same: find the target, find the area, divide by the coverage the maker publishes for that R-value. The tables on this page give the targets so you can check the inputs.

Insulation pages

What R-value does the US code require by climate zone?

The table reproduces the main columns of the 2021 IECC prescriptive table. Ceiling values rise with the zone number, and floors follow a separate scale. Wall entries that give two figures joined by "ci" mean a cavity value plus continuous insulation on the outside of the studs; the alternatives separated by "or" are equivalent ways to comply. Code R-values count insulation only, not drywall, sheathing or air films; cavity and continuous insulation are checked separately.

There is a useful exception at the eaves, where a full-depth layer is hard to fit. Where R-49 is required, R-38 over the whole attic counts if full-height R-38 extends over the wall top plate at the eaves (R-49 likewise for R-60). For homes that already have some insulation, ENERGY STAR publishes how much to add rather than a total. ENERGY STAR suggests adding R-25 to R-49 over 3-4 in of existing attic insulation, depending on zone.

How do UK U-values compare with R-values?

Not directly, and treating them as interchangeable is a common error. A U-value is the reciprocal of the total RSI of all layers including surface resistances, so it cannot be compared directly with an insulation-only R-value. Multiply an imperial R-value by 0.1761 to get RSI (m2·K/W); multiply RSI by 5.678 for imperial R. So a UK limit tells you how the whole wall or roof must perform, while a US R-value names the insulation layer alone.

England's limits for new homes are set in Approved Document L. In England, new-dwelling roofs must not exceed U = 0.16 W/m2K (limiting value). New-dwelling walls in England must not exceed U = 0.26 W/m2K. New-dwelling floors in England must not exceed U = 0.18 W/m2K. Approved Document L's limiting U-values are backstops; some elements usually need to be significantly better to meet overall targets. The calculator works in either system, but for a UK project confirm the build-up with the insulation maker's U-value calculation.

Why does blown-in insulation list both bags and depth?

Because either one alone can mislead. Loose fill can be blown fluffy to reach the depth with too few bags, or packed to use the bags without reaching the depth. A stated blown-in R-value needs both the minimum bags per 1,000 sq ft and the minimum thickness. For blown-in insulation, code compliance uses the manufacturer's settled R-value. The two figures from one fiberglass maker show the scale: AttiCat needs at least 19 bags per 1,000 sq ft for R-38. AttiCat must be blown at least 13.5 in thick for R-38. Cellulose needs more bags for the same R-value because it is denser. GreenFiber INS515LD needs at least 40.9 bags per 1,000 sq ft for R-38.

Batts are simpler. The package states its area, and widths match stud spacing. Batts for 16 in o.c. framing are 15 in (381 mm) wide and for 24 in o.c. framing 23 in (584 mm) wide; 16 and 24 in widths also exist. R-13 fiberglass batts are 3.5 in (89 mm) thick, sized for 2x4 walls. R-21 batts fit 2x6 walls at 5.5 in (140 mm). An Owens Corning EcoTouch R-13 package of 15 x 93 in pieces (11 pieces) covers 106.6 sq ft.

How many homes are under-insulated?

A large share on both sides of the Atlantic. The US Energy Information Administration's 2024 Residential Energy Consumption Survey, still preliminary, found that About 20% of US primary homes (26.8 million of 132.5 million) were reported as poorly insulated (23.3 million) or not insulated (3.4 million) in EIA's 2024 Residential Energy Consumption Survey (preliminary). About 14% of US homes (18.2 million) were reported too drafty in winter most or all of the time, and another 54.8 million some of the time, in EIA's 2024 RECS (preliminary). These are householders' own reports about their primary homes, not measured R-values.

England's housing survey measures loft depth directly. In 2024, 42% of dwellings in England had 200mm or more of loft insulation, up from 39% in both 2014 and 2022 (English Housing Survey 2024-25, MHCLG). Over 2.7 million households in England lived in homes with less than 100mm of loft insulation in 2024-25; 1.2 million of them (43%) also had uninsulated solid walls (EHS home insulation fact sheet, May 2026). 54% of dwellings in England had cavity or solid wall insulation in 2024, up from 48% in 2014; among cavity-wall homes, 73% of owner-occupied but only 60% of private rented had it insulated (English Housing Survey 2024-25).

The payoff is in heating. ENERGY STAR estimates homeowners save an average of 15% on heating and cooling costs (11% of total energy costs) by air sealing and adding insulation to attics, crawlspace floors and basement rim joists. That is an ENERGY STAR average across homes, not a promise for any single house. Material costs have risen sharply: US producer prices for insulation materials were 50.3% above the 2020 average in September 2026 and 2.7% higher than a year earlier (BLS PPI). UK producer prices for thermal and acoustic insulating materials stood at 206.7 in August 2026, roughly double 2015 levels, and 4.3% higher than a year earlier (DBT Table 2, ONS data).

The under-insulated homes data page carries the full survey tables. Price series are on the lumber and drywall price trends page, and debris figures on the construction waste statistics page.

Where do the insulation tools stop?

They count material for a cavity or attic floor you have measured. They do not check ventilation, vapour control or air sealing, and those decide whether insulation works as rated. Moisture trapped in the wrong place can rot framing, and gaps around pipes and wiring can undo much of the gain; the ENERGY STAR savings figure above combines air sealing with insulation for that reason.

Code R-values are minimums for new work and for alterations that trigger the energy code. Local amendments differ, and some states use an older edition. Read formulas and data sources for how each value is checked. Walls also need finishing, covered in the walls category, and roof insulation often goes in during a re-roof: see all roof estimating tools.

Insulation questions