The Insulation Book

How We Check

Where The Insulation Book gets its R-values, bag counts, clearances and code rules, the sources we use with their dates, and how often figures are rechecked.

Insulation figures are easy to get wrong, because the sources keep moving and old numbers stay online. ENERGY STAR’s 2016 do-it-yourself guide says the recommended level for most attics is R-38, while its live retrofit table gives R49 to R60 for an uninsulated attic in climate zones 2 to 8. The 2024 model energy code asks for less than that table in a new home in most zones. The Department of Energy’s consumer pages on insulation and air sealing answered “page not found” when we checked on 10 November 2026. The federal tax credit for insulation ended for work placed in service after 31 December 2025, yet pages still promise it. A wrong clearance or a missed stop can cause a fire, an asbestos exposure or a carbon monoxide problem. This page sets out where our figures come from, how we word what we could and could not read, how we show sources that disagree, how often everything is rechecked, which documents we use, and where our limits are.

General information, not professional advice. Building codes are adopted locally, with changes, and your building department decides what applies to your house. Where one of our pages marks a stop, stop and call a qualified professional.

Where figures come from

We go to the source that made the figure or the rule, and link it at the claim:

  • ENERGY STAR, for targets, steps and stops. Its retrofit table of recommended insulation levels for existing wood-framed homes is the target on our pages and in our tools. Its do-it-yourself guide and project pages give the steps we repeat and the conditions that call for a contractor first. ENERGY STAR’s levels are recommendations it calls cost-effective, not a code.
  • EPA and the Consumer Product Safety Commission, for safety lines. EPA’s pages on vermiculite, asbestos, carbon monoxide and mold, and the commission staff’s fact sheet on spray foam, are quoted in their own words. We don’t soften them.
  • The Code of Federal Regulations, for what a label and a receipt must show. The Federal Trade Commission’s R-value Rule (16 CFR part 460) makes every package of loose-fill insulation carry a chart of bags per 1,000 square feet and minimum thickness by R-value, and makes an installer give a signed receipt. The Consumer Product Safety Commission’s cellulose rules (16 CFR parts 1209 and 1404) set a safety standard for cellulose insulation and the caution label its containers must carry.
  • Makers’ own sheets, for bag counts and thicknesses. Bags, installed and settled depth, and R-value by thickness come from the maker’s coverage chart or data sheet for that one product, named with its publication number and date. The sheets are copyrighted, so we use their numbers as data, with attribution, and describe the rest in our own words. Some makers’ sheets are only online as copies on a retailer’s site; we use those as the maker’s document and say so. A maker’s figure is never a recommendation of the product.
  • Model codes, by section and edition. Vent ratios, vapor retarder classes, foam barriers, clearances around recessed lights and new-home minimums come from the International Residential Code and the International Energy Conservation Code, read on the publisher’s site. The codes are copyrighted, so we paraphrase and cite the section and edition. A model code is not the law where you live: states and cities adopt their own editions with changes, and your building department decides.
  • The Department of Energy, from dated archived copies. The department removed its consumer pages on insulation in 2026. We read them in the Internet Archive, cite them as “DOE, archived” with the date of the copy, and say the live page is gone. Program notices and the weatherization pages that are still live are cited from energy.gov.
  • State rules in official text. Where a state rule matters, such as the knob-and-tube exceptions in Washington and Oregon, we read the state’s own published text.
  • Money and programs. The tax credit comes from the public law and the IRS, energy prices and heat-content factors from the Energy Information Administration, rebate program status from the state energy officials’ association table, and the Zonolite trust’s terms from the trust’s own pages.
  • Contractor, installer, retailer how-to, cost-guide, forum and video pages: never a source for a rule, a figure, an example or a statistic.

How a figure gets on the page

Each figure and rule is sorted by how directly it was read, and that decides how we word it:

  • Read. The primary document was opened and read on the date shown. We state the figure plainly, with its source and the date. Before a writer uses it, the passage is read again in full, because conditions and exceptions often sit a line or two further down.
  • Partly read. We read only part of the document, or read it through an official text that repeats it. For example, we have not opened the National Electrical Code itself, so its knob-and-tube rule is described “as Oregon’s 2023 code prints it” and its recessed light rule as the 2024 International Residential Code carries it. The page says which text we read.
  • Not read. The figure stays off the page, or the page says “not read” and sends you to the source, the maker or your building department.

Why some items say “not read”

“Not read” means we have not opened that item in a primary source we would stand behind. It says nothing about what the rule or figure is. Some standards sit behind an account or a paywall. Some makers publish no coverage chart we could find on their own site. Some official sites refuse the connection. In those cases the page says so, rather than borrowing a number from a contractor, a retailer’s how-to page or a forum.

The same goes for gaps inside a source. The climate zone guide we use leaves out four counties, and we don’t fill them in from memory. One open-cell spray foam data sheet gives its R-value at 1 inch only, so our thickness tool does not print a figure for it at 3.5 inches. When we can’t find a rule on a point, we say “none found in the sections we read”, not “there is none”.

How we show sources that disagree

Primary sources contradict each other on this topic more than on most. We name both sides, say which one we follow and why, and date each:

  • The attic target. ENERGY STAR’s retrofit table (R30 in zone 1, R49 in zones 2 and 3, R60 in zones 4 to 8 for an uninsulated attic) is our target. The 2024 International Energy Conservation Code’s ceiling minimum for new homes (R-30 in zones 0 and 1, R-38 in zones 2 and 3, R-49 in zones 4 to 8) is shown beside it and labelled as a model-code minimum. ENERGY STAR’s table says it is based on the 2021 edition of that code, whose ceiling row it matches. ENERGY STAR’s 2016 guide (“R-38 or about 10 to 14 inches”) and a second, older table of ranges still on its site are explained and never used as the target.
  • Vermiculite. ENERGY STAR’s 2016 guide says not to disturb vermiculite “unless you’ve had it tested”. EPA’s asbestos pages say to assume it contains asbestos and not to disturb it. We follow EPA.
  • Savings. ENERGY STAR’s methodology models an average saving of 15% of heating and cooling costs, 11% of total energy costs, for a typical house built from 1970 to 1989. Another page on the same site says “up to 10%”. We quote the methodology, with its model, and always with “savings vary”.
  • A maker’s sheet and the code. Where a maker’s sheet sums up a code rule differently from the code section we read, we cite the code for the rule and the sheet only for the product’s own figures.

How figures are dated

A figure is right only for a certain table, edition, product or period, so we give that, not just the number:

  • Targets with their zone and table. An R-value target names its climate zone and ENERGY STAR’s table, with the day we read it, because the live table prints no date.
  • Code figures with section and edition. For example “IRC 2024 R806.2”. The 2024 edition moved the foam plastic rules from section R316 to R303, so we give the 2024 number and note the older one.
  • Makers’ figures with publication number and date. For example, Owens Corning’s AttiCat fact sheet (10011287-E, September 2019) prints 25 bags per 1,000 square feet at R-49 and Greenfiber’s SANCTUARY cellulose sheet (PM-6.3328 Rev C, May 2020) prints 72.6. A single “bags per square foot” figure is wrong for most products.
  • Archived pages with the copy’s date. For example “DOE, archived 22 June 2026”.
  • Prices with their period. For example, the US residential electricity price of 18.16 cents per kWh is the Energy Information Administration’s preliminary figure for August 2026 (Electric Power Monthly, Table 5.6.A, read 10 November 2026).
  • Programs with their status date. A rebate program’s status carries the date of the table it came from, and “launched” is never written as “money available”.
  • A checked date on every page. Each page ends with the date its sources were checked.

How calculators work

Our calculators give estimates and show their working. How much insulation do I need estimates the R-value of what you have from its depth, using ENERGY STAR’s multipliers for existing insulation, and sets it against ENERGY STAR’s target for your zone, with the 2024 model-code minimum beside it. The gap is our arithmetic and is labelled so. The blown-in insulation calculator reads bags and minimum thickness from the maker’s chart for the product you pick, and lists only products whose chart we read. The thickness tool lists only materials with a data sheet we read. The attic ventilation calculator applies the vent ratios in section R806.2 of the 2024 International Residential Code to the area and vent sizes you enter. The savings calculator applies ENERGY STAR’s modeled percentage for your zone to your own bills and your own job price.

Each formula is tested against answers worked by hand before the calculator goes live. Every default that is a rate or a rule says where it comes from and when we read it, or that it is our assumption, and you can change it. No tool prices a job, and no tool says that a house is up to code or safe.

How pages are checked

After a page is written, a separate fact-check compares each target, thickness, bag count, clearance, ratio, percentage, price, date, code section and safety line with its source, and reworks every sum on the page. A final review looks for errors, gaps and anything that could mislead a reader about a clearance, a stop or what a figure applies to. A page with a serious problem still open does not go out.

How often figures are rechecked

  • ENERGY STAR’s tables and the code tables: each January and July.
  • Energy prices: at each update of a page or tool that uses them, from the Energy Information Administration’s latest release.
  • Rebate program status: monthly.
  • The Zonolite trust’s terms: at each update of a page that cites them, because the trust says its payment percentage can be reduced at any time.
  • Makers’ charts and data sheets: each January and July, and when we learn a maker has issued a new sheet.
  • Archived Department of Energy pages: each January and July we check whether a live page has returned, and cite it if it has.
  • Everything else: at least every six months, and sooner when we learn a source has changed.

A page shows an updated date only when its content really changed. Between rechecks a source can change without our knowing, which is why each figure shows the day we read it.

What is an assumption

Where we work something out ourselves, we say so. Arithmetic is shown step by step. A gap between a target and an estimate is labelled as our arithmetic. A count we made from a source’s table is labelled as our count. A conclusion a source does not state is labelled as ours. ENERGY STAR’s retrofit table has a column headed “If you already have 3–4 inches of insulation” and does not say whether its figures are a total or an amount to add, so we print the column under its own heading and don’t choose a reading for it.

How we treat claims seen elsewhere

Claims from sellers are reported as claims, never as fact, and we don’t name a seller when we question one. Some lines you may see, checked as of 10 November 2026:

  • “R-38 is the recommended attic level.” That is the figure in ENERGY STAR’s 2016 guide (EPA 430-F-04-024). Its live retrofit table gives R49 for an uninsulated attic in zones 2 and 3 and R60 in zones 4 to 8. See how much insulation you need.
  • “You can still get 30% back on insulation.” Not for work placed in service after 31 December 2025 (Public Law 119-21, section 70505, amending 26 U.S.C. 25C). While the credit ran, the IRS says, labor for installing insulation did not count. See the insulation tax credit.
  • “Insulation can cut your energy bill in half.” ENERGY STAR’s methodology models an average of 15% of heating and cooling costs from air sealing and insulating a typical 1970 to 1989 house, from 7% in zone 1 to 20% in zone 4C. The Federal Trade Commission requires a seller who makes a savings claim to have a reasonable basis for it and to say “Savings vary” (16 CFR 460.19). See the savings calculator.
  • “Test your vermiculite, then decide.” EPA says you “should assume that vermiculite insulation is from Libby and treat the material as if it contained asbestos by not disturbing it” (EPA). See vermiculite insulation.
  • “You need so many bags per 1,000 square feet.” It depends on the product. The count on the bag’s own chart is the one the federal rule requires (16 CFR 460.12), and charts we read range from 21.5 to 72.6 bags per 1,000 square feet at R-49. See the blown-in insulation calculator.
  • “A radiant barrier saves 10%.” The Department of Energy’s page said “some studies show” 5% to 10% of cooling costs in a warm, sunny climate (DOE, archived 17 June 2026; the live page was removed). See radiant barriers.

The sources we use

These are the main documents behind our pages, as of 10 November 2026. Each page names the ones it relies on, at the claim. “Read” dates are on the pages themselves.

ENERGY STAR (EPA)

EPA, CPSC and other health and safety agencies

Code of Federal Regulations

  • 16 CFR part 460, Labeling and Advertising of Home Insulation (the Federal Trade Commission’s R-value Rule): sections 460.5 (R-value tests), 460.12 (labels), 460.13 (fact sheets), 460.17 (what installers must tell their customers), 460.19 (savings claims) and 460.20 (R-value per inch claims)
  • 16 CFR part 1209, Interim Safety Standard for Cellulose Insulation
  • 16 CFR part 1404, Cellulose Insulation: section 1404.4, the caution label on every container of cellulose

We read these in the electronic Code of Federal Regulations as current on the day shown on each page.

Department of Energy

The department’s Energy Saver pages below returned “page not found” on energy.gov on 10 November 2026. We cite these Internet Archive copies:

Still live on energy.gov:

Model codes and state rules

Model codes are read on the International Code Council’s site. Your state or city may enforce a different edition, with changes.

  • 2024 International Energy Conservation Code, chapter 4: Table R402.1.3 and sections R402.2.1, R402.2.2, R402.2.4, R402.2.5, R402.2.9, R402.2.13 and R402.5.4; chapter 5: section R503.1.1 on alterations
  • 2021 International Energy Conservation Code, chapter 4: Table R402.1.3 and section R402.2.1
  • 2024 International Residential Code, chapter 3: sections R302.5 and R302.6 (garages), R302.10.1 (insulation facings) and R303.4 and R303.5 (foam plastic; section R316 in earlier editions)
  • 2024 International Residential Code, chapter 4: sections R408.1 to R408.4 (crawl spaces)
  • 2024 International Residential Code, chapter 7: section R702.7 and its tables (vapor retarders)
  • 2024 International Residential Code, chapter 8: sections R806.1 to R806.5 (attic ventilation and unvented attics) and R807.1 (attic access)
  • 2024 International Residential Code, chapter 15: sections M1501.1 and M1504.3 (where exhaust fans discharge)
  • 2024 International Residential Code, chapter 40: sections E4003.5, E4004.8 and E4004.9 (recessed lights)
  • Oregon, 2023 Oregon Electrical Specialty Code, Table 1-E, as amended 1 January 2026: the state’s knob-and-tube exception, and the text through which we read section 394.12(5) of the National Electrical Code
  • Washington, WAC 296-46B-394, effective 1 April 2024
  • California’s knob-and-tube exception, read in the state housing department’s final rulemaking text for the 2025 California Electrical Code (an archived copy, which prints only its opening line) and, for its six items, in the City of Berkeley’s form and the City of Menlo Park’s summary; the published state code itself was not read

Makers’ charts and data sheets

Each applies only to the product named. A product is listed because we read its sheet, not because we recommend it.

Money and programs

Our limits

Some sources can’t be read in full, because they sit behind an account or a paywall, refuse the connection or were never published online. As of 10 November 2026 we have not read the National Electrical Code itself, so pages describe its knob-and-tube and recessed light rules only through Oregon’s official code and the 2024 International Residential Code. We have not read California’s knob-and-tube exception in the state code, or the rule in any state beyond Washington, Oregon and California. We have not read a primary standard for testing fuel-burning appliances after air sealing, so pages give ENERGY STAR’s and EPA’s lines and no test method. We have not read a coverage chart for every loose-fill product, so our bag tool lists only the ones we have. We have no data sheet for polyiso board at low temperatures or for open-cell spray foam beyond 1 inch, so pages give no figure for either. We do not know which code edition any state or city enforces, and we do not say. We do not know how much rebate money is left in any state. We have not inspected, installed or tested anything ourselves.

No check catches everything, so please tell us if you find a mistake. Fixes are logged on our corrections page, and our editorial policy covers the rest of how pages are made.

Before you rely on a figure here, check it against its source, the chart on the bag or the label on the product in your hands, and ask your building department which rules apply. If you find vermiculite, old wiring, wet insulation or rot, stop and call the right professional. Don’t wait for a recheck of ours.

Sources on this page checked on 10 November 2026.