If you have ever compared insulation options, you have seen the term R-value stamped on every product spec. It is the single most useful number for judging how well insulation will keep your home comfortable — but only if you understand what it does and does not tell you.
What R-Value Actually Measures
R-value is a measure of thermal resistance — how well a material slows the flow of heat through it. The higher the number, the harder it is for heat to escape in winter or push in during summer. Two facts make the number practical to work with:
- R-values add up. Layer one material over another and their R-values combine, so an attic with R-38 of existing insulation plus an added R-11 reaches R-49 total.
- It is rated per inch. Every material has an R-value per inch of thickness, so the total you achieve depends on both the product and how deeply it is installed.
One honest caveat: R-value is measured in a lab and only accounts for heat that conducts through a material. It says nothing about air leaking around it — which is why the highest R-value in the world underperforms in a drafty house. More on that below.
R-Value by Insulation Type
Different materials deliver very different R-values per inch, and each has its place. Here is how the options we install compare:
- Closed-cell spray foam — about R-6 to R-7 per inch. The highest R-value per inch of any common insulation; our foam tests at roughly R-6.88 per inch. It is rigid, adds structural strength, and acts as both an air barrier and a vapor barrier, making it ideal for tight spaces, rim joists, and below-grade walls.
- Open-cell spray foam — about R-3.5 to R-3.8 per inch. A softer, more breathable foam at a lower cost. It expands to fill and seal irregular cavities and is a strong choice for interior walls and attic roof decks where vapor permeability is wanted.
- Fiberglass batt — about R-3 to R-4 per inch. The familiar, budget-friendly option for standard-depth wall and attic cavities. It performs well when it fills the space completely, with no gaps or compression.
- Blown-in and rigid board. Blown-in cellulose or fiberglass blankets an attic floor evenly around obstructions, while rigid foam board adds continuous R-value over walls and foundations.
Because closed-cell foam packs the most resistance into the fewest inches, it is often the right answer where depth is limited. Where there is room to build up thickness, a lower-cost material can reach the same total R-value for less.
Where R-Value Matters Most — and Why Air Sealing Comes First
Heat leaves a home wherever the barrier is weakest, so it pays to target the biggest offenders:
- The attic. Warm air rises and escapes through the roof, making the attic the highest-return upgrade in most homes. Guidelines for our New York and New Jersey climate generally call for roughly R-49 to R-60 here.
- Exterior walls, rim joists, and band boards. Often under-insulated in older homes and a major source of cold rooms.
- Basements and crawlspaces. Uninsulated foundation walls and floors pull heat down and out year-round.
Just as important: a great R-value only performs when air is not bypassing it. Gaps around wiring, plumbing, top plates, and rim joists let conditioned air leak out and drag your insulation's real-world performance down. That is why we pair insulation with air sealing and weatherization — the combination is what delivers the comfort and the savings, not the sticker R-value alone. Sealed and properly insulated, most of the homes we serve see 20% or more come off their monthly heating and cooling bills.
Choosing the right insulation is not about chasing the biggest R-value on the box — it is about matching the right material and the right air-sealing plan to your home. A free energy audit from The Home Insulators pinpoints exactly where your home is losing energy and what it will take to fix it. Family-owned and serving Westchester and Rockland County, NY and Bergen County, NJ since 2015, we are happy to walk you through it. Call 914-233-3084 to schedule your free audit.