The right R-value is not one number — it depends on where you live, which assembly you are insulating (attic, wall, or floor), and whether the insulation will actually reach its rated performance once installed. This guide walks through all three so you buy the right amount of the right material, and install it in a way that gets you the R-value you paid for.
What's in this guide
Find Your DOE Climate Zone
The US Department of Energy divides the country into climate zones 1 through 7, plus a small zone 8 for the far north of Alaska, based on heating and cooling degree days. Zone 1 is hot and humid (south Florida); zone 7 is cold (northern Minnesota, interior Alaska). The zone number is the single biggest input into how much insulation makes sense for your house — a target that is generous in zone 2 can be genuinely inadequate in zone 6.
| Zone | General Region | Character |
|---|---|---|
| 1–2 | South Florida, south Texas, Hawaii, coastal Gulf | Hot, cooling-dominated; insulation matters less than air sealing and radiant barriers |
| 3 | Southeast, coastal California, most of the Southwest | Mixed-hot; both heating and cooling loads, cooling usually larger |
| 4 | Mid-Atlantic, Ohio Valley, Pacific Northwest | Mixed; meaningful heating and cooling both |
| 5 | Upper Midwest, New England, mountain states | Cold; heating-dominated most of the year |
| 6–7 | Northern Plains, far north New England, interior Alaska | Very cold; heating dominates almost entirely |
You do not need a map to find yours — search "DOE climate zone" plus your county, or check with your local building department, since the zone also determines your energy code requirements for new construction and major renovations. Coastal areas sometimes fall in a "marine" subzone that follows slightly different rules than the interior zone at the same number; marine zone 4, for instance, generally follows zone 5 wall requirements.
R-Value Targets by Assembly
Attic, wall, and floor are three different problems, and they do not share a target. Heat rises, so the attic is almost always your highest-value R and the cheapest to add — it is one flat, open, accessible surface, versus a wall you have to open up or a floor you have to work from underneath.
| Climate Zone Group | Attic | Wall Cavity | Floor (over unconditioned space) |
|---|---|---|---|
| Zones 1–2 | R-30 to R-49 | R-13 to R-15 | R-13 |
| Zone 3 | R-49 to R-60 | R-13 to R-21 | R-19 to R-25 |
| Zones 4–5 | R-60 | R-20 to R-30 | R-25 to R-30 |
| Zones 6–7 | R-60 | R-30 | R-30 to R-38 |
These are DOE and ENERGY STAR retrofit ranges, not a code citation — the IECC edition your jurisdiction has adopted is the number that governs a permitted job, and it changes every few code cycles. Treat this table as your planning starting point and confirm the specific requirement locally before you buy for anything that needs a permit.
Two things worth knowing before you shop: wall targets above about R-21 usually mean two layers, since a standard 2×6 cavity tops out around there with fiberglass — reaching R-30 means cavity insulation plus continuous rigid board outside the studs (written as "R-20 + R-5ci"). And the attic is almost always the best place to spend the budget first. Dollar for dollar, blown-in over an open attic floor delivers more comfort and energy savings per dollar than the equivalent work in a finished wall.
Get exact depth, batt, and bag counts for your zone and assembly
Insulation Calculator →Insulating an attic specifically? Use the dedicated calculator
Attic Insulation Calculator →R-Value per Inch, by Material
Once you know your target R-value, the material you choose decides how many inches of depth it takes to get there — which matters enormously in a shallow 2×4 wall or a low-clearance floor cavity.
| Material | R per Inch | Notes |
|---|---|---|
| Blown fiberglass (loose fill) | R-2.2 to R-2.7 | Common for open attic floors; settles slightly over time |
| Fiberglass batt | R-3.1 to R-3.4 | Standard walls, floors, accessible attics |
| Blown cellulose | R-3.2 to R-3.8 | Attics and dense-packed existing walls |
| Mineral wool batt | R-4.0 to R-4.3 | Holds shape, does not burn, roughly 30% more R per inch than fiberglass batt |
| Open-cell spray foam | R-3.5 to R-3.9 | Air seals and insulates in one pass; vapor-permeable |
| EPS rigid board | R-3.6 to R-4.2 | Stable over time; slabs, exterior sheathing, budget continuous insulation |
| XPS rigid board | R-5.0 new, drifting toward R-4.5 | Blowing agent diffuses out over years; good below grade |
| Polyiso rigid board | R-5.6 to R-6.5 (warm), drops in cold | Highest new R per inch, but underperforms its label in cold-climate winters |
| Closed-cell spray foam | R-6.0 to R-7.0 | Highest stable R per inch; also an air and vapor barrier at 1.5–2 in |
Closed-cell spray foam is the only material that gets meaningful R-value into a genuinely shallow cavity — a 2×4 wall filled with closed-cell performs close to what a 2×6 fiberglass wall does. It costs several times as much as batts per square foot, which is why it tends to show up in specific spots (rim joists, cathedral ceilings, below-grade walls) rather than whole houses.
Why Compression Destroys R-Value
R-value comes from still air trapped in the material's structure, not from the material itself. Compress it and you remove the air pockets that do the work — R-value drops roughly in proportion to the thickness you lose, not gently. An R-19 batt folded over at the top of a wall cavity is worth something closer to R-13 in that spot. An R-30 batt crammed into a 2×8 floor bay it does not fit is R-30 on the label and closer to R-25 once it is squeezed in.
This shows up in two common ways. First, buying the wrong length: an 8 ft wall's clear stud height is about 92.5 in, so a 93 in batt fits flush and a 96 in batt folds over at the top of every bay, losing R-value in exactly the spot where ceiling heat loss is worst. Second, the classic 2×6 mismatch — R-19 fiberglass is manufactured at 6.25 in thick and gets squeezed into a 5.5 in cavity, landing around R-18. R-21 batts are manufactured at 5.5 in specifically to fill a 2×6 bay without compression; if you have a 2×6 wall, that is the batt to buy, not R-19.
The other loss mechanism is gaps, not compression: a batt that covers 95% of a cavity does not deliver 95% of its R-value, because the uncovered strip becomes a path for air to bypass the insulation entirely — closer to 70% of the rated performance. Cut batts to fit snugly wall-to-wall and split them around wiring and plumbing instead of compressing the batt behind the obstruction.
Air Seal Before You Insulate
Insulation slows conductive heat flow through a material. It does almost nothing to stop air movement — fiberglass, in particular, is close to an air filter, not an air barrier. A tightly sealed attic at R-38 routinely outperforms a leaky one at R-60, because the leaky one is losing conditioned air straight through gaps that no amount of R-value fixes.
The sealing order that actually works: seal the big holes first — open chases around chimneys and flues, dropped soffits above kitchen cabinets, the top plates of interior walls, and the gap around plumbing stacks. These few openings leak more air, combined, than every small penetration in the house. Then seal the small ones: wiring holes, ceiling boxes, recessed can housings, bath fan bodies, and the attic hatch perimeter. Do all of it before insulation goes down — once it is covered, none of it is reachable again without pulling insulation back out.
If you are insulating a vented attic, install a baffle in every rafter bay along the eaves before blowing or laying insulation, not just a few. Baffles keep the soffit-to-ridge air path open; skip them and insulation packed out to the eave blocks the vents, the attic stops drying, and in cold climates that leads to ice dams at the exact eaves you just insulated.
Work out attic depth, bags, and baffle count together
Attic Insulation Calculator →Before You Touch an Old Attic
A handful of situations mean stop and get an assessment before you insulate at all, rather than push ahead:
- Vermiculite. Loose, pebbly, grey-brown granules — often sold as Zonolite — are frequently contaminated with asbestos from the source mine. Do not disturb it, sweep it, vacuum it, or blow insulation over it until a lab has tested a sample.
- Knob-and-tube wiring. This older wiring type sheds heat into open air by design, and covering it with insulation is prohibited by code in most jurisdictions because it can overheat. It needs to be replaced before the space is insulated, not worked around.
- Non-IC-rated recessed lighting. Older can lights not labeled "IC" (insulation contact) need at least 3 in of clearance on every side. Burying one is a real fire risk. Either build a dam around it or, better, swap it for an airtight IC-rated LED housing before insulating — that also closes a significant air leak.
- Chimneys and flues. Keep the code-required clearance (commonly 3 in to masonry chimneys and B-vent flues) using rigid, non-combustible material — never expanding foam against a metal flue.
Once the attic is assessed, sealed, baffled, and clear of anything that needs to stay uncovered, insulating it is one of the highest-return jobs in a home — and, unlike a finished wall, one most homeowners can safely do themselves.
Frequently Asked Questions
How do I find my DOE climate zone?
Search "DOE climate zone" plus your county, or ask your local building department — they use the same zone to set energy code requirements. The US runs zones 1 (hot, south Florida) through 7 (cold, northern Minnesota and interior Alaska), with zone 8 covering the far north of Alaska.
What R-value do I need for my attic?
Typically R-30 to R-49 in zones 1–2, R-49 to R-60 in zone 3, and R-60 across zones 4 through 7. The attic is usually the cheapest, highest-return place to add R-value in a house — confirm the exact target for your zone and any local code requirement before buying.
What R-value do I need for exterior walls?
Roughly R-13 to R-15 in zones 1–2, R-13 to R-21 in zone 3, R-20 to R-30 in zones 4–5, and R-30 in zones 6–7. Targets above about R-21 usually require both cavity insulation and a layer of continuous rigid board outside the studs, since a standard 2×6 cavity tops out around R-21 on its own.
Does compressing insulation reduce its R-value?
Yes, roughly in proportion to the thickness lost, because R-value comes from trapped still air that compression squeezes out. A folded-over batt at the top of a wall, or an R-19 batt squeezed into a 5.5 in 2×6 cavity, performs noticeably below its rating — the fix is buying the batt manufactured for that cavity depth (R-21 for a 2×6) rather than compressing a thicker one.
Should I air seal before or after insulating?
Before, always. Insulation slows conductive heat loss but does very little to stop air movement. A well-sealed attic at R-38 commonly outperforms a leaky one at R-60. Seal the big gaps (chases, dropped soffits, top plates, plumbing stacks) and the small ones (wiring holes, can lights, the attic hatch) before any insulation goes down, since none of it is reachable once covered.
What is R-value per inch and why does it matter?
It is how much R-value a material delivers per inch of thickness, and it matters most in shallow cavities. Fiberglass batt runs about R-3.1 to R-3.4 per inch, mineral wool about R-4.0 to R-4.3, and closed-cell spray foam R-6.0 to R-7.0 — the only material that gets meaningful R-value into a genuinely thin 2×4 wall.
Is it safe to insulate over old attic insulation?
Usually, yes — blowing loose fill over old fiberglass or cellulose is normal. Two exceptions: never insulate over wet insulation without first finding and fixing the leak, and never insulate over vermiculite until it has been tested for asbestos, since a large share of it came from a contaminated source mine.
Can I insulate over a non-IC-rated recessed light?
No, not directly. Non-IC-rated housings need at least 3 in of clearance on every side or they present a real fire risk. Build a dam around it with rigid material, or replace it with an airtight, IC-rated LED housing before insulating — the swap is usually cheaper than building dams and closes an air leak at the same time.
What R-value do I need for a floor over a crawl space?
Roughly R-13 in zones 1–2, R-19 to R-25 in zone 3, R-25 to R-30 in zones 4–5, and R-30 to R-38 in zones 6–7. Support batts with wire or netting so they stay tight against the subfloor rather than sagging and losing contact, which creates the same convective bypass problem as a gap in a wall.
Why does my attic have ice dams after I added insulation?
Almost always missing or blocked baffles at the eaves. A vented attic relies on airflow from the soffits to the ridge to stay cold and dry; insulation packed to the eaves without a baffle in every rafter bay blocks that path, the roof deck stays warmer and damp, and meltwater refreezes at the cold eaves as an ice dam.