How to Use the Insulation Calculator
- Choose your units. USA mode uses square feet, inches and R-values. UK mode uses square metres, millimetres and roll depths.
- Choose the area you're insulating: attic/loft floor, walls (between studs), or floor (over a crawlspace or unheated basement).
- Enter the area. For an attic or loft, that's the floor area of the space; for walls, the wall area minus windows and doors.
- Enter what's there now (existing depth or R-value) so the calculator only adds what you need.
- Pick your climate zone (USA) or target depth (UK). The calculator suggests a target.
- Choose the material: batts or rolls, blown fiberglass, or blown cellulose.
- Add a price per roll, batt pack, bag or square foot (optional) to see the cost.
How Much Insulation Do I Need? (USA)
In the USA, insulation is rated by R-value, its resistance to heat flow. Higher is better. ENERGY STAR's recommendations, based on the 2021 International Energy Conservation Code (IECC), are:
| Climate zone | Example areas | Attic, uninsulated | Attic, 3–4 in already there (add) | Floor |
|---|---|---|---|---|
| 1 | South Florida, Hawaii | R-30 | R-25 | R-13 |
| 2 | Gulf Coast, Phoenix | R-49 | R-38 | R-13 |
| 3 | Atlanta, Dallas, Southern California | R-49 | R-38 | R-19 |
| 4A, 4B | Washington DC, Kansas City | R-60 | R-49 | R-19 |
| 4C, 5, 6 | Seattle, Chicago, Denver, Minneapolis | R-60 | R-49 | R-30 |
| 7, 8 | Northern Minnesota, Alaska | R-60 | R-49 | R-38 |
For walls, a 2×4 wall holds R-13 to R-15 batts and a 2×6 wall holds R-19 to R-21. When the siding comes off, ENERGY STAR recommends adding insulated sheathing: about R-5 in zone 3, and R-5 to R-10 in zones 4–8.
Depth needed for each material
R-value depends on the material's thickness:
| Material | Approx. R-value per inch | Depth for R-38 | Depth for R-49 | Depth for R-60 |
|---|---|---|---|---|
| Blown fiberglass | R-2.5 | 15.2 in | 19.6 in | 24 in |
| Blown cellulose (settled) | R-3.5 | 10.9 in | 14 in | 17.1 in |
| Fiberglass batts | about R-3.1–3.3 | 12 in (R-38 batt) | about 14–16 in | two layers |
| Mineral wool batts | about R-4 | 9.5 in | 12.3 in | 15 in |
Common batt sizes: R-13 (3½ in, for 2×4 walls), R-19 (6¼ in), R-21 (5½ in, high density, for 2×6 walls), R-30 (about 9½–10 in) and R-38 (about 12 in).
Worked example (USA): topping up an attic
A 1,000 sq ft attic in zone 5 with about 3½ inches of old fiberglass. ENERGY STAR suggests adding R-49.
- Blown cellulose: 49 ÷ 3.5 = 14 inches more; about 58 bags at a typical 1.18 bags per 1,000 sq ft per R
- Blown fiberglass: 49 ÷ 2.5 = 19.6 inches more; about 25 bags (a typical fiberglass chart shows about 19 bags per 1,000 sq ft for R-38)
- Installed cost: at about $1–$3 per sq ft, roughly $1,000–$3,000
Bag coverage varies a lot between brands, so the calculator lets you enter the "bags per 1,000 sq ft" figure from the chart on your bag.
Worked example (USA): wall batts
800 sq ft of 2×4 wall (after subtracting windows and doors), with studs at 16 inches on center, using 15 × 93 inch R-13 batts:
How Much Loft Insulation Do I Need? (UK)
The UK recommendation for lofts is at least 270mm of mineral wool (350mm in Scotland). The usual method is two layers:
- About 100mm between the ceiling joists, which are typically about 100mm deep.
- About 170mm laid across the joists at right angles, which covers the joists and stops heat escaping through the timber.
At a typical thermal conductivity of 0.044 W/mK, 270mm gives an R-value of about 6.1 m²K/W and a U-value of roughly 0.16 W/m²K, which meets the Building Regulations target for upgrading a loft.
Loft roll sizes and coverage
Typical coverage for a popular brand of 0.044 mineral wool loft roll:
| Roll thickness | Approx. coverage per roll |
|---|---|
| 100mm | 13.89 m² |
| 150mm | 9.18 m² |
| 170mm | 8.01 m² |
| 200mm | 6.84 m² |
Different brands use different roll lengths, so check the coverage on the packaging. The calculator defaults to these figures and lets you change them.
Worked example (UK): 45 m² loft
Empty loft, insulating to 270mm
Topping up from 100mm to 270mm: you only need the 170mm cross layer, so 7 rolls.
Cost: materials are roughly £10 per m², so about £450; professional installation typically costs about £27–£40 per m² for rolls, or about £1,200–£1,800.
Blown-In vs Batts vs Rolls
- Batts and rolls are the easiest DIY option for open attics and stud walls. They work best where joists are evenly spaced and there are few obstructions.
- Blown fiberglass is light, doesn't settle much, and is fast to install over large attics. You need more depth than cellulose for the same R-value.
- Blown cellulose is made from recycled paper, has a higher R-value per inch, and fills gaps around wires and pipes well. It settles over time, so installers blow it slightly deeper.
- Spray foam seals air leaks as well as insulating, but costs more and needs a professional. In the UK, spray foam on the underside of roofs can cause problems with mortgage valuations and surveys, so take advice first.
Whichever you choose, air sealing gaps around light fittings, pipes, the loft hatch and top plates first makes the insulation work far better. Air sealing typically costs about $200–$1,500 if you hire it out.
Insulation Costs
USA (installed, per sq ft)
| Material | Typical cost |
|---|---|
| Fiberglass batts | $0.80–$2.50 |
| Blown fiberglass | $1–$2.50 |
| Blown cellulose | $1.20–$3 |
| Mineral wool | $1.50–$3 |
| Spray foam | $1.50–$5 |
Removing old, damaged insulation adds about $1–$2 per sq ft.
UK (loft insulation to 270mm, installed)
| Home | Typical cost |
|---|---|
| Flat or 1-bed | £200–£500 |
| 2-bed terrace | £300–£700 |
| 3-bed semi | £500–£1,200 |
| 4-bed detached | £900–£1,800 |
Insulating an empty loft to 270mm can save a gas-heated home roughly £210–£390 a year; topping up from 120mm saves much less, around £20–£35, so check what you already have before you spend. Some households can get insulation funded through energy company or local council schemes; check eligibility with your council or energy supplier.
Common Mistakes to Avoid
- Blocking ventilation. Keep insulation back from the eaves or use baffles so air can flow through the roof space. Blocked vents cause condensation and rot.
- Squashing insulation. Compressed insulation loses much of its R-value. Don't board over a full-depth loft without raising the boards on loft legs.
- Covering hot fittings. Keep insulation away from non-rated recessed lights, flues and chimneys.
- Forgetting the hatch. Insulate and draught-strip the loft hatch or attic door.
- Insulating over damp. Fix roof leaks and damp first.
- Wrong vapor barrier placement. In cold US climates, the vapor retarder goes on the warm (interior) side. Don't add a second one when topping up.
If you're also replacing windows, the Window Replacement Cost Calculator and Double Glazing Cost Calculator estimate those costs. For a loft you plan to turn into a room, see the Loft Conversion Cost Calculator, because the insulation goes in the roof slope instead of the floor.
Frequently Asked Questions
How much insulation do I need in my attic?
ENERGY STAR recommends R-30 to R-60 depending on climate zone. Most of the USA (zones 4–8) should aim for R-60 in an uninsulated attic, or add R-49 on top of 3–4 inches already there.
How many inches is R-49?
About 14 inches of blown cellulose or about 19½ inches of blown fiberglass.
How many bags of blown-in insulation do I need?
Multiply the attic area (in thousands of sq ft) by the bags-per-1,000-sq-ft figure for your target R-value on the bag's chart. For R-38 fiberglass, that's often about 19 bags per 1,000 sq ft.
How much loft insulation do I need in the UK?
At least 270mm (350mm in Scotland), usually 100mm between the joists and 170mm across them.
How many rolls of loft insulation do I need?
Divide the loft area by the roll's coverage and add 10%. For a 45 m² loft, that's about 4 rolls of 100mm and 7 rolls of 170mm.
Is it worth topping up loft insulation?
If you have less than about 100mm, yes. If you already have 200mm or more, the extra savings are small.
What R-value do I need for walls?
R-13 to R-15 in 2×4 walls and R-19 to R-21 in 2×6 walls, plus insulated sheathing in colder zones when re-siding.
Get quotes from local insulation installers
An installer can check ventilation, air leaks and the right material for your home.