Start with R-value, then convert to inches
Insulation thickness is not a number you can pick on its own. You need two figures: the R-value your wall must reach, and the R-value per inch of the material you choose. Thickness is then simple division: target R-value divided by R-value per inch.
R-value per inch depends on the material. Typical figures published by Pacific Northwest National Laboratory, a U.S. Department of Energy laboratory, are roughly 2.5 to 4 for fiberglass batt or board, 3 to 4 for mineral wool board, 3.8 to 4.4 for expanded polystyrene (EPS), about 5 for extruded polystyrene (XPS), and about 6 for polyisocyanurate. Use the figure printed on the label of the product you actually buy, because density, facing, and manufacturing change it.
Find your climate zone first
The 2021 International Energy Conservation Code, the model energy code that many U.S. states and cities adopt, sorts the country into eight climate zones, some with moisture subdivisions. Find your county on the zone map in the Department of Energy’s home insulation guidance. Your local building department tells you which edition of the code applies to your project.
For a wood-frame wall, the code lists combinations rather than one value. The first number is the cavity R-value (insulation between the studs). The second is the continuous R-value (insulation placed outside the framing, such as rigid board or insulated sheathing). R-13 + R-5 means an R-13 batt in the cavity plus R-5 of continuous insulation. The summary below is based on Department of Energy guidance and industry summaries of the 2021 table:
| Climate zone | Wood-frame wall options |
|---|---|
| 1 and 2 | R-13 cavity, or R-10 continuous with no cavity insulation |
| 3 | R-20 cavity, R-13 + R-5, or R-15 continuous with no cavity insulation |
| 4 and 5 | R-20 + R-5, or R-13 + R-10 (zone 5 also lists R-15 continuous with no cavity insulation) |
| 6 to 8 | R-30 cavity, or R-20 + R-5 |
In zones 4 and 5, the 2021 edition raised the options for above-grade framed walls, so cavity insulation alone no longer meets the table there. Each row is an alternative. You pick one option that fits your framing depth and detailing. Some state and local amendments change these values, and the table in your adopted code governs. Treat this summary as a planning guide and confirm the exact row before you buy material.
Why continuous insulation matters
Wood studs, plates, and headers conduct heat much better than batt insulation does. A cavity-only wall therefore loses heat through the framing, which is why the code often asks for insulation outside the studs as well. Continuous insulation covers that framing, so the whole wall performs closer to its listed value. It also keeps the sheathing on the wall’s inner side warmer, which lowers the risk of condensation in cold climates.
The cavity R-value depends on stud depth. A standard 2x4 wall is 3.5 inches deep and an R-13 batt fills it. A 2x6 wall is 5.5 inches deep, which leaves room for a thicker batt. Check the batt’s listed thickness against your actual stud depth before you order.
Worked examples
New 2x4 wall, zone 3, R-13 cavity plus R-5 continuous. The R-13 batt fills the 3.5-inch cavity. For the continuous layer, divide 5 by the R-value per inch of XPS, which is about 5. The result is 1 inch of XPS. The total is about R-18, which meets the R-13 + R-5 option.
New 2x4 wall, zones 4 or 5, R-13 cavity plus R-10 continuous. The R-13 batt fills the cavity. For the R-10 continuous layer, the thickness depends on the material:
- XPS, at about 5 per inch: 2 inches.
- EPS, at 3.8 to 4.4 per inch: about 2.3 to 2.6 inches.
- Mineral wool board, at 3 to 4 per inch: about 2.5 to 3.3 inches.
- Polyisocyanurate, at about 6 per inch: about 1.7 inches.
These figures are the arithmetic from typical values. Your product label replaces them.
New 2x6 wall, zones 6 to 8, R-20 cavity plus R-5 continuous. The 5.5-inch cavity takes an R-20 batt sized for 2x6 framing. A 1-inch layer of XPS adds the R-5. If you prefer the cavity-only option of R-30, you need a considerably deeper cavity, so confirm the framing plan and the manufacturer’s listed thickness first.
Retrofit: when the siding is off
When you already have siding and a wood-frame wall, the cheapest moment to add insulation is when the siding comes off. The ENERGY STAR guidance for existing wood-frame walls describes two cases. For an uninsulated wall, you blow insulation into the empty cavity through holes drilled in the sheathing, then install insulative wall sheathing under the new siding. That sheathing is R-5 in zone 3 and R-5 to R-10 in zones 4 to 8. For an insulated 2x4 wall in zones 4 to 8, the guidance calls for R-10 insulative sheathing.
Using the material figures above, R-5 is about 1 inch of XPS and R-10 is about 2 inches of XPS. The fasteners for the new siding must still reach the studs through the added layer. Use the manufacturer’s installation instructions to set the fastener length, because a longer fastener needs its own rating for the added thickness.
Safety and conditions before you start
- Permits and inspections. Call your local building department before you remove siding or change the wall depth. Rules vary by city and county.
- Old materials. If the house was built before 1978, assume the paint may contain lead and follow lead-safe work practices. Older siding can also contain asbestos. Have a sample tested before you cut, scrape, or remove it.
- Electrical. Outlets, switches, and fixtures on exterior walls must be extended to the new surface. Shut off the circuit, confirm it is dead with a tester, and do not cut into live conductors.
- Gas, water, and vent lines. Locate pipes and ducts before drilling for blown-in fill. A hole in the wrong place can cause a leak or a gas hazard.
- Moisture. Exterior rigid insulation changes where the wall dries. Flashing, window and door trim depth, drip edges, and vent openings must be redone with the new thickness. Do not seal over soffit or attic vents. Ask a designer for the vapor control approach that suits your climate zone.
- Working at height. Use stable ladders or scaffolding, a helper for long boards, and fall protection on tall walls.
When to check the code and the manufacturer
The code table gives the minimum for compliance, and the manufacturer’s data sheet gives the installed performance of the exact product. Use the code table to choose the option, then use the product label to convert R-value into inches. Installation details for fasteners, sealing, and maximum thickness of a given siding system come from the manufacturers of those components. For masonry, steel-frame, or other non-wood walls, the framing and R-value per inch differ, so the wood-frame figures in this article do not apply directly.
Frequently asked questions
Is thicker insulation always better?
No. Once you reach the target for your climate zone, each extra inch adds less benefit. Thicker exterior layers add weight, change window and door depth, and can move the spot where moisture condenses. Stay at the table value unless a designer shows a specific reason to go higher.
Can I add exterior insulation to a brick or metal-frame wall the same way?
Not directly. The figures in this article apply to wood-frame walls. Brick, masonry, and steel-frame walls have different framing and different performance, so ask the manufacturer of your insulation for installation details for your wall type and check the code table for that wall.
Do I need a permit to add exterior insulation?
Often yes, especially when you remove siding or change the wall’s depth or exterior. Call your local building department before you start, because rules vary by city and county and an inspection may be required.
