Insulation calculator: R-value, thickness and climate zone

What R-value do you need?

It depends on where you live, and the US Department of Energy publishes the recommendation by climate zone through ENERGY STAR.

Zone	Attic	Floor
1	R30	R13
2	R49	R13
3	R49	R19
4	R60	R19
5 and 6	R60	R30
7 and 8	R60	R38

Spec ENERGY STAR Recommended Home Insulation R-Values, administered by the US Department of Energy and EPA, based on the 2021 IECC residential provisions. Checked August 2026.

How thick does that need to be?

Very different depths depending on the material, which is the practical question the R-value alone does not answer.

Material	R per inch	Inches for R49
Fibreglass batt	3.1 - 3.8	about 14
Blown fibreglass	2.2 - 2.9	about 19
Blown cellulose	3.2 - 3.8	about 14
Open-cell spray foam	3.5 - 3.7	about 14
Closed-cell spray foam	6.0 - 7.0	about 8
EPS rigid board	3.6 - 4.2	about 13
Polyiso rigid board	5.6 - 6.5	about 8

Spec R-per-inch values are manufacturer ranges and vary by product. Check the specific product data sheet. Polyiso performs less well in cold temperatures, which matters in unheated attics in northern zones.

Why the wall never performs as well as the batt

Because the studs are not insulated. Heat takes the easy path through the timber, which is roughly R-1 per inch against R-3.5 for the insulation beside it. This is thermal bridging, and it means a wall built with R-13 batts performs closer to R-12 overall once the framing is counted.

It is also why continuous exterior insulation is increasingly specified: a layer of rigid board over the studs breaks the bridge in a way that no amount of cavity insulation can.

Common questions

What is RSI, and how does it relate to R-value?

RSI is the metric equivalent, in square metre kelvin per watt. Divide an imperial R-value by 5.678 to get RSI. R49 is about RSI 8.6. The UK uses U-values instead, which are the inverse: a lower U-value is better insulation, and U equals 1 divided by the total R including surface resistances.

Can you over-insulate?

Not in terms of heat loss, but there are diminishing returns and real practical limits. Going from R30 to R49 saves noticeably less than going from R11 to R30 did. Compressing batts to fit reduces their R-value, and filling a vented attic so deep that it blocks the soffit vents causes moisture problems.

Does insulation need a vapour barrier?

It depends on your climate, and getting it wrong is worse than omitting it. In cold climates the barrier goes on the warm side, facing the interior. In hot humid climates it may need to go the other way or be left out. This is one to check locally rather than follow a general rule.

Is spray foam worth the extra cost?

Closed-cell foam gives roughly twice the R-value per inch and acts as an air and vapour barrier, which matters where depth is limited, such as a cathedral ceiling or a van conversion. In an open attic where depth is free, blown cellulose reaches the same R-value for considerably less.