How Much Insulation Do I Need?

Standard batt cannot reach R-13 in a 2×4 wall

R-value is additive with thickness, so the maths is simply target ÷ R-per-inch:

R-13 ÷ 3.2 per inch = 4.06 inches.

A 2×4 stud cavity is 3.5 inches deep. That number does not move.

So the R-13 batts sold for 2×4 walls don't get there by being thicker — they get there by being denser. Fitting R-13 into 3.5 inches requires exactly 3.71 per inch, which is what the calculator's R per inch you'd need output returns, and what those batts are built to deliver. (The general high-density figure in the material list is 3.8, slightly better again — it reaches R-13.3 in the same cavity.) That density is the entire reason high-density batts exist and cost more.

The cavity is the constraint, not the R-value you want

Once the space is fixed, the material is your only lever:

In a 3.5″ 2×4 cavity	Best R available
Blown fibreglass (2.8)	R-9.8
Standard batt (3.2)	R-11.2
High-density batt (3.8)	R-13.3
Closed-cell foam (6.5)	R-22.8

Same space, twice the performance, and a very different price.

That's why the tool shows cavity it would fill as a percentage. On the defaults it reads 116% — over 100 means it does not fit, and no quantity of the wrong material fixes that. Quantity is the easy half.

Enter the area to insulate as well and it returns the volume of material — cubic feet, which is the unit blown insulation is actually sold against. A bag states its coverage at a given depth, and that qualifier is the one people skip: a bag covering 40 sq ft at R-19 covers far less at R-49, because it is the same material blown deeper.

Never compress a batt to fit

The single most common insulation mistake.

Compressing a batt does not give you its rated R-value. It gives roughly the R of that thickness of that material — often somewhat less, because compression collapses the trapped air pockets that are doing the actual work.

Stuffing an R-19 batt into a 2×4 cavity gets you something around R-13, at R-19 prices, with a worse fit.

Buy the batt made for the cavity.

The attic is the opposite case, and that's where the value is

In an attic, depth is usually free. Nothing stops you adding more — so the constraint that dominates walls simply doesn't apply.

Which means the cheap materials win on cost per R. R-49 takes about 13.2 inches of blown cellulose at R-3.7. That's a matter of blowing more in, not a design problem.

The same R-49 in a stud wall would be impossible at any density.

If you have a fixed budget and both are unfinished, the attic almost always returns more.

When is a high R-per-inch worth paying for?

Only when space is the binding constraint — and that's worth being clear about, because the expensive materials are sold on this number.

Closed-cell foam at R-6.5 per inch is genuinely twice the performance of standard batt in the same cavity. That matters enormously in a 2×4 wall and hardly at all in an open attic where you could just add depth.

Pay for R per inch when inches are scarce. Pay for cost per R when they aren't.

Two things that matter more than the number

Installation quality. Batts compressed to fit, gaps at the edges, material stuffed around wiring, missed patches behind pipes — all of it drags real performance well below the figure on the packet. A carefully installed R-13 outperforms a badly installed R-19.

Air sealing. Frequently matters more than adding R-value, and costs far less. Insulation slows heat moving through a surface. It does nothing about air moving through a gap.

What this doesn't tell you

Deliberately, because guessing would be worse than declining:

• Your target R — it varies by climate zone and by where in the building you are. Your local code and energy office publish it.
• Vapour control — climate-specific, and where buildings genuinely get hurt when it's guessed at.
• Thermal bridging through the studs themselves, which is why continuous exterior insulation exists.

Print the planning sheet — it has the R-per-inch table and the cavity depths side by side, which is the comparison that decides what to buy.

Sources: Oncor / DOE — insulation R-values per inch · LearnMetrics — R-value chart