# Insulation Calculator

Area divided by package coverage, plus an R-value thickness guide.

- Page: https://jianzhujisuan.com/en/insulation-batts
- Category: Finishes (https://jianzhujisuan.com/en/c/finishes)
- Chinese version: https://jianzhujisuan.com/zh/insulation-batts.md
- Data checked: 2026-10-25

## Quick result

With the default inputs (Area to insulate 1000 ft², Coverage per bag 88.1 ft², Framing spacing 16 in, Waste 10%): **Insulation required 13 bags**

## Inputs

| Parameter | Type | Default | Unit | Note |
|---|---|---|---|---|
| Area to insulate | number | 1000 | ft² |  |
| Coverage per bag | number | 88.1 | ft² |  |
| Framing spacing | number | 16 | in |  |
| Waste | number | 10 | % |  |

## Formula

- **Bags = area × (1+waste) ÷ coverage per bag**: round up
- **Batt width = framing spacing − 1 in**: 15 for 16 in, 23 for 24 in
- **R-value ≈ thickness ÷ conductivity**: fibreglass about 3.7 R per inch

## Worked example

1000 ft² with 88.1 ft² per bag (R-13 at 15 in wide) and 10% waste: 1100 ft², so 13 bags. Switch to 23 in wide batts for 24 in centres and the coverage per bag changes - read the new package figure.

## Questions

### What R-value do I need?

It follows climate zone: walls commonly R-13 to R-21, ceilings R-30 to R-60 and floors R-19 to R-30 in North America. Cold climates want R-49 or better overhead because heat rises and ceiling insulation gives the best return. Wall values are capped by stud depth - 2x4 takes R-13 to R-15 and 2x6 takes R-19 to R-21 - so going higher means exterior or continuous insulation.

### What happens if the batts do not fit tightly?

Performance falls off a cliff rather than degrading gradually: a 5% gap can lose 20-30% of the effective R-value because air convects through it. The usual culprits are batts not pressed against the stud face, boxes, pipe penetrations and the narrow strips between window frames and studs. Fill those with offcuts or foam. Compressing batts also hurts - stuffed in too tight, the R-value drops.

### Do I need a vapour barrier?

Cold climates do: the barrier goes on the warm side of the insulation to stop interior moisture entering the cavity and condensing. In warm humid climates such as the south-eastern US it is more nuanced - put it on the wrong side and you trap moisture in the wall, which is worse than none. It has to be continuous, with seams lapped and taped and proper gaskets at outlets and penetrations. Get this wrong and the mould shows up years later.

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