The measurements for insulation are plain geometry, but the shapes are not always plain. A wall has windows, an attic has a roofline, and a house often has several identical sections.
Walls: length × height, minus openings
A 12 ft wall with 8 ft ceilings is 96 sq ft. Subtract the openings that will not be insulated, such as a 3 × 7 ft door at 21 sq ft. The planner’s wall mode does this for you: 75 sq ft for one wall here.
Wall area = length × height
Area to insulate = wall area − openings
Identical sections: multiply
If four walls match, measure one and enter a count of four. The planner multiplies instead of making you re-enter, so four such walls total 300 sq ft.
Worked example Four identical 12 × 8 ft walls, one door each, 40 sq ft per package
| Material | Calculated | Buy |
|---|---|---|
| Insulation40 sq ft per package (the value you entered) | 7.5 packages | 8 packages |
Attics and floors: length × width
For an attic floor or a crawl space, use the rectangle you are covering: a 30 × 20 ft area is 600 sq ft. Measure the area the insulation covers, which may be smaller than the whole footprint if parts are boxed in or blocked.
Triangles and gables
A gable end is a triangle: half of the base × the height. A gable that is 24 ft wide and 8 ft tall is 96 sq ft. Enter the total as a known area. For a sloped ceiling, treat the sloped plane as its own rectangle.
What changes the estimate?
- Openings you will leave uninsulated
- Whether you are covering the full footprint or only part of it
- Framing, ducts and obstructions that reduce the usable area
- How many identical sections you are repeating
From area to packages
Once the area is known, divide by the coverage on your package. That is the part that depends on the product, and the quantity guide shows how much it can vary. This page measures; it does not advise on what R-value or thickness your project needs.