Attic Insulation Calculator
Last reviewed How we calculate
Attic insulation is measured by its R-value, its resistance to heat flow, and each material gives a different R-value per inch of depth. This works out how much R-value an attic already has, how deep new insulation needs to be to reach a target, and how many bags that takes.
Insulation to reach a target R-value
How deep to add insulation to reach a target R-value, and how many bags that takes.
This calculator needs JavaScript to work. The measurements below are the ones you would need to do the sum by hand.
Depth to add
- Existing R-value
- R-value to add
- Bags
- Total depth afterwards
The existing insulation already meets this target. More would add little; check instead that it is evenly spread, and that the attic hatch and the gaps around pipes and light fittings are sealed.
Seal air leaks from the house into the attic before adding insulation. Keep insulation clear of recessed lights not rated for contact, and do not block the soffit vents, because the roof needs that air. Wear a mask, eye protection and long sleeves when working with loose-fill, and only walk on the joists.
How the calculation works
Multiply the depth of the existing insulation by its R-value per inch to get what is already there. Take that from the target to find the R-value to add, and divide by the R-value per inch of the new material to get the depth. Bag labels list how much area one bag covers at each R-value, and that gives the bag count.
A worked example
A 1,000 sq ft attic with 4 in of loose-fill fibreglass has about R-10. To reach R-49, it needs another R-39. In loose-fill cellulose at about R-3.5 an inch that is roughly 11.1 in more, for a total depth of about 15.1 in. If one bag covers 40 sq ft at that R-value, the job takes 25 bags.
Choosing a target
The right target depends on your climate. ENERGY STAR publishes recommended levels by climate zone, ranging from around R-30 in the warmest parts of the US to R-49 to R-60 in colder ones. Local energy codes may also set a minimum.
Before adding insulation
Seal the gaps where warm air from the house leaks into the attic first, around the hatch, pipes, wiring and light fixtures, because insulation does little against moving air. Keep insulation clear of recessed lights that are not rated for contact, and keep the soffit vents open so the roof can breathe. In the attic, step only on the joists, and wear a mask, eye protection and long sleeves when handling loose-fill.
Frequently asked questions
How much attic insulation do I need?
ENERGY STAR recommends levels by climate zone, from around R-30 in the warmest parts of the US to R-49 to R-60 in colder areas. Local energy codes may also set a minimum.
Can new insulation go over the old?
Usually, as long as the old insulation is dry and in good condition. Use unfaced insulation on top, because a facing laid over existing insulation can trap moisture between the layers.
Will more insulation stop ice dams?
It helps, but only as part of the fix. Ice dams come from heat reaching the roof deck, so sealing air leaks into the attic and keeping the ventilation working matter as much as the insulation depth.