Gutters & ventilation
Attic insulation R-values by climate zone, and how much to add
Most attics already have some insulation. Measure it, convert it to R-value, and the zone map tells you how much more to add.
R-value in one paragraph
R-value measures resistance to heat flow: the higher the number, the slower heat passes through. R-values add: R-11 of old fiberglass plus R-49 of new cellulose is R-60. Each material has a typical R-value per inch, so depth converts to R-value and back. Blown fiberglass is about R-2.5 per inch, blown cellulose about R-3.5, fiberglass batts about R-3.1 to R-3.4 depending on density, and mineral wool about R-3 to R-4. The bag’s label gives the exact figure for the product.
How much the attic should have
Energy Star publishes recommended attic insulation levels by climate zone, using the US Department of Energy’s climate zone map (zone 1 is the southern tip of Florida and Hawaii; zone 7 northern Minnesota; zone 8 Alaska).
| Climate zone | Uninsulated attic | Attic with 3 to 4 in already |
|---|---|---|
| 1 | R-30 to R-49 | R-25 to R-30 |
| 2 and 3 | R-49 to R-60 | R-38 to R-49 |
| 4 to 8 | R-60 | R-49 |
The “already insulated” column is what to add on top of an existing 3 to 4 inches; the total ends up near the uninsulated figure.
Measuring what is there
From the attic hatch, with a ruler and a flashlight, measure the depth in several places: between joists, near the eaves, over the middle. Identify the material:
- Fiberglass batts or loose fiberglass: pink, yellow or white glassy fibres; about R-3 per inch for old batts, R-2.5 loose.
- Cellulose: grey, paper-like, dense; about R-3.5.
- Rock wool: grey or brown, woolly; about R-3.
- Vermiculite: light, pebble-like granules, grey-brown or gold. Do not disturb it. Vermiculite insulation from before about 1990 may contain asbestos; the EPA advises leaving it alone and having it tested by a professional. This site gives no handling procedures.
Topping up a zone 5 attic
1,200 square feet, target R-60, 3½ inches of old fiberglass at R-3 per inch, adding blown fiberglass.
- Existing: 3.5 × 3 = R-10.5; to add: R-49.5
- Depth: 49.5 ÷ 2.5 = 19.8 inches, for a finished depth of about 23 inches
- Bags: about 40 by a typical chart; use the chart on the bag you buy
- Depth markers: 4 (one per 300 sq ft, IRC N1102.2.1)
From the attic insulation calculator.
Bags: read the chart
Blown insulation is sold by the bag, and a bag’s coverage depends on how deep it is blown: the deeper the layer, the fewer square feet each bag covers. Every bag carries a coverage chart giving, for each R-value, the minimum thickness, the maximum square feet per bag and the bags per 1,000 square feet. Buy by that chart, at the R-value you are adding, not by the number printed largest on the bag. Many stores lend the blowing machine free with a minimum purchase.
Before you insulate
The order matters more than the material.
- Air-seal the attic floor. Seal the gaps around top plates, plumbing and wiring holes, light fixtures and the chimney chase (with metal and fire-rated sealant), and weatherstrip and insulate the hatch. Air leaks carry more heat, and moisture, than missing inches of insulation stop.
- Fit rafter baffles in every vented bay so the new insulation does not block the soffit vents. They must reach at least 2 inches above the finished depth; the rafter baffles calculator checks whether one per bay is enough.
- Keep clearances. Insulation stays 3 inches from recessed lights not rated for insulation contact (IC-rated fixtures can be covered), from chimneys and flues as the code requires, and from knob-and-tube wiring, which should be inspected first.
- Check the ventilation. Deep insulation makes a cold attic colder, which is the point, and its ventilation more important: the 1:150 rule covers it.
- Staple depth markers to the trusses, one per 300 square feet, facing the hatch.
Fiberglass or cellulose?
Both reach any target. Cellulose has the higher R-value per inch and slows air movement through the layer better; it is heavier, settles 10 to 20 percent (the bag chart allows for it) and holds water if a roof leaks. Blown fiberglass is lighter, does not absorb water, settles little, and needs more depth for the same R-value. Batts suit open, regular joist bays and DIY without a machine, laid in two layers crossing each other to cover the joist tops.
Powered fans and insulation
A powered attic fan in a well-insulated, air-sealed attic has little to do; in a leaky one it pulls conditioned air out of the house. The attic fan calculator sizes one with the intake it needs, but sealing and insulating usually make it unnecessary. Better attic insulation also reduces the heat that melts roof snow, which helps with ice dams; the ice barrier guide covers the roof side of that problem.
When to combine with a reroof
Insulating is an attic job, independent of the roof, but a reroof is a good moment to fix the ventilation that goes with it: cut a ridge vent, add soffit intake, replace damaged decking from above. The repair or replace guide discusses timing the two together.