Insulation Depth to R-Value Chart: Inches to R for Blown Cellulose, Blown Fibreglass, Batts and Spray Foam, How to Read the Ruler in Your Attic and What Settling Does to the Number

The chart

Per-inch figures are mid-range: cellulose 3.6 to 3.8 settled, loose fibreglass 2.2 to 2.8 (some newer products higher; read the bag), fibreglass batts 3.0 to 3.7, mineral wool 3.7 to 4.3, closed cell R-6 nominal and about R-5.6 aged, open cell R-3.7. Nobody puts 10 in. of closed cell in an attic; that column is there so you can read a cathedral ceiling or a rim joist off the same page. The R-value basics page covers what R actually measures.

How to read your own attic

1. Four places, not one. By the hatch, at the far end, at an eave and in the middle. Attics are never even; the far end over the bedrooms is usually thinner because the installer ran out of hose or patience.
2. Push the tape to the drywall, not to the top of the joist. The joist top is 2×6 or 2×8, so 5.5 or 7.25 in. above the ceiling; if the fluff is level with the joists, that is your depth. If it is below them, the joists are bridging and the attic is worse than the depth says.
3. Identify the product. Grey or tan, papery, packs in the hand: cellulose. Pink, yellow or white, fluffy, itches: fibreglass. Grey-brown pellets, shiny flakes, lightweight, like cat litter: vermiculite, and the tape measure goes back in your pocket until a lab test comes back. That one is not negotiable.
4. Read across the chart from the depth to the R. 5 in. of cellulose is about R-18; 5 in. of fibreglass is about R-13. Both get written on the quote as “existing R-20” by an installer in a hurry, and the difference is five R-points of top-up at $0.065 each.
5. Look for the depth markers. Cardboard or plastic rulers stapled to the trusses every few hundred square feet, marked at the target depth. If the last contractor left them, you can read what he promised against what is there. If he did not, you cannot, which is the point of asking for them this time.

The bag chart is the real number

Every bag of blown insulation has a chart on it, required by the labelling rules, that says for each R-value: the minimum settled thickness, the installed thickness to blow, the minimum weight per square foot and the number of bags per 1,000 sq ft. The last column is the one that matters. An installer can blow fibreglass fluffy, with the hose held high, and reach 23 in. with two-thirds of the bags the chart calls for; it looks like R-60 and it is R-40 with a lot of air in it. He cannot fake the bag count. The invoice should say how many bags went in, and you can count the empties in the truck.

Cellulose works the other way. The chart says blow to about 20 in. for a settled 16.5; an installer who blows exactly 16.5 and leaves has given you R-50 by spring. Either way, depth alone is not the proof. Depth plus bags plus the chart is. The cellulose vs fibreglass page goes through what each one does over time; the inspection checklist puts the bag count on the list of things to get in writing.

Settling, in inches

Cellulose: up to 20% in the first year, mostly in the first few months, then it stops. 20 in. blown becomes 16 to 17 in. Fibreglass: 2 to 4%, which is an inch on a deep attic. Batts: nothing, unless they were compressed when they were installed, in which case the loss already happened on day one; an R-20 batt stuffed into a 3.5 in. bay is about R-13, because R-value comes from the thickness, not the label. Closed cell foam: nothing, though it does shrink away from the framing if it was sprayed off-ratio, and that gap matters more than the R.

The old attic you are reading now has done all its settling. A 1985 house with 5 in. of cellulose was probably blown to 6 or 7 in. for R-20 and has sat at R-18 for thirty-five years. Read it as it is, not as the original permit said.

The numbers the depth has to reach

OBC 2024 with SB-12, Zone 1, the common prescriptive package: ceiling below an attic R-60, cathedral or flat roof R-31, exposed floor R-31, above-grade wall R-22 nominal or an equivalent effective value, full-height basement wall R-20. The attic figure is where every renovation ends up as well, not because the code reaches back into an existing house but because the rebate programs and the energy advisors use it, and because the arithmetic from R-40 to R-60 is cheap: 20 points at $0.065 is $1.30 a square foot. The attic R-value page argues the whole thing out.

Two depth rules that are not about R. Insulation stays 3 in. clear of a masonry chimney, with a metal dam. And in a vented attic the insulation has to stop short of the roof deck at the eave, which with 17 in. of cellulose on a 4-in-12 roof means the baffle has to be there or the last 3 ft of attic over the outside wall gets R-10 and ice dams. That is also where the depth drops first if nobody dammed the top plate, and where the frost line on the ceiling in a cold snap shows you the thin spot from inside the bedroom.

Effective R, the number the heat-loss calculation and the SB-12 form use, is different from the chart again, because the framing bridges the insulation. In an attic with trusses at 24 in. the framing is a small share and R-60 blown is close to R-60 effective. In a wall it is not: a 2×6 with R-22 batts is about R-17 effective. The wall insulation hub has the table.

Questions people ask

Sources and further reading

• Keeping the Heat In: Natural Resources Canada: the plain-language Canadian guide to air sealing, insulation materials, basements, walls and ventilation.
• Ontario’s Building Code, Supplementary Standard SB-12: the energy-efficiency packages that set the R-values for attics, walls, basements and slabs in a new or renovated Ontario house.
• Canadian Construction Materials Centre: National Research Council: the product evaluations behind spray foam and other insulation products, and what an evaluation number means.