Attic Insulation and R-Value Guide: Depth, Bags, and Payback Math

Heat rises, and in a Canadian winter a poorly insulated attic is one of the clearest paths for that heat to leave the house. Topping up attic insulation is therefore one of the most common home projects: the area is accessible, the work is measurable, and the target is a number you can check with a ruler. This guide from Mississippi Maple explains what R value and RSI mean, how depth relates to R, why bag coverage shrinks as your target R rises, and how to count bags without guessing. It also shows a simple payback framing that uses your own heating bill numbers, so you can judge the project on your figures rather than on anyone else's claims.

This guide explains how the math works. Product ratings, local code targets, and installation requirements vary, so verify locally and follow the coverage chart on the product you actually buy.

R value and RSI, in plain language

What R value measures

R value measures how strongly a material resists heat flow. Higher R means more resistance, which means heat moves through more slowly. Insulation does not create heat and it does not stop heat flow completely. It slows it down, hour after hour, for the life of the building.

R values add in layers, when the layers are installed properly and not compressed. If an attic already has insulation that provides R-20 and you add a layer that provides R-30 on top, the combined target is R-50, provided the existing layer is in reasonable condition, evenly distributed, and not crushed by the new work. That adding rule is why top up projects are described by the R you need to add, not only by the final R you want.

RSI is the metric version

Canada works in metric, and the metric measure of the same resistance is RSI. The conversion used in this guide is:

RSI equals R divided by 5.678.

So R-50 in RSI is 50 divided by 5.678, which equals about 8.81. Working it out: 5.678 multiplied by 8.8 equals 49.9664. The remainder from 50 is about 0.0336, and 0.0336 divided by 5.678 is about 0.006, giving about 8.81.

A few common attic targets converted the same way: R-40: 40 divided by 5.678 equals about 7.05 RSI. R-50: 50 divided by 5.678 equals about 8.81 RSI. R-60: 60 divided by 5.678 equals about 10.57 RSI.

You may see either unit on product packaging, on energy assessments, and in code related material in Canada, sometimes both on the same label. Knowing the single division lets you move between them without treating them as different properties. They are the same resistance, expressed in different unit systems. Imperial R uses square feet, Fahrenheit, and BTU based units. RSI uses square metres, Celsius, and watts based units, which is why the conversion factor is what it is.

Targets in Canada, and why to verify locally

Many Canadian homeowners aim for about R-50 in an attic, and some newer targets and programs go higher, while older homes often sit well below that. The right target for your home depends on your province, your local building requirements, the age and construction of the house, available attic depth, and any program requirements you are trying to meet. The Ontario Building Code sets insulation requirements for building work it covers, and Natural Resources Canada publishes guidance on home insulation and EnerGuide assessments. Treat any single number in a guide, including R-50 in the worked example here, as a chosen example target, and verify locally what applies to your home and your project.

Depth versus R value

R comes from depth, for blown in products

For blown in insulation such as cellulose or blown fibreglass, you do not buy R directly. You install a depth of material, and that depth delivers an R, according to the manufacturer's coverage chart for that specific product. The relationship is:

Depth needed equals target R divided by R per unit depth for that product.

If a product chart states its R per 25 mm (about 1 in), you divide the target R by that figure to get how many units of depth you need, then multiply by 25 mm. If the chart works in R per 100 mm, you use that basis instead. Always work from the chart printed for your product and bag size. Two products that look similar in the attic can have different densities and different R per unit depth, and settling after installation is accounted for in proper charts as a settled depth or settled R figure. Use the settled values for planning, because those describe the insulation after it has been in place, not only in the first hour after blowing.

Compression works against you. Insulation that is packed down into a smaller depth than its chart assumes will not deliver the full R on the label. This matters around eaves, around stored items, and anywhere someone has walked or placed boards directly on loose fill. Keep storage off insulation where possible, or use a properly raised platform detail that maintains depth, and restore any areas that have been compressed or displaced.

Measure what you have before you add

Before a top up, measure the existing depth in several places, including near the eaves and near the centre of the attic, and note whether it is even. Attics are rarely uniform. One end may be deeper than the other, and insulation may be missing entirely around a hatch, a fan, or a chimney chase. A single measurement beside the hatch, which is the easiest place to stand, is often the least representative spot in the whole attic.

Record the product type if you can identify it, its depth, and its condition. Wet, matted, or contaminated insulation is a different problem than thin insulation, and adding new material on top of a problem does not fix the problem. Also look for the air sealing issues described next, because topping up is the best and often the only easy moment to reach them.

Air sealing, baffles, and hatches come first

Insulation slows heat, air sealing stops the leak

Warm indoor air leaking into a cold attic carries both heat and moisture. Gaps around plumbing stacks, wiring, light fixtures, top plates, and the attic hatch can leak a surprising amount of air even when the insulation layer looks adequate. Sealing those gaps with appropriate materials before adding insulation is a standard step in Canadian guidance on attic work, and it is much harder to do well after another 200 mm to 300 mm of loose fill is covering everything. Verify locally which fixtures and clearances need special treatment. Pot lights, chimneys, and exhaust fans have clearance and rating requirements, and insulation contact rules for them are safety matters, not preferences.

Keep ventilation air moving at the eaves

Many Canadian attics ventilate through soffit vents at the eaves and exit vents near the roof peak or ridge. Blown in insulation that spills into the eaves can block the intake path. Baffles, also called vent chutes, hold a channel open from the soffit into the attic while insulation is placed around them. Count baffles as a separate material line, one for the rafter spaces that need them along vented eaves, and confirm your roof's ventilation approach before you add depth near the edges. An attic needs both insulation and its intended ventilation. Doing one at the expense of the other creates moisture risk.

The hatch is part of the ceiling

An uninsulated, unsealed hatch is a hole in both the insulation and the air barrier. Weatherstripping the hatch, adding insulation to its back, and making sure it closes evenly are small jobs compared with blowing an entire attic, and they close a gap that sits directly above heated space. Include the hatch in the project list from the start, not as an afterthought if material is left over.

Bag coverage, and why it shrinks as R rises

Coverage is area at a stated R

A bag of blown in insulation does not cover a fixed area. It covers an area at a stated R value and depth. The packaging coverage chart will list, for a given bag weight, how many square metres that bag covers at several different R values. At low R, the layer is thin, so one bag spreads over a large area. At high R, the layer is thick, so the same bag is used up over a smaller area. Nothing changed about the bag. The depth per square metre changed.

In formula terms, for the same product:

Coverage at a new R equals coverage at a known R multiplied by known R divided by new R.

That inverse relationship is the key idea in this whole guide. Double the target R, and you halve the area one bag covers, assuming the same product and the same installed density. This is why a bag count from a friend's R-30 project cannot be carried over to your R-50 project, even if your attics are the same size.

Bags needed, rounded up

Bags needed equals attic area divided by coverage per bag at your target R, rounded up to whole bags.

Round up because you cannot buy part of a bag in a useful planning sense, and running short near the end of a job, with the blower rented and the attic open, is the most frustrating outcome in a top up project. Also allow a little extra for uneven areas, settling to the chart value, and the material that stays in the hose and machine. How much extra is a planning choice. State it on your list, as with any waste allowance, so you know what was calculation and what was cushion.

Worked example

This example uses clearly stated assumptions so you can repeat it with your own attic and your own product chart.

Assumptions: Attic area: 100 square metres (about 1,076 square feet) Target: R-50 total, calculated here as a full R-50 layer for bag counting Product assumption: one 28.5 kg bag of cellulose covers about 3.7 square metres at R-40 Conversion: RSI equals R divided by 5.678 Conversion: 1 square metre equals about 10.764 square feet

Check the area conversion first: 100 multiplied by 10.764 equals 1,076.4 square feet, or about 1,076 square feet, which matches the assumption above. The target in metric: 50 divided by 5.678 equals about 8.81 RSI, as worked out earlier in this guide.

Step 1: scale the bag coverage from R-40 to R-50

Coverage at R-50 equals coverage at R-40 multiplied by 40 divided by 50. 40 divided by 50 equals 0.8. 3.7 multiplied by 0.8 equals 2.96. So, under this stated assumption, one bag covers about 2.96 square metres at R-50. Notice what happened: the target R went up by one quarter, from 40 to 50, and the coverage per bag went down, from 3.7 to 2.96 square metres. That drop is exactly the shrinking coverage described above.

Step 2: divide the attic area by the coverage per bag

Bags before rounding equals 100 divided by 2.96. 2.96 multiplied by 33 equals 97.68. The remainder is 100 minus 97.68, which equals 2.32. 2.32 divided by 2.96 equals about 0.78. So 100 divided by 2.96 equals about 33.78 bags.

Step 3: round up to whole bags

33.78 bags rounds up to 34 bags. That is the count to buy for the full R-50 layer under these assumptions, before any extra allowance you choose to add for uneven areas or machine losses.

Optional check on total material weight: 34 multiplied by 28.5 kg equals 969 kg of material in total. 28 multiplied by 30 would be a rough cross check only. The exact multiplication is: 28.5 multiplied by 30 equals 855, and 28.5 multiplied by 4 equals 114, and 855 plus 114 equals 969 kg. This weight check is useful for planning transport and for confirming with your installer or supplier that your attic structure and access are appropriate for the planned load and depth. Verify locally if you have any structural questions.

Step 4: adjust the method if you are topping up

The steps above count a full R-50 layer, which fits a bare attic or a planning estimate. Most top up projects add only part of that. If an assessment confirms your attic already performs at, for example, R-20, and your target is R-50, the layer you add must provide R-30, because R-20 plus R-30 equals R-50. In that case you would scale bag coverage to R-30 instead: coverage at R-30 equals 3.7 multiplied by 40 divided by 30, which equals 3.7 multiplied by about 1.333, which equals about 4.93 square metres per bag. Bags would then be 100 divided by 4.93, which equals about 20.28, rounded up to 21 bags. The method is identical. Only the R you assign to the new layer changes, so confirm your existing R with measurements or an assessment before you choose which version of the calculation describes your project.

A simple payback framing using your own numbers

Use your bill, not a promised saving

This guide makes no claim about what you will save, because savings depend on your home, your heating system and fuel, your weather, your indoor temperatures, and how much of your heat loss was actually going through the attic. Instead, here is a framing you can fill in with your own figures, before and after the project, or as an estimate you label clearly as your own assumption.

Simple payback in years equals total project cost divided by annual heating saving per year.

To use it: Take your annual heating cost from your own bills for a full year. Decide, as your own stated assumption, what share of that cost you are estimating the attic project might affect, or, better, compare a full heating season after the project with a similar season before it, keeping in mind that a colder or milder winter will move the bill on its own. Whatever annual saving figure you arrive at, write down where it came from.

Then divide. As a pure arithmetic illustration with placeholder numbers you must replace: if a project cost 2,000 in your local currency units and your own estimated annual saving was 250 in the same units, the simple payback would be 2,000 divided by 250, which equals 8 years. Those two input numbers are examples for the division only. They are not a price, not a predicted saving, and not a recommendation.

Simple payback also leaves things out on purpose. It ignores energy price changes, maintenance, comfort gains, moisture risk avoided, and the fact that insulation keeps working after the payback point. It also ignores other work done at the same time, such as air sealing, which may contribute part of any improvement. Use the framing as one lens, alongside comfort and building health, not as the sole verdict on the project.

FAQ

How many bags of insulation do I need for a 100 square metre attic at R-50?

Under the stated assumption in the worked example, where one 28.5 kg cellulose bag covers about 3.7 square metres at R-40, coverage scales to about 2.96 square metres per bag at R-50. Dividing 100 by 2.96 gives about 33.78, which rounds up to 34 bags. Your count will differ if your product chart lists different coverage, so always run the same scaling method with the chart on your bag.

What does R-50 mean in RSI?

RSI equals R divided by 5.678, so R-50 equals 50 divided by 5.678, which is about 8.81 RSI. R and RSI describe the same resistance to heat flow in different unit systems, and Canadian labels and assessments may use either or both.

How deep does blown in insulation need to be for R-50?

Depth depends on the specific product, because products deliver different R per unit of depth. Divide your target R by the R per unit depth on the manufacturer's coverage chart for your product, using settled values, to get the depth to install. Do not borrow a depth from a different product, even if the materials look similar.

Why does one bag cover less area at higher R?

Because coverage is area at a stated depth. A higher target R needs a thicker layer, so the same volume of material in one bag is used up over a smaller area. Coverage scales in inverse proportion: coverage at a new R equals coverage at a known R multiplied by the known R divided by the new R.

Should I remove old insulation before topping up?

Often, clean, dry, evenly distributed existing insulation can stay and count toward the total R, with new material added on top. Wet, matted, damaged, or contaminated insulation, and any insulation hiding air leaks that need sealing, should be assessed on its own before you add anything. When in doubt, get an in person assessment of condition before deciding.

Is R-50 the right target for every Canadian attic?

No single target fits every home. R-50 is a common aim and is the target used in this guide's example, but requirements and sensible targets vary with province, local building rules, attic construction, and program goals. Consult the Ontario Building Code where it applies, Natural Resources Canada guidance, or your local requirements, and verify locally before you commit to a target.

Calculators and next steps

Work through your own attic with the Mississippi Maple insulation calculator, then use the related tools to connect the result with heating size and energy cost questions:

Insulation calculator: https://mississippimaple.ca/tools/insulation.html Electricity cost calculator: https://mississippimaple.ca/tools/electricity-cost.html BTU calculator: https://mississippimaple.ca/tools/btu.html

A good order of work is: measure your attic area and existing depth in several places, confirm your existing R and your target R, air seal and add baffles where needed, read coverage at your target R from your product chart, scale the coverage if your chart lists a different R, divide area by coverage and round up, and finally run the simple payback framing with your own bill and your own cost figures. Keep the measurements, the chart values, and the bag count together. If you get an assessment or a quote later, that one page will let you compare it line by line.

Sources and figures

Sources and figures note: All numerical results in this guide come from the arithmetic shown in the text, using the stated assumptions, including the conversion RSI equals R divided by 5.678, the conversion 1 square metre equals about 10.764 square feet, and the assumed product coverage of about 3.7 square metres per 28.5 kg bag at R-40, which must be replaced with the coverage chart of the product you actually buy. No savings, prices, or performance results are claimed for any specific home. Guidance sources to consult and verify locally include Natural Resources Canada publications and EnerGuide resources, the Ontario Building Code where it applies to your work, manufacturer coverage charts and installation instructions, and your municipal building department.

Estimates only, verify before you buy materials. This guide plans quantities and costs; it does not replace local building codes, manufacturer instructions, or professional advice.