Attic Insulation R-Value: How Much You Need and What It Saves

Attic insulation is the highest-return efficiency work in most houses, because heat rises and the attic is where it leaves. It is also one of the few jobs where the material is cheap and the labour is the cost — which makes it attractive to do yourself.

What R-value means

R-value measures resistance to heat flow. Higher is better, and it adds: R-19 plus R-19 is R-38.

Depth needed per R-value differs by material:

Material	R per inch	Inches for R-49
Blown fibreglass	2.5	~20”
Blown cellulose	3.5	~14”
Fibreglass batts	3.1	~16”
Closed-cell spray foam	6.5	~8”

Targets by climate

US Department of Energy recommendations for attics, uninsulated:

Climate	Target
Hot (southern coastal)	R-30 to R-49
Mixed / temperate	R-38 to R-60
Cold (northern)	R-49 to R-60

Two useful reference points: a house built before 1980 typically has R-11 to R-19, and R-49 is roughly 14 inches of cellulose or 20 inches of fibreglass. If your joists are visible when you look across the attic, you have well under target — joists are 8–10 inches deep and insulation should be well above them.

Air sealing comes first

This is the part people skip, and it undoes the rest. Insulation slows conduction; it does nothing about air moving through it. A house with R-49 and open penetrations to the attic performs worse than one with R-30 that is sealed.

Before adding any insulation, seal:

• Top plates where interior walls meet the attic.
• Plumbing and electrical penetrations. Fire-rated caulk or foam.
• The attic hatch or door, which is usually an uninsulated hole. Weatherstrip it and insulate the back.
• Recessed light housings. Only IC-rated fixtures may be covered; others need a clearance box, and this is a genuine fire consideration.
• Bathroom and kitchen fans, which must vent outside, not into the attic. Venting moist air into an attic causes mould and rots sheathing.
• Chimney and flue chases, with sheet metal and high-temperature sealant, not foam.

Do not block the ventilation

Attics need airflow from soffit vents to ridge or gable vents. Insulation pushed into the eaves blocks the soffits and causes condensation and, in cold climates, ice dams.

Fit rafter vents (baffles) at every eave before blowing insulation. They are inexpensive cardboard or foam channels that keep an air path open.

Batts vs blown

Blown cellulose or fibreglass is better for topping up. It fills irregular spaces and covers the joists without gaps. Rental blowers are usually free or cheap with a bulk material purchase, and it is a two-person job — one feeding the hopper, one on the hose.

Batts make sense for a new install in clean, evenly spaced joists, or where you want to lay a second layer perpendicular over existing batts. Never compress them; compressed batts lose R-value proportionally.

Do not lay faced (paper or foil-backed) batts over existing insulation. The facing is a vapour retarder and a second one traps moisture between layers. Use unfaced.

What it actually saves

Going from R-19 to R-49 in a cold climate typically cuts heating energy by 15–20%. On a house spending $1,800 a year on heating, that is $270–360 annually against a material cost of a few hundred dollars for a typical attic. Payback is usually one to three heating seasons, which is unusually good for building work.

Air sealing alone, done properly, often delivers a further 10%.

Safety notes for the attic itself

Work on a board laid across the joists, not on the ceiling drywall. Wear a respirator and long sleeves — old fibreglass and cellulose dust are both unpleasant. Check for vermiculite before disturbing anything: some vermiculite insulation contains asbestos and needs testing, not a shop vacuum.

Estimating what you need to buy

Blown insulation is sold by bag, and each bag states its coverage at a given R-value on the label — for example “covers 40 sq ft at R-30”.

The calculation:

1. Attic area — length by width of the floor, minus anything you cannot cover.
2. R-value to add — your target minus what you have. Measure existing depth with a ruler and multiply by the R-per-inch figures in the table above.
3. Bags = area ÷ coverage per bag at that R-value.

A 1,200 sq ft attic going from R-19 to R-49 needs R-30 added, which at typical cellulose coverage works out around 30 bags. Buy 10% extra — settling and uneven joist bays always consume more than the arithmetic suggests.

Mark the target depth on the rafters with a marker before you start blowing. Once the hose is running you cannot judge depth by eye, and this is how attics end up at R-30 when the owner paid for R-49.

Cellulose vs fibreglass, practically

Cellulose is recycled paper treated with borate. Higher R per inch, better at resisting air movement through it, and it fills gaps well. It is heavier, it settles a few percent, and it absorbs and releases moisture — which is fine in a ventilated attic and a problem in a wet one.

Fibreglass is lighter, does not settle, and does not absorb water. It is more prone to convective looping in very cold attics, and it performs worse in windy conditions if the attic is not well sealed.

For a topped-up, air-sealed, ventilated attic, either is fine and cellulose gives you more R per inch of headroom. Do not mix loose fill types in one layer.

Do not insulate over these

• Recessed lights that are not IC-rated. They need a 3-inch clearance, and a purpose-made cover box is the right solution. Burying a non-IC fixture is a documented fire cause.
• Chimneys and B-vent flues. Metal clearance and high-temperature sealant, not insulation against the pipe.
• Knob-and-tube wiring, if you have it. It relies on air circulation to shed heat, and covering it is unsafe. That wiring wants replacing anyway.
• Bathroom fan housings in some cases; check the fixture’s rating.
• The attic hatch, which needs a separate insulated cover rather than loose fill dumped near it.

Signs your attic has a moisture problem to fix first

Insulating over a moisture problem accelerates the damage. Before adding anything, look for:

• Dark staining or mould on the underside of the roof sheathing.
• Rusty nail points, which indicate condensation cycles.
• Frost on the sheathing in winter.
• Compressed, discoloured, or damp existing insulation.
• A bathroom or kitchen fan discharging into the attic.

Any of those means find the moisture source — usually air leakage from the house or a fan vented indoors — and fix it before insulating. Insulation traps the problem where you cannot see it.

Ventilation ratio

The usual requirement is 1 sq ft of net free vent area per 150 sq ft of attic floor, split roughly evenly between low (soffit) and high (ridge or gable) vents. Balanced intake and exhaust is what makes the airflow work; all-ridge with no soffit intake will pull air from the house instead.

Fit baffles at every eave before blowing, and count your vents rather than assuming they are adequate. Blocked soffits are the most common reason a well-insulated attic develops condensation.