Choosing an attic insulation system by the house, not the label

Material comparisons usually end in a winner. This one does not, because the question that decides an attic is not which product is better but which product suits this assembly, this access and this budget.

Everything below assumes the ceiling plane is being air sealed either way. That step affects performance more than the choice between two loose-fill products ^[1].

Blown cellulose

Recycled paper fibre, treated with fire retardant, blown loose over the attic floor.

Where it does well. Irregular framing, lots of obstructions, and attics where covering everything evenly matters. It packs around things well and tends to leave fewer voids at the edges. Denser than blown fibreglass, so it resists air movement within the layer somewhat better, though it is still not an air barrier.

Things to know. It settles, which is why the specification is installed density and settled depth rather than the depth on the day. It is heavier than blown fibreglass, so total added depth is worth a thought on older framing. It absorbs and releases moisture, which is fine in a dry attic and a problem in one with an unresolved leak.

Blown fibreglass

Loose glass fibre, blown in the same way.

Where it does well. Deep, straightforward attics where the target is a high R-value and weight matters. Lighter than cellulose, does not absorb water in the same way, and modern blown fibreglass performs well at depth when installed to the stated density.

Things to know. Lower density means it can be more sensitive to being installed thinly or unevenly. As with cellulose, the number that matters is the installed R-value, not the depth ^[2].

Batts

Pre-cut fibreglass or mineral wool laid between joists.

Where they do well. Small, tidy, accessible attics. A hatch cover. A knee-wall section where loose-fill would not stay put. Mineral wool in particular where a specific fire or clearance detail is being addressed.

Things to know. Batts are only as good as the fit. Gaps at the ends, compression under a wire, and voids around cross-bracing all cost real performance, and attic framing is rarely as regular as batts assume. In an existing attic full of obstructions, loose-fill usually gives more consistent coverage. Batts over existing loose-fill is generally not the approach.

Spray foam, at the attic floor

Uncommon and rarely appropriate. Spraying foam onto the attic floor over a finished ceiling is expensive, hard to reverse, and buries everything it covers.

Where a small amount of foam genuinely belongs is as an air-sealing material at specific penetrations — not as the insulation layer — and not against flues or unrated fixtures.

Spray foam, at the roof deck

A different proposition entirely: foam applied to the underside of the roof, bringing the attic inside the building envelope and removing the soffit-to-ridge ventilation.

Where it is considered. Attics containing ductwork or an air handler, where conditioning the space rather than the ceiling has a real argument. Complex roof geometry where the ceiling plane is hard to seal. Some knee-wall situations.

Things to know before anyone quotes it. This changes how the roof assembly handles moisture and drying, and the design has to account for that — it is not a thicker version of the same job. Costs are substantially higher. It affects future roof work and inspection. And the guidance about vented attics on the rest of this site, including the ENERGY STAR and Building America sequencing, is written for conventional vented assemblies and does not transfer unchanged ^[1].

It is a legitimate approach in the right house. It should never be the default recommendation for a homeowner whose actual problem is four inches of settled fibreglass and an unsealed ceiling.

Hybrid approaches

Common and often sensible: air seal thoroughly, then blow loose-fill to the target. Or foam at specific detail points, loose-fill everywhere else. Or batts in a small knee-wall area and blown material in the main attic.

The mix is a response to the building, not a product loyalty.

Side by side

	Blown cellulose	Blown fibreglass	Batts	Foam at roof deck
Typical use	Retrofit over existing	Retrofit over existing	Small or tidy areas	Unvented, conditioned attic
Coverage around obstructions	Good	Good	Poor	Good
Relative weight	Higher	Lower	Low	Low
Settles	Yes — spec settled depth	Some	No	No
Acts as an air barrier	No	No	No	Depends on type and thickness
Relative cost	Lower	Lower	Lower	Much higher
Changes the assembly	No	No	No	Yes — attic becomes conditioned

Relative, not absolute. Actual pricing depends on the scope, and this table is not a substitute for a proposal.

What this means for comparing quotes

If two contractors propose different materials, that is not automatically a red flag. Ask each of them:

• Why this material for this attic?
• What installed R-value, and what settled depth does that correspond to?
• How will coverage be verified?
• What is the air-sealing scope, and is it the same in both proposals?

If one proposes roof-deck foam and the other loose-fill on the attic floor, those are two genuinely different projects at two very different prices, and the comparison has to be made on that basis rather than on the bottom line.

Get the attic evaluated first

Material follows from condition, access and what the attic is being asked to do. Share the address and what is prompting the project.

Related: attic insulation · air sealing · R-value guide · what changes the cost