Insulation Types Compared: Fibreglass, Cellulose and Foam
Updated 2026-08-16 · 8 min read
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The right insulation is decided by the cavity, not the catalogue. A material that's ideal in an open attic is a poor choice in a 2×4 wall, and the one that solves a shallow cathedral ceiling is a waste of money in a space where cheap loose-fill would have done the same job.
Here's what each material actually does well.
Quick comparison
| Material | R per inch | Relative cost | Air barrier? | Best for |
|---|---|---|---|---|
| Loose-fill fibreglass | ~2.2–2.7 | Low | No | Open attics, top-ups |
| Loose-fill cellulose | ~3.2–3.8 | Low | Partially, at density | Attics, dense-pack walls |
| Fibreglass batts | ~3.0–3.8 | Low | No | Accessible standard cavities |
| Mineral wool batts | ~3.0–4.0 | Moderate | No | Fire and sound performance, damp-prone areas |
| Rigid foam board | ~4–6.5 | Moderate–high | Yes, when taped/sealed | Continuous exterior layers, rim joists, foundations |
| Open-cell spray foam | ~3.5–4 | High | Yes | Cavities where air sealing matters, sound |
| Closed-cell spray foam | ~6–7 | Highest | Yes, and vapor retarder | Shallow cavities, rim joists, moisture-prone assemblies |
R-value ranges vary by product and density; check the manufacturer's figures for what you're buying.
When should you use blown-in insulation?
Fibreglass and cellulose blown from a hopper. This is the default for attics and for good reason: it flows around joists, wiring, plumbing and framing irregularities, filling voids that batts leave open.
Fibreglass loose-fill — lighter, doesn't absorb water, settles relatively little, lower R per inch so it needs more depth for a given R-value.
Cellulose loose-fill — recycled paper treated with fire retardant. Higher R per inch, packs denser, and at density it slows air movement through the material, which is genuinely useful in wall cavities (dense-pack cellulose). It settles more than fibreglass, so installed depth must account for it. Paper-based, so a roof leak matters more.
Where it wins: open attics, top-ups over existing insulation, and dense-pack retrofits into closed wall cavities. See attic insulation R-value guide.
Where it doesn't: anywhere you need the insulation to also be an air barrier, or in cavities you can't blow into.
When are insulation batts the right choice?
Pre-cut fibreglass or mineral wool sold in rolls or sheets, sized for standard framing.
The advantage is cost and DIY accessibility — anyone can install batts in an open, accessible cavity.
The catch is that batts are only as good as the fit. Gaps, compression around wiring, and unfilled corners cause real performance loss, and it's easy to install them badly without knowing. A batt compressed to fit behind a pipe delivers well under its rated R-value at that point, and heat takes the path of least resistance.
Two rules:
- Cut around obstructions, don't compress over them. Split the batt around wiring rather than crushing it.
- Unfaced batts for top-ups. Never put a faced batt with the vapor retarder facing up over existing insulation — it traps moisture in the assembly.
Mineral wool batts cost more but are denser, hold their shape better around obstructions, don't burn, and perform better acoustically. They also tolerate damp better than fibreglass. Worth the premium where fire separation or sound matters.
Where do you use rigid foam insulation?
Rigid panels — typically EPS, XPS or polyiso — installed as continuous sheets.
The key advantage is continuity. Batts and loose-fill sit between framing members, and the framing itself conducts heat around them — thermal bridging, which can substantially reduce a wall's effective R-value below the nominal figure. A continuous layer of foam board over the framing interrupts that path.
Where it wins:
- Rim joists — cut to fit, sealed at the edges with foam or caulk. One of the highest-return small jobs in a house.
- Continuous exterior sheathing during a re-siding project.
- Foundation walls and slabs, where the material must tolerate contact with masonry and moisture.
- Interior basement walls, in the right assembly.
Notes: polyiso has the highest R per inch of the three but its performance drops in cold temperatures, which matters in cold-climate exterior applications. Foam boards generally require a code-specified thermal or ignition barrier when used in living or occupied spaces — check the requirement, it's not optional.
Spray foam
Applied as a liquid that expands in place. Two distinct products:
Open-cell — lower density, lower R per inch, permeable to vapor, good acoustically. Air-seals well. Less expensive of the two.
Closed-cell — high density, high R per inch, acts as an air barrier and a vapor retarder at sufficient thickness, adds structural rigidity. The most expensive per R-value by a wide margin.
Where it genuinely earns its cost:
- Shallow cavities — cathedral ceilings, 2×4 walls — where you need R-value the space can't otherwise deliver
- Rim joists, where air sealing and insulating in one pass is ideal
- Irregular or hard-to-seal geometry
- Assemblies where you want the air barrier and insulation to be the same layer
Where it doesn't: an open attic. Loose-fill delivers the same R-value there for a fraction of the money.
Cautions: spray foam is a professional installation — mix ratio, substrate temperature and lift thickness all affect whether it cures properly, and a bad install is expensive to remediate. It also permanently changes the assembly; foamed roof decks, for example, convert a vented attic into an unvented one, which has consequences for roof drying and requires the assembly to be designed for it. Get that right at design stage, not afterwards.
Choosing by location
| Location | Usual best choice |
|---|---|
| Open attic floor | Loose-fill (fibreglass or cellulose) |
| Cathedral ceiling | Closed-cell foam, or a designed vented assembly |
| Open wall cavity (new/gutted) | Batts, or foam where R-value is tight |
| Closed wall cavity (retrofit) | Dense-pack cellulose or injected foam |
| Rim joist | Rigid board sealed at edges, or closed-cell foam |
| Basement wall | Rigid board in an appropriate assembly |
| Crawl space | Depends on vented vs sealed — see basement and crawl space insulation |
| Between floors for sound | Mineral wool or open-cell foam |
Moisture is the thing that ruins insulation
Every material above loses performance when wet, and some assemblies trap moisture if the vapor control is in the wrong place.
Three principles:
- Vapor retarders belong on the warm-in-winter side of the assembly in heating-dominated climates. Requirements vary by climate zone and are set by code — check yours.
- Never sandwich a vapor-impermeable layer on both sides of a cavity. Moisture that gets in can't dry in either direction.
- Fix the water first. Roof leaks, plumbing leaks and bulk water in a crawl space must be solved before insulating, not after.
R-value isn't the whole story
Two things R-value doesn't capture:
Air leakage. Loose-fill and batts are air-permeable. Air moving through insulation carries heat straight past it. This is why air sealing comes before insulating — it's the single most common reason insulation underperforms its rating.
Thermal bridging. Framing conducts heat around cavity insulation. A wall with R-13 batts has a whole-assembly R-value meaningfully lower than 13 once studs, plates and headers are accounted for. Continuous exterior insulation is the fix.
The bottom line
Match the material to the assembly. Loose-fill for open attics, batts for accessible standard cavities, dense-pack for closed walls, rigid board where you need continuity or moisture tolerance, closed-cell foam where the space is shallow or the air barrier and insulation should be the same layer. Spend on foam only where its properties are actually needed — in an attic, cheap loose-fill does the same job. And whatever you choose, air seal first and keep the assembly dry, because both defeat any R-value.
See what the work costs in attic insulation cost, or check your whole-house picture with the home energy score calculator.
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