Spray Foam vs Mineral Wool: Which Insulation Is Right for You? — Guide

08 May 2024

Choosing insulation is one of those decisions you only want to make once. Two of the most popular options on the market today—spray foam and mineral wool—both deliver strong thermal performance, but they work in very different ways and suit different homes, budgets, and priorities. Here's a practical breakdown to help you decide.

How Each Material Works

Spray foam is applied as a liquid that expands into a foam, filling gaps, cracks, and awkward cavities before hardening. It comes in two varieties: open-cell, which is softer and allows some moisture vapour through, and closed-cell, which is denser, acts as a vapour barrier, and adds structural rigidity.

Mineral wool—made from molten rock or slag spun into fibres—comes in batts, rolls, and semi-rigid boards. It's fitted manually between joists, studs, and rafters, and it's the same family of material many homeowners already have in their lofts.

Thermal Performance

Spray foam generally wins on raw insulating power per inch. Closed-cell foam in particular achieves a high R-value in a thin profile, which makes it attractive where space is tight, such as between rafters or in converted attics. Open-cell foam is less efficient per inch but still performs well once applied at sufficient thickness.

Mineral wool offers solid, dependable R-values that meet most building codes comfortably when installed at standard depths. The trade-off is that you typically need more thickness to match foam's performance—a consideration if you're insulating a shallow cavity.

Installation and Air Sealing

This is where the two materials differ most. Spray foam is applied by professionals with specialized equipment, and once it cures, it seals the space almost completely. That airtightness is a genuine performance benefit: air leakage is responsible for a significant share of heat loss in most homes.

Mineral wool is often marketed as a DIY-friendly option, and batts between standard-spaced joists are indeed manageable for a careful homeowner. However, results depend heavily on installation quality:

Batts must be cut precisely around wiring, pipes, and outlets—gaps dramatically reduce real-world performance.
Compression squashes the fibres and lowers the R-value.
Mineral wool does not seal air leaks on its own, so a separate air barrier strategy is usually needed.

A note on moisture

Mineral wool is water-resistant but not a vapour barrier, and it drains rather than absorbs water if exposed. Spray foam, especially closed-cell, resists moisture penetration directly—useful in basements and against exterior sheathing.

Cost, Longevity, and Practicality

Spray foam carries a higher upfront cost because it requires professional installation and the material itself is pricier. Mineral wool is cheaper to buy and can be installed without a contractor in many cases, which keeps the total bill lower.

Both materials are durable. Mineral wool is inorganic, doesn't support mould growth, and won't sag or settle if properly supported. Spray foam is similarly long-lived when correctly mixed and applied, but a poor installation is difficult and costly to remove—something worth weighing if you're renovating an older property with unknown conditions behind the walls.

Fire performance is another point in mineral wool's favour: it's inherently non-combustible and can withstand very high temperatures without melting or emitting significant smoke.

Which Should You Choose?

There's no single right answer, but these guidelines cover most situations:

Choose spray foam if you're insulating irregular or hard-to-reach spaces, converting an attic where depth is limited, or prioritizing maximum airtightness in a new build or deep retrofit.
Choose mineral wool if budget matters, you're working with standard open framing, you want a fire-resistant and DIY-friendly material, or you value easier removal during future renovations.
Consider a hybrid approach—many builders use closed-cell foam for a thin air-sealing layer and mineral wool batts for the bulk of the insulation, combining the strengths of both at a moderate cost.

Whichever route you take, the quality of installation matters as much as the material itself. Get quotes from reputable installers, ask about moisture management in your climate, and make sure the insulation strategy fits the way your home is built. Done right, either choice will keep you comfortable for decades.