Rigid Foam vs Fibreglass Insulation: Which Performs Better? — Guide

06 May 2022

Choosing the right insulation is one of the most impactful decisions in any renovation or new build. Rigid foam boards and fibreglass batts are two of the most common options, and each brings distinct strengths and trade-offs. Understanding how they compare in performance, cost, and installation will help you pick the right material for your project.

Thermal Performance: R-Value Compared

R-value measures resistance to heat flow, and it's the figure most homeowners focus on first. Rigid foam boards—whether expanded polystyrene (EPS), extruded polystyrene (XPS), or polyisocyanurate—generally deliver a higher R-value per inch of thickness than fibreglass. This means you can achieve the same thermal resistance with a thinner profile, which matters in tight spaces like basement walls, rim joists, or exterior sheathing.

Fibreglass batts, by contrast, typically offer a moderate R-value per inch but are available in thickened widths to compensate. In a standard framed wall cavity, both can meet code requirements—the difference is how much space they consume to get there.

Watch Out for Real-World Drift

Manufactured R-values are measured under ideal lab conditions. Fibreglass can lose effectiveness if it settles over time or if air moves through it, because air leakage undermines its insulating ability. Rigid foam holds its rated performance more reliably and also acts as an air barrier in many applications. Polyiso, however, can lose a small amount of insulating capacity in very cold temperatures, so it's best paired with another insulation layer in extreme climates.

Moisture Handling

Moisture is where these materials diverge sharply:

Rigid foam absorbs very little water and does not support mould growth, making it ideal for basements, below-grade walls, and areas prone to dampness.
Fibreglass can hold moisture within its fibres, which reduces its insulating value and can encourage mould on adjacent surfaces if a vapour barrier is missing or damaged.
Foam boards provide their own vapour resistance, though seams must be taped or sealed properly to prevent moisture bypass.
Fibreglass typically requires a separate polyethylene vapour barrier in cold climates, adding a step to installation.

If your project involves concrete, masonry, or humid spaces, rigid foam is usually the safer bet.

Installation and Practicality

Fibreglass batts are lightweight, flexible, and forgiving. They compress around wiring, pipes, and irregular framing, and most homeowners can install them with basic protective gear. The downside is that gaps, compression, and poor fitting dramatically reduce real-world performance—a batt installed carelessly may underperform its rating by a wide margin.

Rigid foam requires more precision. Boards must be cut cleanly, fitted snugly, and sealed at joints with tape or canned foam to function as an air barrier. It's also more brittle and bulkier to transport, and some products require covering with a fire-resistant material such as drywall when used in living spaces.

Cost Considerations

Fibreglass is almost always the cheaper option per unit of R-value, which is why it remains the default for attics and standard wall cavities. Rigid foam costs more upfront, but it can pay for itself in applications where its thinner profile frees up usable space, or where its air-sealing ability reduces the need for supplementary materials. For a whole-house project, the budget gap can be significant, so many builders use a hybrid approach: foam board as continuous exterior or basement insulation, with fibreglass filling framed cavities.

Which Should You Choose?

There's no universal winner—context decides. Choose rigid foam when dealing with basements, exterior sheathing, slab edges, or anywhere moisture and air leakage are concerns, or when space is at a premium. Choose fibreglass for open attics, standard framed walls on a budget, or projects where a DIY-friendly, flexible material makes sense.

For many homes, the best answer is not either/or but both: rigid foam handling moisture and continuous thermal coverage where it matters most, with fibreglass providing economical cavity fill. Assess your climate, the specific area being insulated, and your budget, and you'll land on the right combination.