The Basics of Rigid Foam Insulation – A Beginners Guide

The early onset of sub-zero temperatures will have most Britons struggling to pay for their already high energy bills. Coupled with the fact that over a third of homes predate 1946 (the highest percentage in …

rigid foarm insulation

The early onset of sub-zero temperatures will have most Britons struggling to pay for their already high energy bills. Coupled with the fact that over a third of homes predate 1946 (the highest percentage in Europe) and have meagre or no insulation whatsoever, this spells a long and cold winter for many. 

The good news is that home insulation is still affordable and some insulation products are covered by reduced VAT rates. And when you consider lowered energy bills, the insulation products used up paying for themselves within the first few years of installation. 

The hardest part may be deciding which type of insulation to choose. Older solutions like glass and mineral batts have proven effective in the past, but newer products, in the form of rigid foam boards, are better in every way. Not only do they keep more warmth in, but they can also be much thinner to fit within limited space, especially in the predominantly older building stock. 

Rigid insulation boards are becoming the industry standard, and recent PUR and PIR boards should be high on anyone’s Christmas shopping list. 

What Are Rigid Foam Boards?

Foam boards are one of the most common types of insulation products. They’re of solid closed-cell construction (unlike batts and spray foam) and made of varying materials and in different production processes. The aim is to retain more heat and increase the thermal efficiency of even the oldest and leakiest homes. 

Boards can be used in different building areas, like walls, roofs and floors and come in different thicknesses. Besides keeping more heat in and reducing energy bills, rigid foam boards are favoured for their fire resistance, keeping out moisture and excessive noise, and lending to the structural integrity of building elements. Once installed, they can last for the lifetime of the building.

Also Read: Top Reasons Why Your Home Needs Rockwool Insulation

Types Of Rigid Foam Boards

Different boards behave differently, and that’s reflected in the price. But even the cheapest boards are worth considering if costs are an issue. 


Expanded Polystyrene or beadboard is a lightweight and cheap insulation board made of closed-cell thermoplastic polystyrene foam. It is made by heating polystyrene beads in the presence of a blowing agent. The result is a foam that consists of 98% trapped air and only 2% foam. 

This lends its good thermal properties, with air being a poor heat conductor. To reach the thermal effectiveness of other boards, such as the 75mm PIR insulation products on the market, EPS needs to be optioned thicker and in higher densities and enveloped in foils. In this case, EPS boards need to be 150mm to have comparable performance. 

It doesn’t mean that it’s an overlooked insulation solution. EPS boards are strong and resistant to impact, making them ideal for flooring. And since they’re water and moisture-proof, the thermal performance isn’t affected by liquids (great for external wall insulation) and meets elementary fire standards in that it won’t ignite or spread flames. 


Extruded Polystyrene is an improved version of EPS due to how it’s made. It involves liquifying polystyrene beads under high heat and pressure and in the presence of a blowing agent to create a closed-cell foam. The resulting product is denser and has higher compressive strength, meaning it will be better suited for flooring applications and where high-impact resistance is needed. 

It also has much better water resistance, so it’s preferred in roofing applications (such as inverted roofs). As it also has higher thermal efficiency (or R-values) for not much more means that it’s often the inexpensive thermal insulation board most budget-conscious buyers choose.  

PUR and PIR 

These are two slightly distinct types of polyurethane-based insulation boards. They demonstrate nearly twice the thermal efficiency of polystyrene EPS and XPS boards, which sets them apart from those materials. Rigid polyurethane (PUR) and polyisocyanurate (PIR) boards have similar basic components such as crude oil derivatives, polyols, isocyanates, and blowing agents that expand into a closed-cell foam between an aluminium backer and facer foil.

While PUR boards were the benchmark for thermal performance a few years back, PIR boards have suppressed them in almost all use cases. These contain more isocyanates that lend the boards better compressive strength, much-improved fire resistance (PIR boards slow the spread of flames and reduce smoke) and higher R-values meaning even lower gas and heating bills. 

Besides meeting stricter fire standards, they can also be used in thinner variants where there’s a need for high thermal values. To compare, a 90mm PUR board has the same R-value as a 75mm PIR insulation board. Those 15mm count in areas where space can be an issue, such as external walls and concrete slab floors. 

In addition to the scope of advantages and eco-friendliness due to the absence of harmful CFCs, PIR boards also comprise the insulating core of insulated plasterboard, often used in internal walls. 


These are the most recent boards on the market, and they naturally have the best sheathing properties and the highest prices. They have a high closed cell count and fine cell structure that can prevent heat loss and moisture penetration. 

Phenolic boards are made of phenolic resins and foaming agents under high heat and pressure with the ingredients sandwiched between a metallic facer and backer materials. They exhibit up to 10% better thermal efficiency than comparable PIR boards and have much better fire and moisture resistance. They can also prevent mould and mildew buildup and have slightly higher compression strength. They can also be thinner where needed. 

Uses of Rigid Foam Boards

Rigid foam insulation boards can encase different areas within homes, offices and businesses, such as external, internal and stud walls. For exterior wall sheathing, look to boards with low moisture permeance, like PIR and phenolic for the best results. 

Internal walls can be insulated with PIR-backed plasterboard, and the same is often used in stud walls. PIR, XPS and phenolic foam boards are all used in cavity wall insulation.

EPS and XPS boards are cheap below-concrete slab insulation solutions. On the other hand, PIR and PIR-backed plasterboard often insulate above-floor slabs. Suspended timber floors are also insulated with the same materials. 

For roofing projects, look for PIR or phenolic boards in slanted roofs, while flat roofs require a PIR board with a water-resistant bituminous layer. XPS boards are a cheaper alternative and can be used in both flat and pitched roofs.

Buying Tips

When sourcing foam boards, the first thing is to allocate a budget. EPS boards are good all-rounders and cost-effective when insulating larger surfaces. XPS boards aren’t a lot more expensive but are better in flooring and roofing applications where they can be made slightly thinner. 

PIR boards are high-performance and versatile, and prices are reflected with better properties across the board. They meet some of the highest insulation and fire-resistance requirements, and so are proven choices in older homes. When cost isn’t an issue, opt for phenolic boards. These provide the best result in most applications. 

Thickness correlates with thermal efficiency and price. Boards with higher R-values are more expensive but can be used in thinner variants. For instance, prices are similar for thicker EPS or XPS compared to thinner PIR and phenolic boards with the same or similar efficiency levels. What isn’t is that not all boards can fit within the available space. 

Regardless of price and installation costs, all insulation boards can recoup initial costs within a couple of years of being installed. Homeowners can in the meantime enjoy more warmth and lower energy bills. 

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