Why PIR Insulation Boards May Be the Best Solution for Your Floor

Keeping homes cool in the summer and warm in the winter comes at a cost. Heating and cooling account for more than half of energy bills, largely due to the poor levels of insulation in many UK homes.

While most homeowners see walls and roofs as the main culprits for heat loss, roughly a fifth of heat is lost through floors. This is both in the case of suspended timber and solid concrete floors. So, how can you save some money while also reducing your carbon emissions? Short answer, with the right insulation boards.

What Are Insulation Boards?

These are materials that have been used in some form or another for decades and now come in extremely effective PIR or phenolic varieties. They have largely replaced traditional mineral wool batts and spray foam due to their easy installation, low cost, higher thermal efficiency, and versatility in different building elements.

Insulation boards are essentially closed-cell foam panels, consisting of tightly packed polymers with trapped air that act as a barrier against heat transfer. Boards additionally seal rooms and buildings against drafts and air pockets, while preventing the buildup of moisture. Their structure helps to add rigidity and will keep its form throughout the lifetime of the building.

PIR boards, in particular, have some of the best properties of all insulation boards without being too thick. Standard sizes, like the 90mm PIR insulation boards, are versatile and can fit on roofs, walls, and floors. This is extremely important in areas with space restrictions, meaning older flooring boards like EPS or XPS might not fit if you’re aiming for higher thermal efficiency.

Using PIR Boards in Flooring Applications

Floors may experience lower rates of heat loss compared to walls, roofs, and ceilings, but any rooms above unheated and uninsulated areas, such as garages, will be noticeably colder. Boards in varying thicknesses can suit different floor types. This means they can work well when insulating below concrete slabs and screed, between suspended timber joists, and on block and beam floors. These boards can also be used under floating timber and chipboard floors and suspended timber floors, either above or below the joists.

Insulating Solid Concrete Floors

Insulating concrete and masonry floors is usually done during the construction of new homes. Concrete is a poor-performing insulator, so using foam boards substantially prevents heat loss.

The process involves different layers. First to be poured is the hardcore concrete base layer, followed by a blinding sand layer, a polythene sheet to prevent damp creep, and then PIR boards on top. Any leaks or gaps are taped.

Boards are most effective when laid in a brick pattern and sourced in the right thickness, depending on the available space. Medium thicknesses (75 to 100mm) are most common, but thinner boards applied in overlapping layers are just as effective. The finishing touches include the use of perimeter PIR strips to seal gaps and edges and pouring the concrete slab.

Insulating Below Concrete Screed 

Floors topped with thinner concrete screed layers are insulated similarly. First, the slabs are poured, then compacted, damp-proof sheets go on top, and PIR boards are placed in a brick bond pattern.

Boards are then covered with thinner damp-proof sheeting to prevent vapour, followed by pouring the sand and screed mixture at the recommended thickness. Floating floors without screed, are another instance where PIR boards are a good solution when retrofitting.

Insulating Suspended Timber Floors 

Suspended timber floors are insulated either from above or below the joists. Insulating from below involves less work, and the floorboards remain in place during the entire process.

Insulating from below the floor joists involves measuring and cutting PIR insulation boards to match the gaps between individual joists. Boards should fit snugly and then be secured to the joists either with timber battens or galvanised nails.

Insulating from above the joists is done in new builds with boards cut to exact sizes and matching the distances between joists. They are then secured with softwood battens, hammered in place with nails, or fastened with steel saddle clips at heights that make fitting floor boards simple.

Both methods usually require thicker 90-mm PIR insulation boards, but where space is tight, thinner boards (around 25 or 30mm) either in single or double layers also make for good insulation, provided they are installed correctly. Lastly, close off smaller gaps with expanding polyurethane sealant and larger holes with precisely cut PIR strips.

Why PIR Boards?

You might be tempted to opt for the cheaper XPS and EPS boards, especially for their high resistance to impact, but these have significantly lower thermal resistance (or R-values), so they won’t help much with lower energy bills or keeping rooms and premises comfortable.

Flooring applications are often limited by space issues. For instance, going with 150-mm XPS or EPS boards (to match the R-values of a comparable 90-mm PIR insulation board) may be impractical in the least, or they just won’t fit.

And this is just one advantage of these boards over other varieties. Boards can be much thinner (or of standard thickness) when fitting between joists or under concrete floors without eating into recommended ceiling heights. It means PIR is an easier way to comply with British Building Standards.

The combination of polymers, polyols, and isocyanates (in what is known as polyisocyanurate, or PIR for short) and the rigid closed-cell structure display other benefits too. Boards are lightweight, so standard 2400 by 1200mm boards in 90mm weigh roughly 8 kilos each, are easy to handle and transport, and make installation a one-man job. Additionally, they’re more durable than competing insulation boards, lend structural stability to other building elements, and won’t shrink or rot over time.

Where they perform much better in fire resistance and performance against moisture. Here, the added aluminium facer material means these boards meet the strictest fire standards, and won’t catch fire, emit smoke, melt, or drip. Moreover, they considerably reduce instances of humidity causing floor and wall damage due to mould and mildew. This makes them useful for roofing and wall uses, besides floors.

Lastly, polyisocyanurate is an eco-friendly insulation product. It uses no harmful CFCs during production (unlike related PUR boards), is packed with benefits for what it offers, and with prices coming down due to the selection of brands is the go-to choice when insulating your floor.