How do I ensure these PIR boards are actually airtight under my suspended floor?

Expanding foam is definatly the way to go for the primary seal between the PIR and the joists, rather than relying on tape alone. Even the best foil tape struggles to maintain a long-term bond with raw timber beacuse of the dust and the way wood expands and contracts with the seasons. If you rely on tape on the wood, it eventually peels away and you lose that airtightness you're working so hard for. Use a **low-expansion gun grade foam** specifically. The DIY cans with the straw are too messy and hard to control, but a proper applicator gun lets you run a bead right into those gaps with precision.

Once the foam has cured, you can trim the excess flush with a long blade. If you want a belt-and-braces approach, you can then run **aluminum foil tape** over the joints where the PIR meets the timber, but its main job there is just as a secondary vapor barrier. The foam is what provides the actual structural friction and the thermal break. To prevent the boards from shifting or dropping over time, most of us either use small galvanised nails driven into the joists at an angle to act as 'stops' or screw some thin timber battens underneath the boards. Compared to standard fiberglass batts, this setup is vastly superior because it stops air movement within the floor void. Fiberglass is notorious for letting drafts 'wash' thru the material, wich kills the R-value. By using PIR, you're creating a solid thermal envelope. Just make sure you aren't blocking any air bricks in the external walls; the crawlspace underneath the insulation still needs that cross-flow ventilation to keep your joists from rotting.

For the garage conversion, you might find the joist spacing is less uniform than in the main house. In those cases, cutting the boards about 10mm undersize and filling the entire perimeter with foam is much faster than trying to get a perfect friction fit. It actualy results in a better seal because the foam fills every nook and cranny in the old wood.

Spot on with the gun foam advice. I’ve seen too many DIYers use the cheap 'Soudal' straw cans only to have it shrink and pull away within a year. If you want a real seal, grab some Illbruck FM330; it’s elastic, so when those joists move, the seal stays intact. Also, since you're doing the lounge next, don't overlook the R-value drop if you leave a massive air gap above the board. I always aim for Celotex GA4000 or Kingspan TP10 at 100mm, then shim them tight against the floorboards. If you leave 50mm of dead air, you're just inviting convection currents to steal your heat. Get it snug.

Back in the day, we just stuffed mineral wool up there and hoped for the best, but you’d still feel the breeze on a cold Tuesday. You’re on the right track with the PIR, but watch your fingers on those board edges—I’ve got scars from slipping with a rusty saw back in '92. One thing these youngsters didn't mention is the 'friction fit' myth. Don't bother trying to wedge them in tight; you'll just bow the board. Cut them a good 15mm shy and let the foam do the heavy lifting. Also, check your wiring before you seal it all up. Nothing worse than forgetting a cable run and having to hack through your hard work later.

Don't forget to tape the seams where the boards meet each other, not just the joists. I always use a decent 75mm foil tape for those bits. If you're worried about them sagging later, I've found that those little plastic insulation clips are a lifesaver. They're cheap and way faster than faffing with battens. Also, when you get to the garage, check your head height before committing to thick boards—I once over-insulated a floor and couldn't open the door properly afterwards. Proper nightmare.

Quick update — I took the advice on the Illbruck FM330 foam and it’s a total game changer compared to the straw cans. The seal feels much more robust against the timber. I also picked up those plastic clips to prevent any sagging later on. Feeling way more confident about starting the lounge now. Cheers for the help!