Can you add insulation between floor joists without removing floorboards?

Loose-fill insulation works best if you can access the joist cavities from a basement or crawl space below, but if you're working from above with floorboards in place, you'd need to drill holes through the subfloor between each joist bay and use a pneumatic blower to fill them - messy and labor-intensive, which is why most people end up removing boards if they want decent coverage.

If you've got access from below - basement, crawl space, whatever - that's your best shot. You can drill holes through the rim joist or band board and blow in loose fill pretty easily. From above with floorboards intact, you're looking at drilling a bunch of holes and dealing with a mess, so most folks find it's worth pulling up at least a section of flooring if the insulation gap is really bad enough to bother with.

Drilling access holes through the floorboards is genuinely your best bet here, and it's way more doable than people make it sound! You just need a spade bit or hole saw to create openings between joists, then blow in your loose-fill insulation (cellulose works great) and patch the holes afterward with wooden plugs or just fill them flush. It takes some patience to get the spacing right so you're hitting the joist cavities properly, but the payoff in energy efficiency is totally worth the effort! The key is going slow and measuring twice so you're not drilling into pipes or wiring running underneath.

The key thing people miss is that you really need to seal those access holes properly after you're done, or you're just creating air leaks that defeat the whole purpose. If you drill through floorboards and blow in loose-fill from below, the insulation settles and shifts over time - so you want to use a backing material (like cardboard or rigid foam) under the joists first to catch and support it, then seal your holes with caulk or foam once you're finished.

From above is trickier because gravity works against you, but it's doable if you're patient. You'd drill holes between joists, use a shop vac with a collection bag attached to the fill hose to control the mess, and work slowly so the material settles properly. The annoying part is making sure you hit actual cavity space and don't accidentally hit wiring or plumbing - so knowing your floor layout beforehand saves a lot of frustration.

One detail the others didn't mention: check whether your floorboards are original or if there's a subfloor underneath. If there's a plywood subfloor below your finished flooring, drilling becomes more complicated because you've got two layers to punch through, and you risk splintering or creating pathways for air movement between the two layers.

The cleanest approach is spray foam if you can access from below - it fills gaps way better than loose-fill and sticks to everything without settling over time. Drilling holes through floorboards works too, but you'll want a shop vac running because loose-fill gets messy and some settles, leaving top cavities empty. If basement or crawl space access exists, that's definitely easier than working from above.

The access question is really the whole game here - if you've got a basement or crawl space, blowing in loose-fill from below is genuinely your easiest path and doesn't touch those floorboards at all. But if you're stuck working from above, drilling strategically placed holes through the floor (say, one per joist bay) is totally workable; you're not committing to major surgery on your floors, just creating temporary access points. Just make sure you're drilling between joists, not through them, and seal those holes back up once you've blown in your insulation. The real limitation isn't the method - it's whether you can actually reach those cavities from either direction.