Floor Insulation Guide For Suspended Timber And Solid Floors
Lifting the boards and filling the gaps between the joists is one of the simplest ways to stop a room feeling cold underfoot.
Written by David White
Updated
The short answer
Floor insulation stops heat escaping through the floor and cuts the draughts that come up between floorboards. On a suspended timber floor, you lift the boards and fit mineral wool or rigid insulation board between the joists, supported from below, while keeping the airbricks clear. Solid concrete floors are insulated with rigid boards laid on top, which raises the floor level and is a bigger job.
Most people think of the loft and the walls when they talk about insulation, but a draughty floor can make a room feel cold even with the heating up. Insulating the floor is a job you can do yourself, and the difference shows as soon as the boards go back down.
How you go about it depends on what sort of floor you have. A suspended timber floor, with boards on joists and a void underneath, is a job most competent DIYers can take on. The only hard part is lifting the floorboards, and once they're up, the insulation itself goes in quickly. A solid concrete floor is a different job altogether, which we cover further down.
Why Insulating Your Floor Is Worth Doing
The Energy Saving Trust lists floor insulation as one of the ways to cut heating bills, particularly on the ground floor of a house with a suspended timber floor. Beyond the savings, it makes rooms more comfortable in a few ways:
- Draughts: air rising through gaps between the boards and under the skirting is cut down
- Warmth: heat stays in the room instead of escaping into the void below
- Comfort underfoot: floors feel noticeably less chilly on bare feet
- Summer temperatures: insulation slows heat moving in both directions, so rooms stay steadier
- Sound: denser insulation also reduces noise travelling between floors
Suspended Timber Floors Or Solid Concrete
If you can lift a floorboard and see a gap with joists running across it, you have a suspended timber floor. These are the easiest to insulate, as the insulation sits between the joists and the boards go straight back on top. Upstairs floors are almost always built this way too.
A solid floor is concrete laid directly on the ground. You can't get beneath it, so insulation has to go on top in the form of rigid boards with a new floor surface over them. That raises the floor level, so doors, skirting boards and thresholds need adjusting, and it's worth getting advice before you start.
The Drawbacks To Plan For
Floor insulation isn't a tidy job. The room has to be emptied, the carpet and underlay rolled back or taken up and the floorboards lifted, so it makes sense to do it while you're already redecorating or replacing the flooring. Our guide on removing carpet and gripper rods covers that first stage.
Lifting the boards can also uncover problems you didn't know about. Rotten or split boards, damp joists or old wiring all need dealing with before the insulation goes in, so allow time and budget for surprises.
Thermal Or Acoustic Insulation
Insulation rolls and slabs are sold as thermal or acoustic. Both keep heat in, but acoustic insulation is denser, which makes it better at stopping sound passing through the floor.
On a ground floor, thermal insulation is all you need. Between floors, acoustic insulation is the better choice, as it cuts footsteps and voices from the room above as well as keeping the heat where you want it. That's especially handy in rentals and shared houses.
Choosing The Insulation Material
Mineral wool rolls, made from glass or rock fibres, are the most common choice between joists. They're cheap, easy to cut with a long knife and squeeze in snugly, filling uneven gaps well. Rigid foam boards give more insulation for the same thickness but need cutting accurately to fit tightly between the joists, with any gaps sealed.
Natural options like sheep's wool are pleasant to handle and don't irritate the skin, but they cost more. Whatever you use, check its thickness against the depth of your joists, as it needs to fit without pushing the boards up.
How To Insulate A Suspended Timber Floor
The steps below are how we tackle a suspended timber floor. Before you start, put on a dust mask, safety glasses and gloves, because cutting and tearing mineral wool sends fine fibres into the air and they irritate the skin, eyes and lungs.
- Clear the room of furniture so you have the whole floor to work on.
- Roll back or remove the carpet and underlay, and pull up any gripper rods along the walls.
- Lift the floorboards, starting with any that are already loose or have been cut before. Number them as they come up so they go back in the same place.
- Check the boards and joists for rot, splits or damp, and replace anything that isn't sound.
- Take out any old, damp or flattened insulation.
- Clean out dust and debris from between the joists with a vacuum.
- Staple breathable netting or fix timber battens along the sides of the joists to hold the insulation up, if there's no ceiling below to support it.
- Measure the gaps between the joists and cut the insulation to size, leaving it slightly oversized so it friction-fits.
- Push the insulation snugly between the joists, working around pipes and cables rather than squashing it over them.
- Refit the floorboards, screwing them down to stop creaks, then relay the underlay and carpet.
As the photo shows, the void under the boards can be shared with pipes and cables. Clip cables so they sit above the insulation where possible, as cables buried in insulation run hotter. Wrap any water pipes in foam lagging too, because the void will be colder once the room above is insulated.
On a ground floor, never block the airbricks in the outside walls. The void needs that airflow to stay dry and keep the joists free of rot.
Insulating A Concrete Floor
Insulating a solid floor means putting down a damp-proof membrane and rigid insulation boards over the concrete, then covering them with a floating chipboard floor or a screed. Because the floor comes up in height, internal doors need trimming and the skirting boards have to come off and go back on at the new level.
It makes most sense when you're already replacing the floor or renovating the room. If you're planning a new floor finish anyway, our guide to Karndean flooring covers one option that sits well over an insulated floor.
Other Ways To Keep The Heat In
Floor insulation works best alongside a few smaller jobs that cut draughts around the room:
- Gaps under skirting boards: seal them with a flexible decorator's caulk or sealant
- Doors: fit draught excluders along the bottom and brush strips around the frame
- Floors without insulation: lay thick rugs with a good underlay
- Windows: hang lined or thermal curtains and close them at dusk
- Old windows and doors: upgrade them to energy-efficient units when the budget allows
To find out where the heat is escaping, a camera from our roundup of the best thermal imaging cameras shows cold spots and draughts along floors and skirting boards clearly.
Is Floor Insulation Worth It
For a suspended timber floor, insulating between the joists pays you back in comfort from the first cold evening. The insulation itself doesn't cost much, and most of your effort goes into getting the boards up and back down again.
If the boards are up anyway, it's also the ideal moment to think about warmth from below. Our guide on installing underfloor heating explains how it works alongside floor insulation, and our pick of the best electric underfloor heating kits covers systems that suit a single room.
Your Questions Answered
Is it worth insulating under floorboards?
Yes, insulating under floorboards is worth it on a suspended timber ground floor, especially if the room feels draughty or cold underfoot. It cuts heat loss into the void and blocks draughts coming up between the boards. Time is the real cost rather than materials, because lifting and refitting the floorboards is the slow part, so it pays to do it when you're changing the flooring anyway.
Can you insulate a floor without lifting the floorboards?
You can insulate a suspended floor without lifting the boards if there's a cellar or a crawl space deep enough to work in. Insulation is fitted between the joists from below and held up with netting or battens. Without that access, the boards have to come up, though you can lift every few boards and slide insulation along the gaps between joists.
Do you need a vapour barrier for floor insulation?
A suspended timber floor doesn't need a vapour barrier under the insulation, as the void beneath has to stay ventilated so moisture can escape through the airbricks. Use breathable netting to support the insulation instead. A concrete floor is different, and rigid insulation boards laid on top of the slab go over a damp-proof membrane.
Can floor insulation cause damp?
Floor insulation won't cause damp on its own, but blocking the ventilation under a suspended floor can. Keep every airbrick clear and don't pack insulation hard against the outside walls, so air still moves through the void. If the joists or boards are already damp, find and fix the cause before insulating, or the problem gets sealed in.
Which insulation material would you use between the joists?
Mineral wool rolls are the one to use between joists for most jobs. They're cheap, easy to cut with a long knife and squeeze in snugly, so they fill uneven gaps well. Rigid foam boards give more insulation for the same thickness but need accurate cutting and sealed gaps. Whatever you buy, check its thickness against the joist depth so the boards sit flat.
Do I need acoustic insulation or thermal insulation?
Use thermal insulation on a ground floor and acoustic insulation between floors. Both keep heat in, but acoustic insulation is denser, so it also cuts footsteps and voices from the room above. That makes it the better choice in rentals and shared houses, while on a ground floor thermal insulation is all you need.
What should I do about pipes and cables in the void?
Work the insulation around pipes and cables instead of squashing it over them. Clip cables so they sit above the insulation where possible, because cables buried in insulation run hotter. Wrap water pipes in foam lagging as well, since the void is colder once the room above is insulated and the pipes are more exposed.
What's the biggest mistake people make when insulating a floor?
The biggest mistake is blocking the airbricks in the outside walls of a ground floor. The void needs that airflow to stay dry and keep the joists free of rot, so keep them clear when you push the insulation into place. Check the boards and joists for rot, splits or damp before anything goes in.
Dave, or Demolition Dave as the team calls him, worked as a builder for over 10 years before joining DIY Works. He now concentrates on his own properties and writes most of our construction content, along with tools, carpentry and plenty more.