Insulate under a suspended timber floor — HomeGraft

Lift boards, support and fit insulation between joists with an airflow gap, then refit boards and seal draughts for a warmer floor.

IUF-01Plan the job and buy your materialsWork out whether you can get under the floor from a cellar or crawl space, or whether you need to lift boards from above. Measure the room and count the joist bays to work out how much insulation you need, and measure joist depth so you leave a 50mm airflow gap below the boards. Buy mineral wool insulation cut to width, plus netting or battens to support it, and screws to refit the boards.

6 each · Mineral wool insulation, 100mm x 1200mm x 600mm roll

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1 each · Galvanised support netting, 25m roll

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1 each · Torch or head torch

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1 pair · Knee pads

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IUF-02Find your stopcock and check for pipes and cablesFind your stopcock, usually under the kitchen sink, and check it turns freely before you start, so you can shut off the water fast if you hit a pipe. Run a cable and pipe detector over the floor area, but treat it as a guide only — detectors miss cables in metal conduit, cables run deep, or pipes outside the usual routes. If you are not certain what is under a board, ease it up gently by hand rather than forcing it or drilling blind, and isolate the circuit at the consumer unit first if in any doubt.

1 each · Cable and pipe detector

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IUF-03Clear the room and lift the floorboardsMove furniture out of the way and lift boards working across the room in one direction, using the technique for lifting a board without wrecking it. Number or mark each board on the underside as you go so it goes back in exactly the same spot. Stack lifted boards flat somewhere safe.

1 each · Chalk or pencil for marking boards

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IUF-04Inspect the void before you insulateCheck joist spacing (commonly 400mm or 600mm centres) and joist depth, and look for damp, rot, or pest damage. Note any pipework or cabling running through or across the joists, and locate air bricks — these must stay clear. Deal with any damp or rot before you insulate over it, since covering a problem makes it worse and harder to spot later.

IUF-05Fit support between the joistsStaple galvanised netting to the sides of each joist about 50mm below the top edge, making a shelf to hold the insulation. As an alternative, screw 25x50mm timber battens to the joist sides at the same depth using 40mm wood screws driven into the centre of each joist. This stops insulation sagging or dropping over time.

1 each · Heavy-duty staple gun and staples

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8 each · Timber batten 25x50mm x 2.4m

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1 box · Wood screws, 40mm, box of 100

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IUF-06Cut and lay the insulationMeasure each bay between joists and cut the insulation slightly oversize so it holds in place by friction, without squashing it flat — compressed insulation performs worse. Lay it onto the netting or battens, leaving a clear 50mm gap of air between the top of the insulation and the underside of the boards. Wear gloves and a dust mask; mineral wool fibres irritate skin, eyes and lungs.

1 box · Disposable dust masks, box of 10

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1 pair · Work gloves

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IUF-07Work around pipes and cables safelyKeep any pipes on the warm side of the insulation, nearer the room, so they're less likely to freeze in cold weather. Wrap any pipe that ends up on the cold, outer side of the insulation with pipe lagging. Never staple or nail through a cable — route netting around it instead, and don't compress insulation directly onto exposed wiring.

4 each · Pipe lagging, 1m lengths

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IUF-08Check ventilation is not blockedMake sure the insulation and netting don't cover or block air bricks or the underfloor airflow route. A suspended timber floor relies on that airflow to stay dry — blocking it lets damp build up and can rot the joists over time. Leave a clear path for air right across the void, not just near the edges.

IUF-09Refit the floorboardsLay each board back in its original position using your marks from earlier. Screw down through the existing pilot holes with 50mm wood screws rather than renailing, so the boards can be lifted again easily in future. Walk the length of each board once fixed to check for squeaks or movement.

1 box · Wood screws, 50mm, box of 100

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IUF-10Seal gaps for draughtsRun a flexible decorators' caulk along the gap where the boards meet the skirting, and fill any wide gaps between individual boards with flexible wood filler or a rope draught excluder. This stops warm air leaking upward through the gaps now that the void below is insulated.

2 each · Flexible decorators' caulk, 310ml

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1 each · Rope draught excluder, 10m

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IUF-11Clear up and dispose of wasteVacuum up dust and loose fibres from the room and check the floor for stray staples or screws. Bag up insulation offcuts and take them to a household waste site if your council won't collect them in the normal bin. Remove and wash gloves and any reusable PPE.

3 each · Rubble sacks

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