In Victorian and Edwardian times it was common to design homes with suspended timber floors at ground level. These floors rested on timber joists above an earth void — sometimes creating a ‘crawl space’ several feet deep beneath the house. Air vents at the bottom of exterior walls helped prevent dampness.
Suspended timber floors can also be found over unheated passageways in Victorian buildings or within medieval structures, over a carriageway or jetty. These floors can be a source of heat loss and draughts, but insulation and draughtproofing can help cut energy bills; take care, though, not to harm your building’s character or impede sub-floor ventilation, which could encourage timber decay. Before beginning any work, consider conducting an airtightness test to pinpoint trouble spots.
Permeable insulation – such as a sheep’s wool, hemp fibre, cellulose or wood fibreboard – works well with pre-1919 buildings, where the design relies on moisture being able to pass through walls. But don’t discount non-breathable insulation when adequate ventilation can be achieved.
Keep a space of at least 150mm below any insulation installed within an externally ventilated floor void (the presence of air vents will point to this). Lag sub-floor cold or hot water pipes and route electrical cables outside the insulation.
Working on the floor
If access is available to a suspended floor from an area below, or via a crawl space beneath the floorboards, insulation batts or boards might be pushed up snugly between the joists from below. Battens, plastic netting or, for better draughtproofing, a breather membrane can be fixed to the joist undersides to hold the insulation. Alternatively, additional insulation, such as wood or hemp fibreboard, can be fixed underneath.
Floorboards are frequently lifted to add insulation between the joists in the form of batts, boards or loose fill cellulose. Damage is easily caused when removing old floorboards and their fixing – usually iron nails, so employ a skilled carpenter to do this job.
Battens, netting, a breather membrane or fibreboard may be used to support the insulation. Fibreboard is sometimes fixed on top of the joists to increase insulation.
The risk of almost inevitable damage may mean that lifting historically important boards to fit insulation is unviable. The space available within a sub-floor void can also limit the thickness of insulation possible. Special considerations exist for traditional buildings, regardless of whether they are listed. These allow lower insulation levels where full compliance would cause unreasonable harm to the property.
Where insulation cannot be installed from below the floor, consider putting it on top. Your options include laying thin, flexible aerogel blankets over floorboards, or wood fibre boards. The disadvantages of this approach are that the original floor will be covered up, and the raised floor level may interfere with doors and steps.
Sometimes only draughtproofing is possible. There are products on offer such as Stopgap (stopg-p.co.uk) and DraughtEx (draughtex.co.uk). Finally, of course, laying a thick carpet or a large rug will reduce air infiltration.