In this new series, Chartered Surveyor Ian Rock FRICS – Author of the Haynes Home insulation Manual –  explains how to cut your energy bills by boosting your home’s thermal efficiency.




How To Insulate Timber Floors


…..and stem draughts and heat loss from under your feet


In a typical home around 15% of the property’s total heat loss is through the floors.

Many older houses have ground floors made of suspended timber which can be particularly cold and leaky. To protect timber ground floors from damp they need a permanent flow of air to ventilate the void underneath so that any build up of moisture can evaporate away. This is provided air via airbricks in the lower walls.

But this constant stream of cold air just beneath your feet can be a source of considerable heat loss, particularly where there are draughty gaps between the boards or around the edges of the room. Although timber has moderate insulation properties,the relatively thin floor boards can transmit cold from the void below.

On the plus side, upstairs floors don’t usually need insulating since they’re already located inside the building.  Only where there’s an unheated space below, such as an integral garage would upgrading the floor be worthwhile. However boosting sound insulation may be worth considering – see


Planning the job

Carrying out insulation work to floors can obviously be fairly disruptive, particularly if it involves moving fitted units in kitchens and cloakrooms etc. So this is a job that needs to be planned carefully, perhaps as part of a larger refurbishment project. There are a number of other points that also need to be considered:-


* The condition of the floor   

There’s no point fitting insulation if it’s going to cover up existing defects, storing up a lot of trouble for the future. So while the floor is exposed check that the joists haven’t been weakened by having excessively large notches and holes cut in them for central heating pipes etc. Also check that external ground levels aren’t too high (a common cause of damp problems) – they should be at least 150mm below the damp proof course (DPC).


*  Sub-floor ventilation        

Check that the airbricks in the lower main walls aren’t blocked as this will allow stagnant air to build up under the floor promoting damp and rot. On an average size house there should be at least 3 on each external wall. New vents may need to be inserted where they’re lacking or defective.


* The floorboards

What kind of floorboards have you got? Some more expensive older houses have tongue and grooved boards with concealed nail heads, which can be hard to lift without damage. Original floorboards should be retained especially in period properties where they have historic value. So where lifting them could cause damage, there may be some other means of access to the void below, such as via a cellar, or by cutting a neat hatch in the floor.


* Services

While the void is exposed, it’s well worth insulating any pipes running under floors. Unless lagged, hot water pipes (e.g. to radiators) can lose a lot of valuable heat, and cold pipes can freeze and burst. Also any draughty gaps around pipes and cables where they pass through the floor should be sealed, e.g. with a flexible mastic.


Choosing materials

Insulation materials commonly used for timber floors includes:-


*  ‘Floppy’ materials (e.g. mineral wool quilt)

*   Semi-rigid batts (e.g. mineral wool or sheepswool)

*   Rigid foamed plastic insulation boards (e.g.polyurethane or polystyrene )

*   Loosefill  (e.g cellulose fibre)


Thick foamed plastic boards are one of the best performing insulators and can be cut to fit between floor joists. Quilts, such as mineral wool are relatively cheap and easy to handle, although dearer natural sheep’s wool is a more pleasant material to work with. Loose-fill and floppy materials obviously need some form of support to be provided to hold them in place, as described below.

The Building Regulations set a U-value target of 0.25 W for upgrading existing floors.  With typical 8 x 2 inch (200 x 50mm) floor joists there should be more than sufficient space for all types of insulation to meet this target.


Fitting the insulation                                               

There are several ways you can insulate a suspended floor. But probably the simplest and most cost-effective single improvement is to carry out draught-proofing



Draught-proofing can achieve very good results with minimal disruption. Sealing gaps will  boost comfort levels by reducing the amount of cold air blowing in. Larger gaps can be filled with small strips of wood cut to match the shape. Smaller gaps can be filled using ready-made strips of artificial foam rubber which are sold in a range of diameters. These are easy to fit by pressing them into place between floorboards etc.

Gaps around the edges of the room can sometimes be quite large; these can be filled  using compressible draught seal strips, or where visual appearance isn’t so important (such as behind fitted units) by squirting expanding foam into the gaps. For smaller gaps silicone mastic sealant can be used.

Gaps around pipes or cables can be sealed with tightly-packed mineral wool, expanding foam or silicone sealant. In cases where the floorboards have excessive numbers of gaps, it’s probably simpler to cover the surface with hardboard sheets screwed in place as a base for carpeting.


Insulating from above

Having lifted the floorboards, strips of insulation can be inserted between the joists. Options include:

 * Mineral wool or sheepswool insulation (similar to that used in lofts) is sold in quilt form which being ‘floppy’ will need to be supported when fitted in floors, for example from netting draped over the tops of the joists or on battens fixed along the undersides of joists.

* Insulation batts (e.g. mineral wool or cellulose) are compressible so they can be fitted between joists, but again some form of support is needed, such as battens fixed to the lower joists.

* Rigid boards (e.g. polyurethane) can be cut to size and wedged between joists and supported on horizontal batten rails fixed along the sides of each joist.


Insulating from below

If you happen to have a cellar under your floor it obviously opens up the possibility of installing insulation from below. But even without a cellar, it’s  often possible to insulate from below because in older properties the sub-floor voids can be several feet deep. This can be useful where floorboards are hard to lift. To get access you may only need to lift about 3 boards, or cut a small hatch in the floor.

Insulation materials such as mineral wool quilt, compressible batts or rigid boards cut to size can can then be pushed up from below and squashed between the joists, held in place with timber straps nailed across their undersides.


Finishing the job

Once all the insulation is in place, the floorboards can be re-laid in their original positions.

But before doing this it might be worth fitting a thin layer of rigid plywood board across the tops of all the joists prior to re-instating floorboards to further improve draught-sealing. And bear in mind that raising the floor height by even a centimetre or so could necessitate the need for adjustments to doors and skirting.


NEXT MONTH:     Cold feet? What can be done to make solid concrete floors warmer?  


Ian Rock’s home insulation tips are taken from the new Haynes Home Insulation Manual. For further information see