Without proper insulation, as much as 50% of your heating could disappear straight through your walls and roof. And leaking heat means leaking money.
Surprisingly quick and easy to install, loft insulation is one of the most effective steps you can take to make your home more energy-efficient and bring down your bills. In fact, if every UK household fitted it to the recommended thickness, it would save enough energy to heat 750,000 homes each year.
Tools & materials required
How much insulation do you need?
Most homes have some insulation, but it might not be as effective as it could be. Although loft floors should have at least 270mm insulation, there's often as little as 25mm - particularly in older houses. Also, existing insulation can sometimes be compressed by storage boards laid over the top, which makes it much less effective.
You can easily top up your insulation by using blanket insulation (also known as loft roll). This comes in different thicknesses and widths that are designed to be laid between and on top of joists. But if you want to be able to walk around your loft or store boxes there, you'll need to lay solid polystyrene board insulation on top of the insulated joists, and secure chipboard loft boards on top.
If you fit insulation at the joists (on the loft floor), it reduces the amount of heat that rises into your loft space. Therefore, in winter it will get colder up there than it used to. If you use the space for storing perishable items like books or photographs, you can insulate the sloping roof to keep more heat in the loft.
The insulation can either be expanded polystyrene panels that you fit between the rafters, or reflective foil that you staple to them. Alternatively, you could install a multi-foil insulating quilt at the rafters. Although this is more expensive than conventional products, it's far more efficient and gives a much higher level of insulation than the equivalent thickness conventional insulating material. However, you should only use rafter insulation as a top-up. You'll also need 270mm insulation at the joists, otherwise you'll be wasting energy and money on heating an uninhabited loft space.
If you convert your loft into a room, the insulation requirements are different. Therefore, you'll need to get professional advice as well as Building Control approval.
The 'R' value
A particular material's insulating properties are shown by the thermal resistance or 'R' value. This is how much the material stops heat passing through it. The higher the 'R' value, the better the insulation. A minimum 'R' value of 6.1 is recommended in a loft, ideally up to 7.0. Simply add the 'R' values of different products together to get their combined 'R' value. To work out the approximate 'R' value of your existing insulation, see the table.
Depth of existing insulation Approximate 'R' value 25mm 0.5 50mm 1.1 100mm 2.2 150mm 3.4 200mm 4.5
How to lay blanket loft insulation
Rolls of blanket insulation are fairly straightforward to lay and fit. To work out how many rolls you need, measure the length and width of your loft. If you can't do this in the loft itself, you could try measuring the ground floor if it's the same size, or even measuring around the exterior walls of your house. Just multiply the length by the width to find the area, and then divide that figure by the coverage per pack of insulation to work out how many packs you need.
Rolls of insulation can be heavy. So if you're using larger ones, either ask someone to pass them up to you or cut them to size before you take them into the loft. When you've laid your second layer of insulation, you won't be able to see the joists. This method is only suitable for a loft (or the part of a loft) that you won't need to walk on or use for a storage. If you're laying mineral wool insulation, you'll need to wear protective clothing and gloves.
Top tip - Walkboards
Make yourself a couple of walkboards to lay across the joists to work safely in your loft area. It's useful to have more than one walkboard so you can stand on one while you move the other (and three if you're working in a larger loft). Use a strong piece of sheet timber - 18mm ply or chipboard is perfect. Don't make the boards too large or they'll be difficult to move around. And as most ceiling joists are fitted at 400mm centres, a walkboard 1700mm long and roughly 500mm wide is ideal. Lastly, try to avoid having too much overhang as the board will be unstable if you stand close to the end.
Safety first - Protect yourself
Wear a dust mask, protective gloves and knee pads when you're working in a dusty loft.
Clear your loft as much as possible to give yourself a large, safe work area, and remove any old storage boards. Use planks to create a flat storage area for a few rolls, close to where you're working. Next, make yourself one or more walkboards, long enough to span three or four joists and wide enough for you to kneel on.
Start in one corner and work back towards the hatch. Put your walkboard along the adjacent joists and lay the first roll at the eaves, leaving a 25mm gap for ventilation and using 100mm insulation between the rafters.
Carry on unrolling between the joists across the loft floor area. If one roll isn't long enough to reach to the other side, start a new roll and join the edges closely together.
Once you've unrolled your first length, lightly push the insulation down against the joists so there aren't any gaps, take care not to compress it too much. Carry on across the complete loft area.
For narrower areas, cut the insulation to the correct width and lay it between the joists in the same way. Try not to pack or compress insulation into awkward corners.
When you reach a cross beam, cut the insulation in line with the beam. Start again on the other side, working in from the eaves.
A second layer
When you've laid insulation between your joists, unroll a second layer of 170mm blanket insulation at a 90 degree angle to the first layer. You can use wider rolls if you want to speed up the process. Also, remember that it's dangerous to walk in the loft when your rafters are invisible. Only cover them in parts of the loft that you'll never walk on - otherwise, lay board insulation.
How to cut blanket loft insulation
Rolls of blanket insulation come in different widths, and you can easily cut them with a handsaw. You might find it easier to do this before you take them into the loft. Once you've rolled the insulation out in the loft, you can cut it with something sharp like a bread knife (or a pair of scissors in tricky areas).
Cut your large rolls using a handsaw. They may be perforated so they can be cut easily, without any need for measuring. You can also cut them in their packaging by following the pre-marked lines. But it's better to leave the roll uncut if you're laying the insulation across the top of the joists.
You can cut blanket insulation by sandwiching it between two offcuts of board. Use the top board as a guide and run a sharp knife along the edge for a straight cut.
For small, difficult cuts or cuts near power cables, use a pair of sharp scissors for accuracy and safety.
One Planet Home - Insulated loft trap door
Don't forget to insulate your loft trap door. You can do this to an existing door by sticking solid insulating board cut to size on its upper side. However, it's better to replace it with an insulated trap door. This is a drop-down door with insulation attached to the reverse side. Most modern homes have a standard size loft opening, therefore you can simply screw a new trap door into place.
How to lay loft insulation around cables and downlights
In your loft you may have electrical cable and downlights for the room below. If your home is an older building, try not to cover cable with insulation as it could overheat. It's better to lay it under the cable, or re-route the cable if you need to. In most new homes, cable is rated for you to cover it and you'll most likely find it clipped to the joists. In this case you can leave it there and carry on insulating. If you're in any doubt, always talk to an electrician.
Some downlights generate lots of heat and can be a fire risk. With downlight fitting, you must leave a sufficient gap either side of it (check the manufacturer's instructions), if you're laying encapsulated insulation or any other insulation except glass mineral wool between the joists. You can lay standard glass mineral wool insulation over the light fitting as long as you've fitted a cylindrical heat diffuser to it to create a gap around the fitting.
If cables in your loft aren't clipped to the joists, carefully lift the cable while you lay the insulation underneath.
If you're using glass mineral wool insulation, you'll need to cut it either side of a downlight fitting. Fit a heat diffuser to the fitting from the room below, following the manufacturer's instructions. Then push it back through the hole into the loft. You can then lay a second layer of insulation over the top of the heat diffuser.
How to lay board loft insulation
If you want to use part of your loft for storage, you'll need to insulate this area in a different way. You can't put heavy boards or boxes on top of blanket insulation, because it'll compress the material and reduce its 'R' value and effectiveness. Instead, use high performance extruded polystyrene board insulation to create an insulated storage deck in your loft. Covering this with chipboard loft boards, will protect it from wear and tear.
Rigid polystyrene board insulation is designed to fit over joists with 100mm loft roll or blanket insulation between the joists underneath. With a double layer of this board insulation on top, you'll have the same 'R' value as a 270mm double layer of blanket insulation. You'll need to be able to get to your storage area directly from the loft hatch, and have access to a water tank if you've got one. If you're putting in blanket installation in the rest of the space, you'll need to lay board insulation in your storage area before you hide the surrounding joists with a second layer of blanket insulation.
Make sure you fit your blanket insulation between and in line with the top of the joists. Lay the board insulation loosely across the joists. Each 1200mm board should typically fit across three of them. If you've got downlights, you'll need to put in heat diffusers and cut a hole in the board insulation to house them.
Cut your boards to size using a handsaw. If you're cutting the boards in the loft, you'll need to be very careful not to cut through any pipes or loose cables.
Mark the centre of the joist on the boards with a marker pen.
Join up the joist centre marks with a straight edge.
Lay a second layer of board insulation in the same way, at 90 degrees to the first layer. Then transpose the joist centre lines onto the second layer. If you've got any downlights, you'll need to cut the board insulation around them, following the manufacturer's recommended guidelines.
Lay chipboard on top of the board insulation and fix it to the joists along every joist centre for a firm fixing. Drill small pilot holes through the chipboard flooring in line with the joist centre marks at each corner and in the centre of each long edge. Screw through the chipboard and both thicknesses of board insulation into the joist, using 150mm screws. Once the storage area is complete, lay blanket insulation over the joists across the rest of the loft to a combined thickness of 270mm.
How to lag pipes
Lagging pipes is pretty simple when you use split foam tubes. Try to lag your cold water pipes in the loft to stop them from freezing, as well as any exposed hot water pipes running through unheated areas of the house for them to retain their heat.
Top tip - Foil rafter insulation
For a quick way to top up your insulation or prevent the temperature in your loft storage space from dropping too low in winter, try fitting insulating thermal foil to the rafters. Make sure you wear safety goggles and a dust mask to protect you against dust. It'll also be easier if you get someone to help unroll the foil. Staple it to the rafters with a staple gun. Each row should also be butt jointed and each join covered with aluminium joining tape.
Open a split foam tube and slide it onto the pipe. If you've got pipes coming up through your loft floor or running along the joists, fold back the loft insulation, fit the pipe tube and then put the insulation back in place.
Carry on insulating the whole length of the pipe run. When you come to a bend in a pipe, cut a 45 degrees mitre join in the foam tube using a small mitre block and small saw.
Tape the join using heatproof aluminium tape.
If you've got a stopcock or tee joint, the easiest way to insulate it, is by making a cut to one side of it, then cutting it horizontally so the foam tube can wrap around the object.
Cut and place a small length of pipe tube over the tap handle, and cover it with tape to hold it in place.
Carry on lagging all the pipes. If the pipes run across a joist, lag them before covering them with a second layer of insulation.
How to fit rafter panel insulation
Expanded polystyrene panels fitted in the rafters are a simple way to improve your insulation. They're easy to cut with a small saw, then you just squeeze them between the rafters. Make sure you wear safety goggles and a dust mask to protect yourself from any dust and debris.
Rafter insulation needs a 50mm air gap between the insulation and the roof felt. Start by attaching 50mm battens in line with the top edge of the rafters, using screws.
The rafter panels have pre-formed grooves to bend and slot them in between the rafters. Then they'll stay in place without the need for extra fixing.
Cover the whole area and leave a 25mm gap along the bottom, at the eaves.