How to insulate your loft
Keep your home warm by insulating your loft
Insulating your loft can seem like an unappealing job, but saving money on your utility bills always sounds good. If your home’s without adequate insulation, you could be losing up to a quarter of your home’s heat through your roof*. This makes insulating your loft one of the most effective ways to reduce heating bills and be more energy-efficient. And even if you already have some existing insulation, it’s key to have the optimum amount for it to be truly effective.
In this guide, we’ll walk you through everything you need to know to lay loft floor insulation. Also known as ‘cold roof’ insulation, insulation is fitted to the floor of the loft trapping the heat in the main part of the house and away from the loft. This is ideal if you don’t use your loft at all, or only for the storage of non-perishable items.
The alternative is ceiling, or ‘warm roof’ insulation. This is fitted above the rafters and keeps the heat in the loft. It uses different types of insulation to loft floor and is used in loft conversions to make the space into a habitable living area. This isn’t explored in this article.
Our guide includes step-by-step advice on how to:
- Work out how much existing loft floor insulation you already have
- Confirm how much new insulation you need
- Lay, and cut, two different types of loft floor insulation – rolls of insulation and insulated loft floor boards
- Lay loft floor insulation around cables and recessed downlights.
Let’s get started…
Choose your tools and materials based on which type of loft floor insulation you're laying.
We’ll discuss two options:
- Loft floor rolls
- Insulated loft floor boards.
Loft floor rolls – these are the traditional option, and, as the name suggests, are rolled out along the floor of the loft. This makes them easier to lay than insulated floor boards, and require fewer tools and take less time. They’re offered as loose or encapsulated (blanket) rolls and are suitable for creating both base and top layers. They can be boarded over using stilts to create a raised platform for storage.
Insulated loft floor boards – these are a two-in-one combination of polystyrene insulation and a rigid top layer made from chipboard. They’re only suitable for creating the top layer of insulation and so still require loft roll be laid for the base layer. They provide a flat and secure surface area - great for storage space and for easy access – but not a structural floor finish to make your loft a liveable room.
Our guide includes advice on laying both of these:
- A base and top layer of loft floor rolls, and
- A top layer of insulated loft floor boards.
The top layer could be a combination of the two (boards where necessary and rolls elsewhere). Configure and calculate the amounts needed based on your loft requirements.
The following lists of tools and materials includes everything you’ll need to lay both types of loft floor insulation. Be sure to check the steps for further detail to avoid buying unnecessary items.
To lay either type of loft floor insulation
- Tape measure
- Insulation guards - also known as heat diffusers, loft caps or loft covers, they protect any recessed light fittings from overheating underneath the insulation. Buy as many as needed.
- Screwdriver – for removing any existing boards fitted to your ceiling joists, if needed. Opt for a hand screwdriver if there are only a few, or an electric screwdriver or combi drill if there are many
- Pen and paper – to note how much existing loft floor insulation you currently have, joist sizes and spacing, if needed.
To lay loft roll insulation
- Loft floor insulation. We love the Eko loft insulation roll - Eko is a new range of insulation available at B&Q. Specifically made for the thermal upgrade of loft spaces at ceiling level. For more info on the options available, check out our insulation buying guide
- Offcuts of wooden board – for cutting insulation
- Panel saw – to cut wide rolls of insulation into multiple slimmer rolls or insulated loft floor boards, as necessary
- Long knife with serrated edge – to cut rolls to the correct length, if needed. A bread knife is ideal.
- Scissors - for making smaller cuts in loft rolls, as needed.
To lay insulated floor boards
- Insulated loft floor boards – this is the only insulation and chipboard loft floor board that we offer. Other insulation boards available don’t work as a similar 2-in-1.
- 150millimetre(mm) wood screws – for securing boards to joists
- Wood adhesive – for joining the boards
- Combi drill – to secure the boards
- Wood drill bit - to fit 150mm wood screws
- Small paintbrush - for applying the wood adhesive to the boards
- Cloth – to wipe away any excess wood adhesive from boards
- Rule or straight edge – for cutting boards
- Jigsaw - for cutting boards to fit around recessed downlights, if needed
- Wood or chipboard jigsaw blade - for cutting boards to fit around recessed downlights, if needed
- 14 to 18mm diameter wood drill bit - for drilling start holes to cut boards around recessed downlights, as needed
- Work bench - to secure the boards when cutting holes for recessed downlights, if needed
- Bin liner/ refuse sack – if removing any existing loft roll
- Dust mask - protects when cutting boards
- Safety googles - protects when cutting boards
Before tackling any loft project, head to our guide on loft safety.
It includes essential advice on how to:
- Choose suitable protective clothing for loft projects
- Access the loft
- Prepare the loft
- Move and work in the loft, including details on using walk boards and how to light the loft
These tips and recommended items aren’t included in this article's list of materials and tools. Be sure to read and check you have everything you need to safely start insulating your loft floor.
Additionally, there are safety considerations specific to laying loft floor insulation.
- Enlist a helper to pass loft rolls and boards to you once you’re in the loft
- Keep loft insulation a minimum of 75mm away from hot flues (including enclosed chimneys, from a boiler or wood-burning stove)
- Take extra care when working close to power cables
- Don’t walk on laid insulated loft floor boards
- Check that any items to be stored on insulated loft floor boards don’t weigh more than the recommended weight limit
- Wear a dust mask and safety goggles when drilling or cutting into the insulated loft floor boards.
Many homes have some existing insulation, but it may not be very effective. This could be because it’s been compressed by storage boards laid over the top or if it’s not been topped up in a long time. Some older properties can have as little as just 25mm (deep) on the loft floor.
The good news is that there’s no need to remove any existing loft floor insulation. Just add one or more new layers on top of it to reach the recommended amount. More on how much is recommended later in the article.
To find out how much you currently have, a trip into the loft is necessary.
In the loft, measure the depth of the existing insulation using a tape measure. Make a note so that you’ll remember the amount once out of the loft.
Measure the spacing between your joists. To do so, start at the mid point of one joist and measure to the mid point of the nearest one. This distance is often 400 or 600mm.
This joist spacing will determine what width of loft roll you can use. More on that in the ‘How to calculate the desired width of loft roll insulation’ section.
With the existing loft floor insulation details confirmed, it’s time to work out how much new insulation is required.
To do this, you need to know two things.
The required thermal resistance of the insulation. Alternatively, if you’re laying loft rolls only, there’s a quicker method which measures the insulation’s thickness instead. For advice on this, jump to the section ‘How to calculate the required thickness of loft floor insulation’.
- Width of the insulation rolls.
We’ll show you how to work these out in the sections below.
How to calculate the required thermal resistance of loft floor insulation
The depth of loft insulation (in mm) equates to an R-value rating. This is a unit of thermal resistance and rates the insulating properties of the material. The higher the R-value, the greater the insulation.
A minimum R-value of 6.1 is recommended for loft floor insulation, but ideally it would be up to 7.0.
Use the table below to work out how the depth of your existing loft insulation translates into an R-value. This is just for glass mineral wool insulation - the most common and popular material. Other types have slightly different thermal properties, so check the online product page or product packaging for details.
|Depth of glass mineral wool loft insulation (mm)||Approximate R-value|
Start with the target R-value of 7 and deduct the R-value of the existing insulation. This will give you how much more R-value is needed. We offer glass mineral wool insulation with R-values of 2.25, 3.85 and 4.5.
If using two layers of loft roll – combine loft rolls of different R-values to get as close as possible to the desired R-value. As a rule, we recommend a base layer of 2.25 and a 3.85 top layer.
If using a top layer of insulated loft floor boards – aim for a base layer as close as possible to an R-value of 2.25. On top of this, the boards will provide additional insulation to bring the combined total up to the equivalent R-value of 6.1.
How to calculate the required thickness of loft floor insulation
The Government recommends a minimum insulation depth of 270mm. This is for glass mineral wool insulation only. We offer glass mineral wool insulation in three different roll thicknesses – 100, 170 and 200mm.
Deduct the depth of the existing insulation from the target depth of 270mm. This will give you how much extra depth is needed.
If using two layers of loft roll – try to get as close as possible to the desired extra depth by combining rolls of different thicknesses. If there’s very little existing insulation, we recommend a 100mm base layer and a 170mm top layer.
If using a top layer of insulated loft floor boards – aim for a base layer of loft roll as close as possible to 100mm. On top of this, the boards will provide insulation to bring the combined total to the equivalent depth of 270mm.
How to calculate the desired width of loft roll insulation
Your choice of width roll is determined by your loft floor’s joist spacings. This is because the insulation is rolled between these joists. We recommend choosing one that’s as close to your joist spacing as possible – this minimises the need to trim off excess.
There are two options – narrow or wide loft rolls. Narrow rolls work best in small spaces, while wider rolls can be more cost-effective and can be cut to size to fit unusual, or wider joist widths.
The two most common joist spacings are 400 and 600mm.
If your loft floor joist spacings are 400mm, choose either:
- 370/380mm wide loft rolls, or
- 11400mm wide loft rolls to be cut into three smaller rolls.
If your loft floor joist spacings are 600mm, choose either:
- 570mm wide loft rolls, or
- 11400mm wide loft rolls to be cut into two smaller rolls.
To calculate how many packs of loft floor insulation are needed, measure the relevant loft space.
Measure the length and width of the loft space to be insulated. Keep all measurements in the same unit – metres (m).
This may be difficult or not possible in the loft area, so if the floor below is the same size as the loft, measure that instead.
Calculate the size of the loft area by multiplying the length by the width.
Length (m) x Width (m) = Area of loft in square metres (m2)
Subtract 10%. The base layer won’t cover the entire loft floor as the rolls are laid between, not over the ceiling joists. By deducting this amount, it calculates a more accurate area to be insulated for the base layer.
Area of loft/100 x 90 = Area of loft m2 to be insulated
Divide the area by the coverage per pack, this will be on the packaging and the online product description. This will confirm how many packs are required. Round up to the nearest whole number.
Size of Area of loft m2 / Pack coverage = Number of packs needed.
Repeat Steps 2 and 4 to calculate how many packs of your chosen top layer of loft insulation are required. Don’t complete Step 3 (deducting 10%) as the joists will be covered by the top layer.
Now you have the final number of loft floor insulation rolls and/or boards required to complete your loft insulation project.
Before laying any insulation, it’s important to prepare the loft space for this project. Again, head to our article on loft safety as this includes essential information on how to move safely in the loft.
In the loft, remove any boards fixed to your joists with a screwdriver.
Check for any existing insulation under the cold water tank (if you have one). If the tank is elevated by at least 300mm - leave it where it is. If the tank isn't elevated by at least 300mm - remove it. Don't block the heat coming up through the ceiling as it can actually help keep the tank warmer and prevent freezing during cold winter weather.
Cold water tanks should be insulated separately, read our how to guide for more information.
Clear as much space as possible to give yourself a large, safe work area. Use any available wooden planks or loft boards to create a flat storage area for a few rolls close to where you’ll be working.
Check for any electrical or lighting cables. Covering these with insulation can pose a fire risk, so it’s important to assess them before laying insulation.
Without touching the cables, look to see how much slack is in the cables. Is there enough slack for it to be lifted on top of the insulation you’re about to lay without overstretching it?
If so – the cables can be moved. Don’t do this now. We’ll explain how to do this, and when, in the section ‘How to lay loft insulation around cables’.
If the cables don’t have much slack, are clipped to the joists or you’re not confident handling them – consult a qualified electrician.
If you have any recessed downlights in the rooms below your loft – lift any existing insulation and check whether the lights are covered by insulation guards. Lights can be a fire hazard if covered by insulation without a protective guard acting as a barrier between the two.
Buy as many guards as are needed. We’ll show you how to fit them later in the section ‘How to lay loft insulation around recessed downlights’.
Enlist your helper to pass you the rolls of insulation up into the loft. Don’t remove them from their packaging or cut them before lifting into the loft. If you open them anywhere else, they’ll expand making it very difficult to carry up into your loft. It’ll also release insulation fibres which can be a messy irritant in the home.
The following steps advise on how to insulate the loft floor with rolled or encapsulated insulation. This includes between, and over, the joists and creating base and top layers.
If any cables or recessed downlights are found, jump to the sections ‘How to lay loft insulation around cables' and 'How to lay loft insulation around recessed downlights’ for advice on working around these. It's essential to start doing this now, before any insulation is laid, so that you're not caught out later on.
If also lagging water pipes as part of your loft insulation project, we recommend doing this before laying any loft floor insulation. This is because it can be difficult to access, measure and insulate water pipes once the loft floor is covered - especially insulated floor boards.
For step-by-step advice on insulating water pipes, head to our article.
If you’re laying narrow, pre-cut rolls (370/380 or 570mm) or encapsulated insulation - jump to Step 2.
Cut any larger width rolls (1140mm) to the desired width dependent on your joist’s spacing. (See ‘How to calculate the desired width of loft roll insulation’ for appropriate width sizes). Still in their packaging, cut along the partially cut perforations using a panel saw.
Start in one corner of the loft and work back towards the loft hatch. Position your walk board at right angles to the adjacent joists. (For more on walk boards, visit our ‘How to work safely in the loft’ article.)
Lay the first loft roll at the eaves, leaving a 25mm gap away from the eaves for ventilation to help prevent condensation.
If you’re laying encapsulated insulation, there’s no need to take it out of its silver wrapping. Roll it out, silver side up.
Continue unrolling between the joists across the loft floor area. Once you’ve unrolled your first length, lightly push the insulation against the joists so there are no gaps, taking care not to compress it. Remember that the insulation will be compressed from being in its packaging, and so could take a while to reach its full height, as advertised. Gently ‘fluff’ it up with your hands to help it reach its maximum depth.
If one roll isn’t long enough to reach to the other side of the loft, start a new roll and butt the joints closely together. Do not overlap or compress the insulation.
When you reach a cross beam, cut the insulation flush with the beam then start again on the other side, working in from the eaves.
To cut the insulation, use a long sharp knife with a serrated edge, such as a bread knife. Sandwich the insulation between two boards and, using the top board as a guide, run the knife along the edge for a straight cut.
Use a pair of large scissors for small or difficult cuts, for example around pipes.
For narrower areas, cut the insulation to the required width and lay it between the joists in the same way. Don’t pack or compress insulation into awkward corners.
Continue laying across the entire loft area until all the base layer is complete.
Lay the second, or top, layer.
Don’t cut any wide rolls into narrower ones when laying the top layer across the top of the joists. This will help speed up the laying process and reduce any joins and gaps.
Lay the first roll of the top layer at right angles to the base layer. Continue until the top layer is complete. Ensure all joints are snuggly butted together.
If using insulated loft floor boards – continue to the following section ‘How to lay insulated loft floor boards’.
The following steps advise on laying a complete top layer of insulated loft floor boards. If you’re only laying a section of boards to create a storage deck, remember to cover the rest of it with loft roll to achieve the optimal amount of insulation.
Turn off the electrical supply. This is because boards are fixed with a drill and so the power must be off in case of drilling into any cables.
Go to your consumer unit or fuse box and switch off the main power switch. If you have more than one consumer unit or fuse box, switch off both of them.
For modern consumer units (with miniature circuit breakers) - switch off and lock the main switch with a lockout tag and padlock. Put the key safely in your pocket. Attach a label or note to the consumer unit(s) or fuse box to advise you’re working on the circuit and not to operate.
If in any doubt, or for more information, consult a qualified electrician.
With the electricity off, use a hanging inspection light and/or head torch to make sure you can see what you're doing.
If you have an existing top layer of loft roll that you’re replacing with insulated loft floor boards, roll up and remove from the area you’re to board. The top layer should stay in place next to the finished boarded area.
Put the unwanted loft rolls into large bin liners/ refuse sacks while still in the loft. This will make it easier to remove from the loft and prevent any fibres from making a mess in your home.
Check that the base layer of insulation is laid between, and flush with, the top of the ceiling joists. Adjust as necessary.
Lay the insulated loft boards at right angles to the ceiling joists and with the chipboard surface on the top. Ensure that each 1220mm long board is supported across at least three joists.
If fitting close to the eaves, maintain a 25mm ventilated airspace between the insulation boards and the eaves.
Keep boards a minimum of 12mm away from any hot water pipes.
We recommend using a pen to mark the position of water pipes clearly on the surface of the chipboard for future reference.
If there are any recessed downlights underneath the base layer – jump to the section ‘How to lay loft insulation around recessed downlights’.
Fix each board to the ceiling joists below with 150mm wood screws.
To do this, use four screws per board, one in each corner. Carefully position so that the boards are secured through the centre of each joist and away from any cables or pipes.
Mark the fixing positions on the board with a pen and drill a countersunk hole through the chipboard only.
Steadily hold the screw vertically and push it through the insulation by hand, until you reach the joist and loosely fix for now.
Lay additional boards in the same way, fitting the tongue and groove joints together snugly.
Apply wood adhesive to the timber joints with a small paint brush. Wipe away any excess glue with a cloth. Glue and fit each board as you go, one by one.
Trim boards to size by marking a straight line with a rule and then cut with a panel saw. Do so outside of the loft for ease.
Ensure any cut pieces, or excessive overhangs, are sufficiently supported by joists.
Stagger the joints of the next row of boards (such as in a brick pattern) for extra stability.
Repeat the laying process until all boards are in place.
Once all boards are laid and glued into position, tighten the screws for a secure fit.
Switch the power back on.
In the section ‘How to prepare to lay loft floor insulation’, you checked the slack in the cables. And based on that (and potentially other factors), decided whether you were going to handle them yourself or consult a qualified electrician. Follow the below steps if you are undertaking the job yourself.
Turn off your electrical power supply at the fuse box/ consumer unit before lifting any electricity cables. See Step 1 of ‘How to lay insulated loft floor boards’ for more on this.
Carefully lift the cable to sit on top of the insulation. Do not stretch it. By laying the cables on top of the insulation, it will also make it easier to find and access them again.
Out of the loft, switch the power supply back on.
Top tip: Upgrade to LED bulbs
Recessed downlights with LED bulbs emit far less heat than those with older-style halogen bulbs minimising their fire risk when covered by loft insulation. They’re also more energy-efficient and so last longer and are cheaper to run.
We recommend still using an insulation guard with LED bulbs as it’ll help maintain the life expectancy of your lights.Shop LED light bulbs
In the section ‘How to prepare to lay loft floor insulation’, you checked if you have insulation guards fitted over any recessed downlights. The following steps advise on how to fit new guards (if there are none in place) and lay insulation accordingly.
Fit an insulation guard around the recessed downlight.
Check that the downlight’s transformer is placed clear of the insulation. And that the downlight doesn’t touch the inside of the guard.
Follow the product installation instructions for further details.
Fit the insulation around the guard.
For the base layer - lay the loft roll over the insulation guard. Continue as per ‘How to lay insulation rolls’.
Lay the top layer.
If using loft roll - continue to lay it out over the base layer. Follow the instructions in ‘How to lay insulation rolls’.
If using insulated loft floor boards – lift the base layer of loft roll to approximate the location of the light. There may be a hump in the base layer indicating the light’s whereabouts.
Position the board on top of this. Draw a circle marking the position of the light underneath on the chipboard surface.
Measure and mark 75mm from two edges of the circle. From these, draw a square around the circle (pictured).
Out of the loft, clamp the board in a work bench.
Drill a hole in each corner of the drawn square. Use a wood drill bit that’s wide enough for a jigsaw blade to enter.
Use a jigsaw with a wood or chipboard cutting blade to cut the chipboard surface along the edges of the square.
Use a long, serrated-edge knife to cut the square out of the board’s polystyrene insulation. Check that both layers of the board (chipboard and polystyrene insulation) have both been removed.
With the square cut in the board, lay it in place and secure following Steps 6 and onwards in ‘How to lay insulated loft floor boards’.
Once all boards are secured, fill the holes in the boards with piece of insulation roll cut to size.