How to Insulate a Loft

Loft insulation is an effective way to reduce heat loss and save on your fuel bills. It traps heat inside your home during the winter and keeps things cooler in the summer. And getting the best loft installation done yourself is easier than you might think.

We'll show you exactly how to insulate a loft, as well as how to insulate pipes and cold-water tanks. You can watch our step-by-step how-to video alongside this guide to see exactly how it's done.

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The recommended loft insulation thickness is 270mm to 300mm to effectively trap heat and lower your energy bills.
Installing loft insulation involves laying a 100mm base layer between the joists and a 170mm top layer at a 90-degree angle.
You need to maintain a 25mm loft insulation eaves gap and leave a 75mm gap around hot chimneys, flues, or recessed lights to ensure safe ventilation.

What are the different types of loft insulation?

As there are so many different types of loft insulation, it might seem overwhelming when it comes to choosing yours. These are the most common types of insulation and what they’re best for.

Glass mineral wool rolls (also known as fibre glass loft insulation) are the most common type of insulation used for straightforward loft projects. This material comes in convenient rolls that are typically either 400mm or 600mm wide to match standard joist spacing. Most of these rolls feature pre-cut perforations, meaning you can easily snap them into halves or thirds to fit perfectly between your timber joists. For the best results, it’s recommended to lay this insulation to a depth of 270mm to 300mm.

Solid loft insulation boards are another excellent alternative. If you want to use your loft space for storage and already have an existing boarded area, you can lay these rigid insulation boards directly over the top of the boards rather than trying to use traditional insulation rolls.

Insulation foil is available as another home insulation option, though it serves a slightly different purpose and isn't typically used to fill the gaps between standard floor joists in the same way as mineral wool.

Planning and preparation

Before you start installing loft insulation, it's important to understand your space. This guide focuses on fibre glass loft insulation wool rolls, which is one of the most common types of loft insulation.

If you're wondering how thick loft insulation should be, the minimum recommended loft insulation thickness is 270mm. This gives you a great thermal resistance value. If your loft already has some existing insulation between the timber joists, you'll need to measure its depth.

Ideally, this base layer needs to be 100mm deep. It should come up to no more than 25mm above or below the joist. You can then lay a second layer of at least 170mm over the top of the joists to reach the building regulations guide of 270mm to 300mm.

The gap between joists is usually 400mm or 600mm. Because of this, a loft insulation roll is typically available in a width of 1140mm. Most rolls have pre-cut perforations so you can easily snap them into halves or thirds.

If you’re wondering how much loft insulation you need, start with a simple calculation. Work out the size of the area in square metres by multiplying the length by the width. Check the coverage of the rolls you’re buying and deduct around 10% to allow for the ceiling joists.

Remove any stored items from the loft so you have a clean and uncluttered space to work in. It's best to work in good lighting, so take a portable battery-powered light up with you. You'll also need to measure your cold-water tank to fit an appropriate tank jacket before you lay the main insulation.

Staying safe

When doing a DIY job like this, safety always comes first.

Turn off the power at the mains consumer unit before you touch any cables.
Always move slowly and carefully.
Work from crawling boards and move them along as you go. Never stand between joists and avoid standing directly on them as much as possible.
Remember to wear safety goggles, a suitable face mask and protective gloves when fitting your insulation.

You’ll also want to make sure the loft is safe after you’ve put in the insulation.

Insulation shouldn't be laid over any electric cables. Ideally, you should clip cables to the roof joists or gently lift them out of the way. You can then lay them over the insulation once it is in position. Do not fit insulation underneath your water tank unless it sits in an elevated position.
It's generally recommended to leave a 75mm gap between the insulation and any hot chimneys, flues, or recessed light fittings. But always check the manufacturer's instructions.
Before you begin, make sure your loft has ventilation gaps at the eaves or is fitted with loft ventilation. You need to maintain a proper loft insulation eaves gap during and after installation.
If you want to use the loft space for storage, you'll need to look at loft insulation and boarding. Install raised floor loft legs so you can fix boards above the insulation. Leave a minimum air gap of 50mm beneath the boards. Alternatively, you can cover an existing boarded area with solid loft insulation boards rather than insulation rolls.

How to install loft insulation

Once you’ve prepped your loft, keep reading for a step-by-step guide on how to install insulation in your loft.

Start by lining your cold-water tank with the jacket. Follow the manufacturer's instructions closely.

Next, insulate your pipes. Use a snap-off knife to cut the pipe insulation to size. You can learn more about the full process with our guide on how to insulate pipes.

Then, secure the pipe insulation in place with duct tape.

Remember that insulation shouldn't be laid over any electric cables. Where possible, clip them to the roof joists or gently lift them out of the way whilst you install. You can lay them over the insulation when it's in position.

If you have recessed lights, now is the time to install downlight protectors. Be sure to follow the manufacturer's instructions.

Measure the width of the roll you need to fit between the joists, which tends to be either 400mm or 600mm. Cut the roll to size using a panel saw or large kitchen knife.

Position your crawling boards in the furthest corner of the loft. You'll need to start laying with a minimum gap of 25mm between the eaves and the insulation to ensure a suitable air gap. Maintain this gap along two opposite sides of the loft.

Make sure the wall plate is covered. Then roll out the insulation, moving backwards so you are working towards the centre of the loft. This avoids putting your weight directly onto the insulation.

Gently push the insulation so it fits snuggly between the joists. But don't compress it, as that will reduce its efficiency.

When you reach a cross beam, use a straight edge or metal ruler and a snap off knife to cut the insulation to fit.

To join two lengths of insulation, simply butt them up tightly together.

Continue to lay until the first layer is in position. Cut the insulation as required to fit any awkward spaces.

To reach the recommended depth, a second layer can be added. Unroll the insulation and lay it at 90-degrees to the first layer and the ceiling joists.

When joining two lengths on this top layer, make sure the butt joint is a snug fit. Stagger the joints to avoid them all meeting in the same place.

Continue to lay the rolls until the entire loft floor is covered.

Don't forget about loft hatch insulation. Measure the loft hatch and cut insulation to size. You will probably need two layers to achieve the recommended depth.

Wrap the insulation you've cut in some of the insulation packaging. This will allow it to be secured in position neatly.

Attach the insulation to the loft hatch using a staple gun or large-headed tacks.

Aftercare

To make sure your hard work pays off for years to come, there are a few simple aftercare steps to keep in mind.

First, avoid applying weight to your insulation, and never store items directly on top of the rolls. Squashing the glass mineral wool material will reduce its efficiency and stop it from trapping heat effectively.

If you do want to use the loft space to store items, you'll need to look at loft insulation and boarding. We recommend installing raised floor loft legs. These allow you to safely fix boards above the insulation to hold your belongings. Just make sure you leave a minimum air gap of 50mm beneath the boards to let the insulation breathe.

It's also important to keep an eye on your airflow. Be sure to maintain your ventilation gap at the eaves during and after installation. Keeping this gap clear helps fresh air circulate, which allows trapped moisture to escape and prevents dampness or condensation from building up in your roof.

Adding insulation to your loft is a straightforward weekend project that will make a lasting difference to your home's comfort.

By following these steps and taking the right safety precautions, you'll be able to enjoy a warmer house and lower energy bills for years to come.

If you’re currently looking to improve your home's energy efficiency, check out our Energy Efficient Home for more projects and tips.

FAQs

Loft insulation works by trapping air within its fibres. This creates a thermal barrier that stops heat from escaping through your roof in the winter and prevents your home from getting too hot in the summer.

When installed correctly and kept dry, standard fibre glass loft insulation can last for decades. It's a great long-term investment that requires very little maintenance once it's in place.

Handling insulation is perfectly safe as long as you wear the right protective gear. Fibre glass can cause skin irritation, so always wear gloves, a face mask, safety goggles, and long sleeves when working in the loft.

Properly installed insulation can help regulate the temperature in your home, which reduces the chance of condensation forming on cold ceilings. But you must ensure proper ventilation in the loft to let moisture escape.