Quick answer: Lay a 100mm base layer of mineral wool insulation between the loft joists, then a second layer of at least 170mm on top running at a right angle, reaching the recommended total depth of 270 to 300mm. Leave gaps around vents, hot flues, and recessed lights, and do not compress the insulation, since trapped air is what gives it its insulating effect.
Heat rises, and in an uninsulated home a significant share of it escapes straight through the roof. Insulating a loft is one of the most cost-effective home improvements you can make, often paying for itself many times over across its lifespan. This guide covers how to do it yourself, what depth you need, and when it is worth calling in a professional instead.
Why Insulate a Loft?
Loft insulation reduces heat loss through the roof, which lowers your heating bills and makes the home more comfortable in both winter and summer. Properly installed insulation typically lasts around 40 years, making it one of the more durable home improvements available, and it can also improve your property’s energy performance rating.
What You Will Need
- Mineral wool insulation rolls, sized to fit standard joist spacing
- A tape measure
- A sharp knife or insulation cutter
- Boards to walk safely between joists, if the loft is not already boarded
- Gloves, a dust mask, and old clothing, since mineral wool fibres can irritate skin and lungs
- A torch or head torch for visibility
Step 1: Check Existing Insulation
Before you start, check whether your loft already has insulation and, if so, how deep it is. Use a tape measure to check the depth in a few different spots, since it is often uneven. If the existing insulation is in good condition, you can simply top it up. If it is old, damaged by damp or pests, or badly compressed, it is best to remove it and start fresh.
Step 2: Prepare the Loft
- Clear out any stored items so you have full access to the joists.
- Check for any dampness, leaks, or signs of pests before you begin, since insulation can trap moisture and make an existing problem worse if it is not dealt with first.
- Lay boards across the joists if you need to move around, since stepping between joists directly on the ceiling below is unsafe.
Step 3: Lay the First Layer
- Starting in one corner and working back towards the loft hatch, unroll the insulation between the joists.
- Aim for a base layer of around 100mm, which should come up to no more than 25mm above or below the top of the joists.
- Cut the insulation to fit snugly around any obstructions such as pipes or cables, rather than compressing it to force a fit.
Step 4: Lay the Second Layer
- Add a second layer of at least 170mm on top of the first, laid at a right angle to the layer below.
- This crossing pattern helps cover any gaps left between the joists in the first layer and brings the total depth up to the recommended 270 to 300mm.
- Continue working back towards the hatch so you are not standing on freshly laid insulation.
What to Avoid Covering
A few areas need to be left clear or given extra space, and getting this wrong can create new problems.
- Leave a 50mm air gap beneath any boarding you plan to add above the insulation, using raised loft legs to maintain the gap.
- Keep a 25mm gap around the eaves, since blocking these vents can trap moisture and lead to damp in the loft.
- Leave a 75mm gap around hot chimneys, flues, or recessed light fittings, for fire safety.
- Never insulate directly on top of, or up against, unprotected electrical cables. If needed, cable covers are available to allow safe insulation nearby.
Don’t Forget the Loft Hatch
The loft hatch is one of the most commonly overlooked heat-loss points. Cut a piece of rigid insulation board to fit the hatch and fix it to the underside, then add a draught excluder strip around the frame to stop warm air escaping around the edges.
Insulating Pipes and Water Tanks
Once the floor insulation is down, pipes and any cold water tank in the loft become more exposed to cold air and are at greater risk of freezing in winter. Wrap exposed pipework in foam pipe lagging, and fit an insulating jacket around any water tank, including underneath it, to protect against freezing.
Choosing Between DIY and a Professional
Straightforward lofts with easy access and evenly spaced joists are generally suitable for a confident DIY job. Consider hiring a professional if your loft has restricted access, if you are converting it into a usable room rather than simply insulating the floor, or if you are uncomfortable working around electrical cabling. Any installer you hire should be registered with TrustMark, a government-endorsed quality scheme, and belong to a recognised Competent Persons Scheme.
How Much Does Loft Insulation Cost?
DIY materials typically cost between £5 and £10 per square metre for mineral wool rolls. If you hire a professional for the full job, expect to pay in the region of £300 to £900 in total for an average-sized loft, including materials and labour. Costs vary depending on loft size, access, and the type of insulation used.
Are There Grants Available?
Government support for insulation is available in the UK, including fully funded options for people on lower incomes and low or zero-interest loans for other homeowners, as part of wider initiatives to improve home energy efficiency. Availability and criteria change over time, so it is worth checking current government energy schemes before starting a paid installation to see if you qualify for support.
Related Guides
Frequently Asked Questions
How thick should loft insulation be?
The recommended minimum depth is 270mm to 300mm, typically made up of a 100mm base layer and a 170mm top layer laid at a right angle to each other.
Can I insulate my loft myself?
Yes, if the loft has easy access and evenly spaced joists, laying mineral wool rolls is a manageable DIY job for most confident homeowners.
Do I need a professional to insulate my loft?
Not necessarily for a straightforward floor insulation job, but a professional is worth hiring for restricted access, loft conversions, or work involving unfamiliar electrical cabling.
Are there grants for loft insulation in the UK?
Government-backed schemes offering grants or low-interest loans for insulation exist and are updated periodically, so check current government energy efficiency programmes to see what support is available.
Final Thoughts
Insulating a loft is one of the most straightforward ways to cut heating bills and improve comfort at home, and for most standard lofts it is a job you can tackle over a weekend. Get the depth right, leave the necessary ventilation and safety gaps, and do not forget the hatch and any exposed pipework while you are up there.