6 tips for insulating your loft from Which? Trusted Trader experts

Installing loft insulation is one of the cheapest and most effective ways to prevent heat escaping your home, since heat rises and would otherwise be lost through your loft.

Loft insulation can reduce your energy use and your bills, helping both the planet and your wallet.

Some types of insulation you can install yourself; for others, you’ll need professional help.

We spoke to two Which? Trusted Traders – Dean Taylor from The Loft Boys and Liam Mellor of LCM Flat Roofing – who have 12 and 10 years experience respectively of installing insulation. Read on for their advice.

Not quite ready to install loft insulation? Head to our guide on draught-proofing your home for other tips on keeping your house warm and cutting your energy bills.

1. Pick the right type of insulation for your loft

Different types of loft insulation have different purposes. The main types are

• Blanket or batt loft insulation
• Sheet loft insulation
• Loose-fill loft insulation
• Blown-fibre loft insulation

Blanket loft insulation

Blanket insulation, which insulates effectively, but can be too bulky to fit in small spaces or around obstructions, is the most common type.

Dean told us that it’s all you should need if the temperature of your loft doesn’t need to be regulated: if, for example, you’re just going to use it as storage space.

Blanket insulation comes in rolls and is usually made of rock, glass or mineral fibre. According to Dean, it’s easy to fit, especially between joists.

Sheet loft insulation

Blanket insulation isn’t suitable for insulating the underside of your roof, though.

If that’s what you want to do – let’s say, you’re intending to spend time in your loft or to store temperature-sensitive items there – you can buy insulating sheets made of natural or synthetic materials from home improvement retailers.

They can then be decorated by painting or covering with plasterboard.

While insulating the underside of your roof is usually very effective in terms of heat retention, Dean cautioned that it’s the most expensive type of loft insulation. It also needs to be installed by a qualified professional – more on this later.

Loose-fill loft insulation

This is made from a variety of granular or lightweight materials such as cork granules, mineral wool or cellulose fibre. More sustainable types of loose-fill insulation can be used, such as recycled newspaper.

Loose-fill insulation fits easily between irregularly spaced joists or around obstructions, and is useful for topping up existing insulation in attics.

However, it can come loose in draughty lofts and safety equipment and protective clothing are necessary to install it safely.

Blown-fibre loft insulation

This type of insulation is blown into the gaps between joists. You’ll need a professional tradesperson to install blown-fibre insulation, who should be able to complete the job easily.

This can be more expensive than other times of insulation.

Our guide to the types of loft insulation explains the pros and cons of each kind.

2. Know when to get your loft insulation professionally installed

You can install some types of insulation yourself. We’ve got tips on on putting blanket insulation in your loft later in this article, for example.

But it’s important to know when to call on the specialist skills of a trader.

For example, Dean told us that ‘Installing a warm loft (where insulation is above the roof deck, the panel underneath the roofing material that’s usually made of wood) is definitely not a DIY job. It involves specialised work on the roof and messing it up could have dire consequences for the stability of your whole home. A professional will spot problems such as damp or asbestos’.

As we mentioned before, insulating sheets are the most expensive type of loft insulation and need to be installed by a qualified professional. Loose-fill and blown-fibre will also require a skilled tradesperson with the necessary equipment to install it correctly.

Need help installing insulation? Use Which? Trusted Traders to find a reliable trader near you.

3. How to DIY install blanket insulation

If you enjoy DIY projects, you can install blanket insulation yourself. Dean provided us with a list of tools that you’ll need, a checklist of preparations to ensure you stay safe, and a brief guide on how to actually do it.

Tools for installing blanket loft insulation

• Rubble bag – for any bits of debris you come across
• Face mask
• Tape measure
• An old hand saw
• Utility knife or scissors
• Mobile workbench

What to check for before installing blanket loft insulation

• If working within an un-floored loft space, then you should law crawl/kneel boards so that you can move around safely
• Try to make sure that the loft is well-ventilated and free from condensation
• Ensure that the loft is well-lit so you can see exactly what you’re doing. Ideally, use several LED lights in various locations. LEDs won’t heat up and they give off a powerful bright white light
• Wear a protective face mask to avoid breathing in dust particles
• Crawling around on your knees for any amount of time can get pretty gruelling so you might also benefit from using knee pads
• If your home was built before 2000, it’s possible that your loft may contain materials involving asbestos. If you suspect that asbestos may be present, then don’t take any chances. Talk to a professional about how to safely remove it.

How to install blanket loft insulation

• If you’re working with an un-floored area, you’ll need those crawl/boards we mentioned. These will ensure you don’t put a foot (or your whole body) through the ceiling below and do yourself an injury – or inadvertently become a TikTok sensation!
• Next, check if you have recessed spotlights within the ceiling. If you do, then you’ll need to install loft caps to ensure your new insulation doesn’t come into contact with the hot back of the lights
• The first 100mm layer of insulation will need to be laid between the joists. Check the average distance between joists and cut the insulation while it’s still packaged using an old hand saw to the width required. (Typically the gap between the joists will be either 380mm or 570mm)
• Roll out the cut insulation between the joists over the entire area required and then lightly press the insulation material to fit between the joists. Nest the insulation around the loft caps if present
• The second layer of 170mm – 200mm insulation needs to be laid running at a right angle over the joists. Unless there are any braces (supporting beams) or other obstructions you can lay this layer over the entirety of your loft floor.
• Start at the furthest point from the loft hatch and unroll the insulation over the top of the joists ensuring an even, gap-free blanket over the entire space.

You can find more advice on blanket, loose-fill, sheet and blown-fibre insulation in our guide to installing loft insulation.

4. Loft insulation for flat and pitched roofs

There are three types of insulation for flat roofs (compared to only two for pitched roofs) and a professional will be able to help you determine which one is best for your property.

Warm deck or warm roof has insulation above the roof deck (the panel underneath the roofing material that’s usually made of wood). It’s recommended in damp and cold areas – which, sadly, includes the UK.

Cold deck or cold roof has insulation below the roof deck and the associated joists. Typically, you leave a gap for ventilation, as condensation could form, and lead to rot.

In both warm deck and cold deck options, the weather membrane (typically formed of roofing felt and bitumen) will be the topmost layer, protecting against rain.

Dean told us that: ‘A flat roof should be insulated from above – which is definitely a job for a professional.’

Liam also explained that ‘The best way to insulate a flat roof is with foil backed foam insulation board but more importantly installed as a warm roof deck (on top of the roof surface) not a cold roof deck (between the rafters) which isn’t as thermally efficient.’

Alternatively, your flat roof might be an inverted roof. This puts insulation above the weather membrane, protecting it from heat and cold. It can even protect against wear and tear if there is access to the roof. The top-most layer is generally gravel or a similar material.

Take a look at our guide to roof insulation for more details on the different options available for both pitched and flat roofs.

To help businesses provide good quality work and consumers find the right trader, Which? Trusted Traders are working with the Insulation Assurance Authority (IAA). The IAA require high standards from member traders, such as training, skills and using the correct materials.

5. Avoid common mistakes when installing loft insulation

Most lofts already have some form of insulation. However, it’s often poorly installed, is being squashed by loft boarding and stored items, or is below the recommended thickness of 270mm.

Dean suggests lifting up a loft board and inspecting what’s there.

He said ‘Be wary of cables and pipes, and ensure there’s a gap for airflow above the insulation. Without this, condensation can build up. The air pockets within the insulation should be fully open with nice fluffy insulation.’

Liam added that: ‘A common mistake is cutting the insulation to fit between rafters or joists. It’s much better installed as an independent layer without breaks in the insulation. A warm deck roof design without air flow requires a vapour barrier to prevent condensation, but is much more energy efficient than the older cold deck roof design.’

The loft and roof are not the only parts of your house you can insulate. Depending on the walls in your property, find out more about cavity wall insulation or solid wall insulation.

6. Use Which? Trusted Traders to find a reliable professional

Which? Trusted Traders recognises reputable traders who pass an strict assessment process. We check the trader’s credit reports, customer satisfaction and business procedures as well as conducting an interview and assessment.

Dean’s company The Loft Boys was awarded a Which? Trusted Traders Certificate of Distinction in June 2020. Liam’s business LCM Flat Roofing Ltd was voted Which? Trusted Trader of the Month for August 2020 and April 2022.

Find out more about installing loft insulation.