Knauf Loft Insulation: Which is Best For Your Loft? A UK Guide

Product Comparison Table

The table below summarises the key differences to help you decide.

Meeting UK Building Regulations: Getting the Depth Right

Before starting your project, it is essential to understand the current UK Building Regulations for loft insulation. These standards are in place to ensure homes are energy efficient and safe.

The 270mm Standard Explained  

For new builds and refurbishment projects in the UK, the current recommended minimum depth for glass or mineral wool loft insulation is 270mm. This standard is designed to achieve a target U-value (a measure of the rate of heat loss through a building element) of 0.16W/m2K for roofs, as required by Part L of the Building Regulations. In Scotland, an even greater depth of 350mm is sometimes recommended for optimal performance.

This 270mm benchmark is a relatively recent development. Building regulations have become progressively stricter over the decades:

• In the 1980s, a depth of just 25mm was common.

• By 1995, the recommendation had increased to 200mm.

• Since 2003, the 270mm standard has been in place.

This history is important because it means that millions of UK homes, while having some loft insulation, are insulated to a standard that is now considered inadequate. The vast majority of loft insulation projects today are not new installations but "top-ups" to bring older properties up to the modern standard. This market reality has directly shaped the products that manufacturers like Knauf produce. They sell a 100mm roll, which is the typical height of UK ceiling joists, and then 170mm or 200mm rolls designed specifically to be laid over the top. This product combination is perfectly engineered for the most common UK scenario.

How to Calculate the Thickness You Need

To determine what you need, the first step is to measure what you already have. Carefully and safely, use a tape measure to check the depth of any existing insulation between the joists.

• If you have no insulation: You will need to build up to the full 270mm depth. The standard method is to install a 100mm layer between the joists, followed by a 170mm layer laid across the joists.

• If you have existing insulation: Subtract its depth from 270mm to find out how much more you need to add. For example, if you have 100mm of insulation already, you need to add a top layer of at least 170mm.

The table below provides a simple guide.

Fire Safety: Understanding the Euroclass A1 Non-Combustible Rating

A critical safety aspect of any building material is its reaction to fire. Knauf's glass mineral wool loft rolls achieve a Euroclass A1 Reaction to Fire classification. This is the highest possible rating, meaning the product is classified as non-combustible.

In practical terms, this means the insulation will not contribute to the development or spread of a fire. Should a fire occur, the insulation will not ignite, produce significant smoke, or release flaming droplets. This provides invaluable peace of mind and is a key requirement for meeting UK fire safety regulations.

A Practical Guide to Installing Knauf Loft Insulation

With the right preparation and attention to safety, installing Knauf loft insulation is a very achievable DIY project.

Step 1: Preparation, Tools, and Safety Gear

Before you start, gather everything you need.

• Safety First: Even with the softer ECOSE® Technology, it is best practice to wear Personal Protective Equipment (PPE). This includes overalls, gloves, safety glasses or goggles, and a dust mask (FFP1 minimum is recommended).

• Tools: You will need a tape measure, an insulation cutter or a long-bladed knife (an old bread knife works well), and sturdy crawl boards.

• Preparation:

  • Completely clear your loft of any stored items to give yourself a clear and safe working space.

  • Ensure the area is well-lit, using a head torch or portable work light.

  • Carefully inspect the ceiling joists for any signs of rot or structural damage. If you find any, do not proceed and consult a professional.

  • Lay your crawl boards across at least three joists. This spreads your weight and provides a safe platform to work from. Never step directly on the plasterboard ceiling between the joists.

  • Look for any gaps where air might be leaking from the rooms below (e.g., around pipes or light fittings) and seal them with a suitable sealant.

Step 2: Laying the First Layer Between the Joists 

The first layer of insulation is laid between the ceiling joists.

• This layer is typically 100mm thick, designed to fill the space up to the top of the joists.

• Start at the furthest point from the loft hatch and work your way back towards it.

• If using a combi-cut roll, tear it at the perforations to the correct width for your joist spacing. If not, cut the roll to size. It should fit snugly between the joists without any gaps, but it should not be compressed or squashed into place. Compressing the insulation reduces its effectiveness by removing the trapped air pockets.

• Continue laying the insulation between all the joists until the entire loft floor is covered.

Step 3: Laying the Top Layer Across the Joists 

The second layer brings the total depth up to the 270mm standard.

• This layer, typically 170mm or 200mm thick, must be laid at a 90-degree angle (crossways) to the first layer.

• Laying the insulation in this direction covers the wooden joists. Timber is a poorer insulator than mineral wool, and if left exposed, the joists can act as a "thermal bridge," allowing heat to escape. Covering them with the second layer ensures a continuous, unbroken blanket of insulation.

• As before, start from the furthest point and unroll the insulation. Butt the edges of each new roll snugly against the previous one, ensuring there are no gaps. Do not overlap the rolls or compress them.

Step 4: Safely Navigating Loft Obstacles

Lofts often contain electrical cables, light fittings, and water pipes. It is critically important to handle these correctly to avoid creating a safety hazard.

• Recessed Downlights: This is the most critical safety issue. Most recessed downlights (especially older halogen ones) generate significant heat and can be a fire risk if smothered with insulation. You must NEVER lay insulation directly over them. Instead, you must fit a non-combustible, fire-retardant downlight cover (sometimes called a "loft cap" or "downlight hood") over each fitting. These covers create a safe air gap around the light, allowing heat to dissipate. The insulation can then be laid continuously over the top of the covers. Some modern, fire-rated LED downlights are specifically designed to be safely covered with insulation (look for an "IC" or Insulation Coverable rating), but you must check the manufacturer's instructions. If in any doubt, always use a protective cover.

• Electrical Cabling: Standard lighting cables can usually be left in place under the insulation. However, high-load cables, such as those supplying an electric shower or a cooker, need to dissipate heat to operate safely. These cables must be carefully lifted and re-laid on top of the final, full depth of insulation. Covering them creates a serious fire risk. If you are unsure which cables are which, consult a qualified electrician.

• Water Tanks and Pipework: Your loft will become much colder after it is insulated. To prevent freezing, any cold water tanks and pipes must be insulated with their own dedicated jackets and pipe lagging. However, it is very important that you do not place insulation underneath a cold water tank. The small amount of warmth rising from the house is often what prevents the tank from freezing solid in winter.

Step 5: Insulating and Sealing the Loft Hatch

An uninsulated loft hatch can be a major weak point, creating a cold spot and allowing draughts.

• Cut a piece of insulation to the size of the hatch. It is often easiest to place this inside a heavy-duty plastic bag and then fix the bag to the top side of the hatch door with staples or adhesive.

• Apply self-adhesive draught-proofing strips around the frame of the hatch on the ceiling. When the hatch is closed, it will compress against these strips, creating an airtight seal.

The Importance of Ventilation to Prevent Condensation

Installing loft insulation is a two-part job: adding the insulation and ensuring ventilation. Overlooking the latter can lead to serious problems with damp and mould.

Why a Well-Insulated Loft Needs Good Airflow

When you insulate your loft floor, you stop heat from the house from escaping into the roof space. A direct and unavoidable consequence is that the loft itself becomes much colder, with its temperature getting closer to the outside air temperature.

At the same time, everyday life in the house below—cooking, showering, drying clothes, even breathing—produces litres of warm, moisture-laden air. This water vapour naturally rises and will find its way into the loft. When this warm, moist air enters the newly cold loft space and hits a cold surface (like the roof timbers or the underside of the roofing felt), it will cool rapidly, causing the moisture to condense back into liquid water.

If this process is allowed to continue without adequate ventilation to carry the moist air away, it can lead to damp, mould growth on timbers, and eventually, structural rot. Therefore, good ventilation is not an optional extra; it is a necessary counterbalance to the act of insulating.

How to Ensure Your Eaves Vents Remain Unblocked

Most of the ventilation for a cold pitched roof comes from airflow through vents at the eaves (the lowest edge of the roof). It is absolutely critical that these are not blocked.

• When laying your insulation, you must maintain a continuous 50mm air gap between the top of the insulation and the underside of the roofing membrane or felt.

• Pushing the insulation tightly into the very edge of the eaves will block the airflow. Instead, gently curve the insulation down to meet the top of the wall insulation, but ensure the air path from any soffit vents remains clear.

• To make this easier and to guarantee the gap is maintained, you can install eaves vents or rafter trays. These are simple, inexpensive plastic inserts that fit between the rafters and provide a permanent, unblockable channel for air to flow through.

Frequently Asked Questions (FAQs)

Q: How much does Knauf loft insulation cost?

A: Prices vary depending on the retailer, thickness, and specific product (Loft Roll 44 or 40). As a rough guide, you can expect to pay between £2.00 and £3.00 per square metre for a 100mm roll of Loft Roll 44, and between £4.00 and £5.00 per square metre for a 200mm roll. It is always worth comparing prices from different suppliers.

Q: Where can I buy Knauf loft insulation in the UK?

A: Knauf loft insulation is widely available across the UK. You can find it in major DIY chains such as B&Q (where it may be branded as 'Eko Roll') and Wickes (branded as 'Space Loft Roll'), as well as at builders' merchants like Travis Perkins and numerous online specialist insulation suppliers.

Q: Can I put too much insulation in my loft?

A: From a thermal perspective, the benefits of adding more insulation show diminishing returns beyond about 300-400mm. The main danger of adding "too much" is not thermal but practical: installing it in a way that blocks essential ventilation at the eaves. As long as you maintain the required 50mm air gap for ventilation, adding extra thickness (for example, to 300mm or 350mm) is not harmful and will provide a marginal improvement in performance.

Q: How long does Knauf mineral wool insulation last?

A: When installed correctly in a dry, undisturbed environment, Knauf glass mineral wool insulation is designed to last for the lifetime of the building. It is a highly durable material that does not rot, settle, or degrade over time, meaning it will maintain its thermal performance indefinitely.

Q: Is Knauf Earthwool insulation good for soundproofing?

A: Yes, glass mineral wool has excellent acoustic properties. Its fibrous structure is very effective at absorbing sound waves and reducing sound transmission. While the primary purpose of loft insulation is thermal, it provides a significant secondary benefit of reducing noise from outside, such as rain or traffic. For applications where soundproofing is the main goal (such as in internal walls or floors), Knauf produces specialised 'Acoustic Roll' products with optimised density for sound absorption.

Legal Disclaimer

The information provided in this article is for general informational purposes only and is intended to provide helpful advice. It does not constitute legal, professional, or technical advice and should not be relied upon as a definitive statement of law or UK Building Regulations, which are subject to change.

All building work should be carried out in accordance with current regulations and best practices. Before undertaking any work, you should consult with suitably qualified professionals, such as a certified electrician for any electrical work or a structural engineer if you have concerns about the integrity of your loft structure. Always refer to the manufacturer's specific product datasheets and installation guides for the most current and detailed instructions.

We disclaim all liability in respect to actions taken or not taken based on any or all the contents of this article to the fullest extent permitted by law. You should not act or refrain from acting upon this information without seeking professional counsel.