ROOF INSULATION

Roof insulation is an insulation system that provides a suitable level of indoor thermal comfort and great savings on heating costs. Simply roof insulation can help you keep the heat or cold in when you need to. When a roof is not properly insulated, the rising heat might lead to melting snow on a roof, which can then lead to ice dams. Currently, the recommended R-value for attic insulation is between 6.1 and 7, a combination of thickness and thermal conductivity. Roof insulation is qualified for government grants. Roof insulation grants are available to completely cover the cost of having insulation professionally installed by installers. If you are in receipt of a qualifying benefit, you should be entitled to a 100% free grant that does not need to be paid back. 

TYPES OF ROOF INSULATION

Depending on the available space, your budget and the presence of a roofing underlay, you can choose between 5 types of roof insulation.
 

1. PIR ROOF INSULATION BOARDS
 

PIR insulation boards are the best choice when you want to use the space beneath the roof as efficiently as possible. The major advantage of PIR compared to other insulation materials is the insulation value. The Lambda value of PIR is situated between 0.021 to 0.032 W/mK. This means that PIR offers superior insulation with the same thickness. PIR or POLYISO boards can directly contribute to strategies for conserving energy and significantly reduce the effects of global warming. They have a high thermal conductivity of 0.022 W/mk and are one of the most capable and valuable roof heat resistant materials used in UK roof constructions. The "rigid" part of the name comes from the fact that unlike other forms of insulation, it doesn't come loose and compress after installation. Instead it's a solid, hard foam material. PIR insulation board is often associated with market leader Celotex company as they began producing rigid polyisocyanurate foam as first. In fact, Celotex invented the thinnest thickness of PIR insulation measuring only 12mm. Many PIR insulation brands, such as Celotex, Xtratherm, Kingspan or Recticel come with a grid marked on, which can act as a guide to help ensure the thermal insulation board is cut in a straight line for rafters. Rigid panels are designed to cover all general needs for thermal insulation, fire resistance and sound reduction and because of their versatility and high compressive strength 120kPa, PIR boards are often used in roofing jobs. PIR insulation turned out to be so good and practical that many people choose to buy a bit cheaper seconds PIR boards which, despite the fact that they were transported and stored, are highly durable and good quality products that they can be used second time on roof. Depending of thickness and brand, rigid insulation boards come in multiple name options, including Celotex GA4000, Celotex TB4000, Celotex XR4000, Xtratherm Thin-R, Kingspan TF70, Kingspan TP10, Kingspan TW55, Recticel Eurothane GP, Quinntherm, EcoTherm EcoVersal, IKO Enertherm ALU and Ballytherm PIR. Thinner insulation boards, ranging from 12mm to 50mm are offered by Celotex, Xtratherm, IKO Enertherm, Ecotherm and Kingspan. It is ideal for in-between and under rafters in a pitched roof. For more information on how to cut PIR insulation click here. If you can't install cold loft insulation because, for example, you're using your loft as a living space, you will need to consider warm loft insulation. For a warm loft, you install insulation directly under the roof in the plane of the roof's pitch. Insulation will reduce heat loss and your loft space won't become excessively hot in summer or cold in winter. It's not as easy as laying rolls of insulation, but it can still be done as a DIY job if you'd rather not pay an installer. One important point to remember with a warm loft solution is the need for ventilation immediately below the roof tiles. This prevents condensation build up or water getting in through or around the tiles, which could cause the roof structure to rot. A good time to insulate a roof is when it's being replaced, although in many cases a roof that is in good condition can be retro-fitted with insulation.

Cost: £18.78/m² (ex VAT), £19/m² installation
Advantages: easy to install, space-saving insulation

2. FIBREGLASS ROOF INSULATION ROLL

Fibreglass insulation called 'loft board' or 'loft roll' is essentially an insulating material that is built into your roof usually between or above the rafters. Glass wool is one of the most popular insulation used to insulate roofs. The first layer is laid between the joists – the horizontal beams that make up the floor of the loft, than another layer is laid at right angles to cover the joists and make the insulation up to the required depth. Fibreglass roof insulation roll is probably the most common, dirt cheap and non-combustible type of insulation used to insulate roof, however it’s itchy and gets everywhere with very little effort. If the insulation gets wet, you will need to replace it a soon as possible. To get decent thermal performance, you need a lot of insulation rolls which can quickly eat into your loft space. Roof insulation roll is the cheapest and most favored option for most common types of roof jobs and it is easy to fit between joists, which means it’s commonly used by DIYers wanting to insulate their roofs correctly in the most cost effective way possible. Blanket insulation is also the one that’s best used in conjunction with flagship Loft Legs, in order to create a solid and stable surface above the government recommended depth of insulation. The best cost effective type of roof insulation is definitely the mineral or glass wool roll. If you consider the economic point and the fact that common spacing of UK rafters are 400mm or 600mm, loft roll is absolutely the best type of insulation that can be used to insulate a roof. Rolls are 1200mm wide and ready-cut to produce three, 400mm wide rolls, or two, 600mm wide rolls and manufactured to a minimum density of 10kg/m3 meeting the requirements outlined within related Building Regulations. It means, you have no offcuts and you can insulate the roof yourself. Moreover, rolls work out cheaper therefore they become the most favoured option from all of available roof insulation types. This is one of the most popular types of insulation, mainly due to the fact it’s been around for so long and is widely available from most building merchants such as Screwfix, B&Q or Wickes. Being lightweight, it has a good thermal conductivity of 0.044 W/mK. If you are planning to renew over 25% of your roof space, you must also ensure that your loft insulation meets the minimum values of the regulations. According to current building regulations recommended thickness for loft insulation should be at least 270mm. 

Cost: £2.96/m² (ex VAT) 100mm, £16/m² installation
Disadvantages: irritating during installation, takes a lot of space 

3. ROCK WOOL ROOF INSULATION

Rock wool roof insulation is manufactured in sheets that are pre-cut to fit between any joist spacing to provide extra insulation within your pitched roof. Boards will boost thermal performance, improve acoustics and insulate between rafters. Slabs are provided in sizes that are easily transportable and good for handling measuring 1200mm x 600mm. In England, the required minimum U-value for pitched roofs insulated at rafter level is 0.18 W/m²K or lower. This equates to a 175mm of slab insulation. Rock wool roof insulation slabs are generally added under a vapour control layer, which reduces the risk of mould and condensation forming. When it comes to best soundproof roof insulation boards between the rafters, Rockwool and Knauf are main players in the UK market. General-purpose rock wool insulation boards such as RWA45, RW5 or RW3 are perfect for roof applications. The cost of insulating a roof will depend on whether you are going to do the work yourself or hire a professional to do it for you. By fitting insulation slabs yourself you can save a fortune. As of 2022 most popular insulation slab 150mm Rocksilk RS45 Knauf cost £36.36 per m2. Average size roof in the UK measures 70m2. Taking this into consideration you would be paying around £2545 for materials only. If you decide to do everything yourself, there's no need to worry about the cheapest loft board as you saved money already on labour costs. One thing to remember if you do this yourself is that you should leave a gap between the insulation and the roof slats. This is to ensure that there is enough ventilation to prevent any build-up of condensation and moisture.
Generally speaking, kg/m³ is the weight of the rock wool slabs if they were manufactured in a block of one metre cube (1m x 1m x 1m). In order for them to be identified, they are given a density rating which refers to the weight of a metre cube, displayed as kg/m³. Roof rock wool insulation is available in a variety of densities which range from 45kg up to 100kg per slab. The greater the mass is, the harder it is to vibrate this material, thus the better the insulating property will be. Therefore, it is better to choose dense and heavy material.

45kg/m³ insulation slabs are the low level insulation density slabs available from Rockwool. Although, those slabs still have higher density than other forms of insulation unlike thermal insulation which is lower in density and supplied in rolls. Rockwool RWA45 is named after its density properties in that it has a density of 45kg per cubic meter. The 45kg range slabs include the following readily available product thicknesses: 25mm ǀ 30mm ǀ 40mm ǀ 50mm ǀ 60mm ǀ 75mm ǀ 90mm ǀ 100mm 

60kg/m³ roof acoustic insulation slabs are the middle level insulation density slabs. In fact, it is recommended to use 60kg/m³ slabs as they offer the best insulation value for the money spent. Rockwool RW3 or Knauf RS60 is named after its density properties in that it has a density of 60kg per cubic meter. The 60kg/m³ density slabs are ideal either for loft floors or ceilings to control home acoustics. 60kg density slabs can be supplied either in standard unfaced 1200mm x 600mm size or foil backed on both sides. Unlike unfaced slabs, foils faced boards offer great fire performance classified as Euroclass A1. 60kg range slabs include the following thicknesses:
25mm ǀ 30mm ǀ 40mm ǀ 50mm ǀ 60mm ǀ 75mm ǀ 90mm ǀ 100mm 

100kg/m³ roof acoustic insulation slabs are the high level insulation density slabs available from Rockwool. In fact, it is recommended to use 100kg/m³ slabs in places such as hospitals, auditoriums, stadium acoustics, commercial spaces and doctor offices. Rockwool RWA5 slabs are great examples of high density slabs made especially for commercial and industrial loft spaces.

Cost: £10.51/m² (ex VAT)100mm, £19/m² installation
Disadvantages: irritating during installation, if any moisture settles in the insulation fabric it will eventually rot.

4. PHENOLIC ROOF INSULATION

Phenolic roof insulation boards are the most productive type of rigid phenolic core, foil-composite insulation presently accessible for use in domestic buildings of new and existing warm pitched roof constructions where the ceiling follows the pitch of the roof and encloses a habitable space. The product is also good to be used as an insulation above the rafters and as a thermal insulation board between and below rafters, in tiled and slated pitched roofs. The product has a declared thermal conductivity λ value ranging from 0.018 to 0.021WmK and is light to handle and can be cut easily. However, care must be taken to prevent damage, particularly edge damage. Panels are very often associated with brands such as Kingspan® and Xtratherm®. Kingspan® offers Kooltherm K107 manufactured with a blowing agent that has zero ODP and low GWP. On the other hand Xtratherm® offers a high-class rigid phenolic foam insulation board with a low emissivity foil facings called SR/PR Safe-R with thermal conductivity of 0.021W/mK. However, Kingspan® offers a slightly lower thermal conductivity by a minimum of 0.03W/mK. Phenolic range includes the following readily available product thicknesses: 25mm ǀ 40mm ǀ 50mm ǀ 60mm ǀ 75mm ǀ 80mm ǀ 100mm and 120mm. Boards are manufactured from closed-cell phenolic foam, faced with a (perforated) bilaminate aluminium foil, bonded to glass tissue on both sides, with the sizes 2400mm x 1200mm.

Phenolic insulation boards are lightweight but strong thermosetting plastics and are considered to be one of the best rigid, closed-cell structure materials for thermal insulation at present being up to 50% more effective than other common insulation materials such as PIR. As they’re closed cells they shouldn’t absorb moisture or water vapour, unlike open cell products. Phenolic insulation achieves a lambda value of just 0.018 W/mK across all thicknesses. High density boards have a good compressive strength and highest fire rating among many organic insulation materials. It can, in an appropriate form, achieve all the European fire certifications - UK Class O and Dutch NEN 6065/6066 Class 1. Generally speaking, the board is fire-resistant, flame-retardant, non-combustible in case of open flame, smoke-free, non-toxic, and non-dropping. In fact, phenolic foam used as composite panels can achieve up to 2 hours fire resistance rating in the 3m furnace test. The fire performance of phenolic foam is exceptional. It combines zero or very low flame spread with negligible smoke emission and a very low level of toxic gas emission. However roof phenolic boards do tend to be more expensive but on other hand offer the thinnest insulation boards for a given specific u-value. 

Cost: £41.16/m² (ex VAT)100mm, £18/m² installation
Disadvantages: expensive to buy, causes pipework corrosion, should not be used on buildings in England and Wales that have a store at least 18m above ground level

5. SPRAY FOAM ROOF INSULATION

If your loft is hard to access, you can have blown insulation installed by a professional, who will use specialist equipment to blow appropriate insulation material into any awkward space. They may use mineral wool fibre, treated cellulose or polyurethane foam. Equipment used includes a hose and specialist tank to mix the material with air, in order to propel it into the loft. Coverage is generally great with blown insulation, and it’s far more effective at reaching and covering every little nook and cranny, than traditional rolls of fibreglass. In terms of effectiveness, blown insulation typically has an R-value of around 3-4 per inch. This is the measure of its ability to resist heat loss, and is better than traditional fibreglass rolls.

Cost: £39.50/m² (ex VAT) 100mm, £41.80/m² installation
Disadvantages: much harder to convert your loft into an extra room, difficult to manage and maintain in the future if any settlement occurs, you won’t be able to see the roof joists

ROOF INSULATION COST | PRICES

The amount of money you spend on roof insulation can vary greatly depending on the type, so it's important to know what you're getting before you buy. As of 2022 the cheapest type of roof insulation would be fibreglass (glass wool). For sure, PIR insulation is also a cost-effective building material that will pay for itself over time. Boards are simply a better solution, enabling greater savings and better thermal insulation than standard mineral wool. The price of PIR insulation often depends on two factors. The first is the brand itself. When buying well-known brands, we often pay for the name itself. The second factor is the supplier location ('near me'). When buying PIR from local building merchants such as Wickes, B&Q we pay for 'off the shelf product', we can touch and see physically, while with an online supplier like us you can only get better deals without seeing a product. It is because products come to you directly from the manufacturer. In terms of m2 coverage cost roof insulation can be classified in particular order:

1. Fibreglass Insulation - £2.96/m²
2. Rockwool Insulation -  £10.51/m²
3. PIR Rigid Insulation Boards - £18.78/m²
4. Spray Foam - £39.50/m²
5. Phenolic Boards Insulation - £41.16/m²

But when it comes to performance the list takes the following form:

1. Phenolic Boards Insulation - Thermal Conductivity of 0.018W/mK
2. Spray Foam - Thermal Conductivity of between 0.022 and 0.035W/mK
3. PIR Rigid Insulation Boards - Thermal Conductivity of 0.022W/mK
4. Rock wool Insulation - Thermal Conductivity of 0.035W/mK
5. Fibreglass Insulation - Thermal Conductivity of 0.044W/mK

WHAT IS THE BEST FORM OF ROOF INSULATION?

Firm boards such as PIR or Rockwool RWA45 are the best solution for insulating the underside of your roof. One of our most popular best seller is the Celotex GA4000 general-purpose insulation board. It’s suitable for a wide range of applications including flat or pitched roof insulation. Light, tough, and easy to cut to size with a plasterboard saw they’re easy to install even in a small loft space. Another popular option is Rockwool RWA45 insulation slab, which have been used in pitched roofs for many years. The simplest and cheapest roof insulation is the classic fibreglass loft option. This is often just called loft insulation. It's fairly easy to do as a DIY project.

ROOF INSULATION BETWEEN RAFTERS

Insulation between rafters is the most frequently used method in the UK. It does not take up space when the loft is converted to a room. The best roof insulation for insulating rafters is definitely PIR (Celotex) and Phenolic board. Those boards do tend to be more expensive but on other hand offer the thinnest possible insulation for a given specific roof U-value. 

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