PITCHED ROOF INSULATION

Pitched roof insulation is a type of protection that can be inserted both between and underneath the rafters to extend the thermal performance up along the pitch of the roof, essentially creating a new, heated room within the loft area. Pitched roof consisting of two angled pieces which meet in the middle, with a piece called a gable at either end. In places such as the United Kingdom, where they have turbulent winters, they build with a very deep pitch so that the roof doesn’t accumulate snow.

PITCHED ROOF INSULATION TYPES

In terms of a pitched roof insulation, there are 4 options to choose from:

1. PIR PITCHED ROOF INSULATION BOARD

It is ideal for in-between and under rafters in a pitched 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.

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

2. FIBREGLASS PITCHED ROOF INSULATION ROLL

Glass wool is one of the most popular insulation used to insulate pitched roofs. Fibreglass insulation called 'loft board' or 'loft roll' is essentially an insulating material that is built into your pitched roof usually between the rafters. Fibreglass pitched roof insulation roll is probably the most common, cheap and non-combustible type of insulation used to insulate pitched roof, however it’s itchy and gets everywhere with very little effort.
To get decent thermal performance, you need a lot of insulation rolls which can quickly eat into your loft space. 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 pitched 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.

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. 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 pitched roof applications. As of 2022 most popular insulation slab 150mm Rocksilk RS45 Knauf cost £36.36 per m2.

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 BOARDS

The product has a declared thermal conductivity λ value ranging from 0.018 to 0.021WmK and is 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.  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. To create a warm pitched roof, insulation such as Kingspan Kooltherm K107 Pitched Roof Board, should be fitted between and over the rafters
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.

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

PITCHED ROOF INSULATION COST | PRICES

The amount of money you spend on pitched 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 pitched roof insulation would be fibreglass (glass wool). However PIR insulation boards are a better solution, enabling greater thermal insulation than standard mineral wool. In terms of m2 coverage cost, pitched 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. 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. PIR Rigid Insulation Boards - Thermal Conductivity of 0.022W/mK
3. Rock wool Insulation - Thermal Conductivity of 0.035W/mK
4. Fibreglass Insulation - Thermal Conductivity of 0.044W/mK

PITCHED ROOF INSULATING METHODS

There are several ways to insulate pitched roof:

UNDER RAFTERS & BETWEEN RAFTERS (INTERNAL)

One of the benefits of insulating at rafter level is that insulation can be provided both above and between the rafters. These can combine to achieve relatively high performance standards. The extent and type of insulation that can be provided between the rafters will be dictated by the size of the rafters, which can vary considerably from slender timbers as little as 75mm deep to those that are 225mm deep or more. Obviously, the greater the depth of the rafters the more opportunity there is to provide insulation thereby reducing the amount of insulation required above the rafters and consequently the height the roof needs to be raised. Ideal for projects where there is access to the internal side of the roof and the external roof finish is not being removed or replaced. There are several types of materials suitable for insulation between the rafters. The most appropriate materials are fibreglass, PIR rigid insulation, phenolic boards or natural-fibre based insulation such as sheep’s wool or hemp fibre. 

OVER RAFTERS (EXTERNAL)

Insulating a pitched roof above the rafters is the common method chosen for new builds and re-roofing existing detached buildings as it provides superior thermal performance for the building. Insulating the roof above the rafters is known as warm roof insulation as the rafters are brought into the insulated envelope of the building.
To create a warm pitched roof, insulation should be fitted over the rafters. As this requires fitting insulation above the rafters the roof covering will have to be removed in order to fit the insulation. This roof type will require a breathable membrane to be fitted above the insulation. This over-rafter insulation method reduces thermal bridging, giving an airtight continuous layer of high-performance insulation and results in a well-insulated attic space. Best suited for projects where there is access to the external side of the roof, this could include applications where there is restricted access internally. The ‘warm roof’ system is used extensively in new construction and the systems for installing insulation in modern buildings are well understood. Insulation added above rafters will typically be a Rigid Insulating Board.

This method of installing roof insulation allows you to maximise the headroom by using a thinner layer of insulation beneath the rafters, whilst achieving U-Values which will ensure you comply with Building Regulations and have a roof space or loft which is comfortable for its occupants.

CEILING LEVEL INSULATION

Insulating a pitched roof at ceiling level is a cost-effective way of installing roof insulation where the attic space will not be used as a living area. The thickness of insulation is largely unrestricted by construction so the layer of insulation can be deeper to achieve better performance as required. Airtightness and ventilation in the roof space are important considerations when installing insulation at ceiling level to avoid condensation forming on top of the insulation and other surfaces in the cold space. 

PITCHED ROOF INSULATION BUILDING REGULATIONS

The Approved Document that accompanies Part L of the Building Regulations for existing dwellings (ADL1B 2010) calls for insulation between the rafters of a pitched roof to have a U-value of 0.18 W/m2K. Such levels of insulation are quite high, but are potentially achievable in ways that are not harmful to historic buildings. U-values measure how quickly energy will pass through one square metre of a barrier when the air temperatures on either side differ by one degree. U-values are expressed in units of Watts per square metre per degree of temperature difference (W/m2K).
It is difficult for many older buildings to achieve a U-value of 0.18W/m2K if there are constraints on the thickness of insulation that can be provided above the rafters. For example, to reach a U value of 0.18 W/m2K in a roof with a non-insulating ceiling and 150mm of insulation between rafters requires an insulating sarking board of about 70mm thickness. Part L of the Building Regulations allows for exemptions and special considerations for historic buildings which enables Building Control Officers or Approved Inspectors to take a pragmatic view in order to conserve the appearance and character of the building and avoid technical risks. Achieving a U value of 0.18W/m² will need 120mm thickness of rigid foam or 270mm of mineral wool. Hitting 'passive house' levels of 0.13W/m² will need 300mm of mineral wool or 180mm of rigid foam. 

HOW TO INSULATE A PITCHED ROOF FROM INSIDE?

If there is no access from outside, the only option left would be to insulate from inside. In this case, we would suggest fitting insulation between the rafters and insulated plasterboard to the inside face of the rafters. Usually this construction is a ventilated roof which will have a sarking felt. Ventilated roofs require a 50 mm air gap above the insulation, timber stop battens can be nailed into the side of the rafters to give the correct air gap when installing the insulation. The insulated plasterboard should then be placed with the long edges running across the rafters.

PITCHED ROOF INSULATION THICKNESS

The depth of insulation is dependent on the U-values required and the depth of the rafters. Mineral wool rolls are ubiquitous and cheap, meaning a whole roof on a standard home could be insulated to the recommended thickness of 270mm. When it comes to PIR pitched boards it is recommended to use 130mm thickness. In England, the as mentioned earlier, required minimum U-value for pitched roofs insulated at rafter level is 0.18 W/m²K or lower. This equates to a 175mm of rock wool slab insulation. 
 

Video credit:https://www.youtube.com/watch?v=qzlylgc4sqU

Related articles:

Best loft roll insulation

How to board a loft

Debunking the top 3 myths about vapour barriers and foam rigid insulation

How to cut fibreglass insulation roll

How to choose the right Kingspan insulation board for construction?

Rockwool acoustic insulation slabs overview

Spray foam insulation – is it really that good?

Should I remove 80 years old insulation?

Vapour control later. The ugly truth?