Middle Avenue by Rural Office, photo by Rory Gaylor
Pitched roofs are a common sight in UK domestic architecture, often constructed as either dual pitch (two sloping parts with a central ridge) or mono pitch (a single sloping roof). They are popular for their practicality and versatility.
In this guide, we'll cover key considerations for insulating pitched roofs, including construction methods, types of insulation, U-values, and how to prevent condensation. This information will assist you in making informed decisions when designing a new roof or enhancing the energy efficiency of your home.
The cold roof method is a highly cost-effective way to insulate a pitched roof. In this approach, insulation is placed at ceiling level, usually with economical quilt insulation, leaving the space above the insulation uninsulated, hence the term 'cold' roof.
To regulate moisture entering the roof void, it's recommended to install a vapour control layer (VCL) first, followed by laying a primary layer of insulation between the ceiling joists. Additional insulation layers should then be added at right angles above the first layer.
If you plan to use the roofspace for storage, ensure that the ceiling joists are robust enough to support the extra load. Provide a minimum 50mm gap between the underside of the storage deck and the insulation to allow for ventilation.
Maintaining a continuous ventilation gap at the eaves and ridge is crucial to facilitate air circulation within the roof void. This helps prevent the accumulation of humid air, which can cause roof timbers to rot.
The cold roof method is apractical choice if you:
A typical cold roof build-up:
1- Roof tiles on battens; 2- Waterproof membrane (can be non-breathable); 3- Insulation above ceiling joists; 4- Insulation between ceiling joists; 5- Vapour control layer; 6- Plaster ceiling; 7- Eaves ventilation; 8- Ridge ventilation (recommended).
A ventilated warm pitched roof is often chosen when converting an existing roof space without the need to disturb the roof tiles or waterproof underlay. This method involves installing insulation between the rafters and leaving an air gap above to ventilate away any moisture that might accumulate within the roof structure.
Additional insulation istypically required below the rafters to meet building regulations.
To control moisture enteringthe construction, a VCL is necessary on the warm side of the insulation. The interior lining (such as plasterboard) can then be fixed to counter battens,creating a service void that protects the VCL from potential penetrations. Alternatively, you can use insulated plasterboard that includes an integrated VCL.
A ventilated warm pitchedroof is a great choice if you want to:
An example of a retrofitted ventilated warm pitched roof build-up:
1- Roof tiles on battens; 2- Waterproof membrane (can be non-breathable); 3- Insulation between the rafters leaving a 50mm air gap above; 4- Insulation underneath the rafters; 5- Vapour control layer; 6- Plasterboard interior lining on 25mm counter battens or insulated plasterboard.
Unventilated warm pitched roofs maximise the insulation potential of the roof structure by eliminating the ventilation gap and filling the full depth of the rafters with insulation. Technically, the roof remains ventilated by installing counter battens below the roof tile battens to create an air gap outside the main roof build-up.
For the optimal build-up, installing insulated sarking boards on top of the rafters will eliminate thermal bridges. If raising the roof line is not feasible, insulation can be addedbeneath the rafters.
To prevent interstitial condensation – dampness within the construction caused by condensing water vapour – a breather membrane and an effective vapour control layer are essential. These components enable vapour to safely diffuse to the outside during winter and into the building during the summer when the external temperatures are higher.
An unventilated warm pitch roof is a consideration if you aim to:
An example of an unventilated warm pitched roof build-up:
1- Roof tiles on battens; 2- Counter battens for ventilation; 3- Breather membrane; 4- Insulated sarking board; 5- Full fill flexible insulation between rafters; 6- Vapour control layer on 9mm OSB board; 7- Plasterboard interior lining on counter battens.
In cold roof construction, lower-performance insulation such as mineral wool (Rockwool or similar) is commonly used because it can accommodate larger material thicknesses. For improved environmental credentials, consider natural materials like sheep’s wool (Thermafleece or similar) or flexible wood fibre (such as SteicoFlex).
For ventilated warm pitched roofs, higher-performance insulation is necessary to optimise internal space utilisation. PIR (Kingspan or similar) or a combination of mineral wool and PIR is often specified. In roof spaces with restricted headroom, consider using super high thermal resistance materials such as aerogel (Thermablok or similar) to achieve the slimmest possible profile. Alternatively, for projects aiming to reduce on-site carbon emissions and have flexibility in overall roof build-up, rigid wood fibre insulation with a low carbon footprint is a sustainable option.
Unventilated warm pitched roofs require insulation using vapour-open materials to assist with moisture control. Additionally, using insulation that is hygroscopic helps reduce the risk of interstitial condensation. Install flexible wood fibre insulation between the rafters, with rigid wood fibre sarking boards (such as Pavatex Isolair Multi or similar) placed on top. If additional internal insulation is needed, consider using a universal rigid wood fibre board (like NaturHeld).
Approved Document L Volume 1: Dwellings (ADL1) of the building regulations outlines energy efficiency requirements for residential buildings in England, which vary depending on whether you are constructing a new house, extension, or upgrading an existing building element.
For new build dwellings, ADL1 covers airtightness, thermal mass, thermal bridging, solar gain, and U-values.
Regarding U-values, ADL1 specifies Notional Dwelling U-values as target values to achieve compliance, and Area Weighted worst case U-values as minimum standards which can be met under certain conditions.
The target U-value for a new dwelling's pitched roof is 0.11 W/m2K. The worst-case U-value is 0.16 W/m2K.
For extensions, ADL1 sets Area Weighted Average Maximum U-values as target requirements and Threshold Values as minimum standards.
If any part of your extensionis built to a Threshold value, you must demonstrate through SAP analysis thatcompensatory improvements elsewhere in the construction satisfy overall requirements.
The Area Weighted Average Maximum U-value for an extension's pitched roof is 0.15 W/m2K. The Threshold U-value is 0.35 W/m2K.
When renovating an existing pitched roof, Improved U-values are the target, while Threshold U-values provide allowances if the works are technically unfeasible or lack a payback within 15 years.
The target Improved U-valuefor renovating pitched roofs is 0.16 W/m2K. The Threshold U-value is 0.35W/m2K.
New build house featuring an unventilated pitched roof designed by Mawson Kerr Architects to create voluminous interior spaces
As a general guide, the table below outlines a few example insulation thicknesses that meet U-value requirements for domestic pitched roofs.
Please note that for warm pitched roof constructions, the insulation thickness must align with the depth of the rafters. For example, if you’re building a warm ventilated pitched roof with 150mm deep rafters, you will typically have a maximum of 100mm of available insulation to fit between the rafters, allowing for a 50mm air gap above the insulation layer. If you have shallower rafters, additional insulation will therefore need to be installed underneath.
Approximate insulation thicknesses (mm) required at 400mm rafter centres to achieve domestic U-values for pitched roofs.
To achieve high levels of thermal comfort, it's crucial to eliminate drafts, as insulation alone may not suffice.
Start by ensuring that the insulation between the rafters fits snugly. Flexible insulation is preferred as it can be cut slightly larger than the gap and then compressed into place.
If space permits, consider adding an additional layer of insulation either above or below the rafters (depending on whether the roof is ventilated or unventilated). This supplementary layer will provide a secondary barrier against drafts. Even if the required U-value can be achieved by insulating solely between the rafters, the extra insulation material will significantly enhance thermal comfort.
For optimal airtightness, think about installing a membrane behind the interior lining. Most airtightness membranes also act as vapour control layers, effectively preventing drafts andmanaging moisture levels. These are sometimes referred to as air and vapour control layers (AVCL).
In summer, roofs can overheat, but incorporating thermal mass helps absorb and slowly release excess heat, creating a more comfortable indoor environment. Therefore, when designing roofs, it's essential to consider factors beyond U-values, including thermal mass and airtightness, to achieve optimal thermal comfort.
Materials like dense insulation such as wood fibre effectively store and release heat, moderating indoor temperature fluctuations. This reduces reliance on mechanical heating and cooling systems, improving energy efficiency and lowering utility costs. Thermal mass also enhances thermal comfort by maintaining stable indoor temperatures day and night.
Installing rigid wood fibre sarking boards over the rafters removes thermal bridges and adds thermal mass to a timber structure. Image by Steico.
Cold Roof Method:
Ventilated Warm Roof Method:
Unventilated Warm Roof Method:
About the author
Aron Coates, an architect with over two decades of experience, draws upon a wealth of residential architectural expertise, encompassing projects ranging from the restoration of historic structures to the design of contemporary homes.
The information provided in this article is intended for general guidance and educational purposes only. At Designs in Detail, we advise you appoint a skilled architect to provide, specific expert advice for your project.
