Dormer windows are an ever-popular choice, enabling homeowners to make the most of their loft space, whether they are renovating or self building. This feature can add distinction and character to a home’s external appearance, along with space and light to attic rooms within.
However, they are notoriously difficult to get right when it comes to insulation — given their complicated designs and small build areas it can be a fiddly job to ensure a dormer is correctly specified and installed, which can in turn result in a distinct weak spot in an otherwise highly efficient house. So, how do you get dormer insulation right?
Vacuum Insulated Panels (VIPs)
VIPs feature a microporous core which is evacuated, encased and sealed in a thin, gas-tight envelope. This design allows them to achieve the required thermal performance at a fraction of the thickness of even premium-performance insulation solutions, making them ideal for dormer applications where space is critical — such as dormer cheeks.
Take, for example, a typical timber dormer window construction designed to achieve a U value of 0.15kW/m2. Assuming 38x63mm timber studwork, 60mm VIPs should be fitted tight up against pre-installed OSB or plywood sheeting, ensuring the VIPs are flush with the outside surface of the studs. High-performance PIR insulation infills of the same thickness should be cut and used to accurately fill gaps around the perimeter of the dormer cheek.
A vapour control layer should then be installed with appropriate detailing at joints, penetrations and wall perimeters. In this case, a 72.5mm thickness of premium-performance rigid phenolic insulated plasterboard is chosen to bring the wall up to the desired level of thermal performance.
Insulating a Dormer Roof
- Using a typical plywood deck above 50x150mm timber joists at 600mm centres, the deck should be dry and clean and graded to the correct fall for drainage. A vapour control layer should then be applied over the primed deck in accordance with the manufacturer instructions, ensuring the vapour control layers are adequately sealed with a minimum overlap of 150mm. The vapour control layer should also be turned up at the roof edge to a height appropriate for the waterproof membrane.
- 60mm vacuum insulated panels (VIPs) are then installed above this layer in a chessboard pattern with joints tightly butted, and high-performance PIR insulation infills of the same thickness fitted to fill any gaps. The infills should also be installed at the edge of the roof, or where penetrations are present, in strips no less than 200mm wide, to account for building tolerances and to provide a zone to allow for peel restraint mechanical fixing of the membrane. The insulation should be bonded using a proprietary adhesive system.
- 25mm high-performance PIR insulation infills should then be bonded over this as soon as possible and a waterproof membrane installed over the whole insulated area including any insulation upstands.
Insulating the Dormer Sides
- Assuming 38x63mm timber studwork, 60mm VIPs should be fitted tight up against pre-installed oriented strand board (OSB) or plywood sheeting, ensuring the VIPs are flush with the outside surface of the studs. High-performance PIR insulation infills of the same thickness should be cut and used to accurately fill gaps around the perimeter of the cheek. Any small gaps can then be filled with expanding urethane sealant. To keep the insulation in place, treated softwood battens should be nailed to the side of the timber studs, with care taken to ensure the VIPs are not punctured or damaged.
- A vapour control layer should then be installed with appropriate detailing at joints, penetrations and wall perimeters. In this case, a 72.5mm thickness of premium-performance rigid phenolic insulated plasterboard is chosen to bring the wall up to the desired level of thermal performance.
- Treated battens should then be fixed vertically to the external cheek wall structure along with a breathable membrane. Care should be taken to ensure the battens and fixings coincide with the studs, head rails and sole plates and that they’re suitable for the cladding system, wind loading and wall design.
- Timber cladding can be fixed directly to the vertical battens in accordance with the manufacturer’s recommendations. Alternatively if render or tile-hung cladding are used, the relevant render carrier or horizontal battens can be fixed to the vertical battens.
Case study: Cropthorne Autonomous House
Situated in the heart of the Worcestershire countryside, Cropthorne Autonomous House goes above and well beyond even the strict heating and energy demand requirements of the Passivhaus standard. Almost all of the energy is provided by renewables, with space heating virtually eliminated and mains water and drainage replaced by rainwater harvesting and composting toilets.
While prioritising these sustainable principles, owners Mike Coe and Lizzie Stoodley were also firmly committed to maintaining a clear aesthetic vision for the property, including a dormer window on the rear north-facing roof. In order to bring this area up to the same level of thermal performance as the rest of the roof (it has an exceptional thermal performance with a U value of 0.08kW/m2), 300mm of premium-performance rigid phenolic insulation was installed within a plywood frame, forming the cheeks and roof of the dormer. A copper-clad plywood frame was then fitted outside this, creating a 300mm void around the insulation.
While the dormer in this instance sits well within the dimensions and contemporary design of Cropthorne Autonomous House, some buildings would be better suited to a slimmer solution. As a result, it is worth exploring which alternatives could deliver the required thermal performance without greatly increasing the design thicknesses. One increasingly popular option is to use VIPs.
Kingspan: 01544 387384
Adrian is Head of Marketing and Product Development at Kingspan Insulation and is one of the UK’s leading experts on insulation