Cold Bridging for Rooflight How to Insulate Your Rooflight Upstand Correctly

Proper rooflight installation is about more than daylight and design - it also plays a major role in the thermal efficiency of your property. If a rooflight upstand is not insulated correctly, it can become a weak point in the building envelope, allowing heat to escape and cold air to transfer indoors. This issue is known as cold bridging and is one of the most common causes of poor rooflight performance. 

Whether you are installing a new flat roof rooflight or upgrading an existing system, understanding how to prevent cold bridging can improve comfort, reduce energy bills, and protect the surrounding structure from moisture-related issues. 

What Is Cold Bridging for Rooflight and Why Does It Matter? 

A cold bridge occurs when a thermally conductive material creates a direct path between the cold outside and the warm inside of a building, bypassing the insulation layer. In flat roof rooflights, the upstand - the raised kerb on which the glazing unit sits - is one of the most common and most overlooked cold bridge locations in residential and commercial construction. 

The kerb is typically constructed from timber, concrete block, or steel. Without adequate insulation, these materials conduct heat at a rate dramatically faster than the surrounding roof insulation, causing two serious problems. 

How Cold Bridging Happens at a Rooflight Upstand? 

Most flat roofs are insulated with PIR or mineral wool board, either in a warm roof deck configuration or below the deck. This insulation creates a horizontal thermal barrier across the roof plane. When a rooflight is installed, an upstand is built to raise the glazing above the roof level - typically a minimum of 150mm  to prevent water pooling and ingress. 

The physics are straightforward. Timber has a thermal conductivity (λ value) of approximately 0.13 W/mK. A dense concrete block sits around 1.0–1.5 W/mK. PIR insulation, by contrast, sits at just 0.022–0.023 W/mK. An uninsulated 50mm timber kerb loses heat over fifty times faster than the equivalent thickness of PIR board.  

Step-by-Step: How to Correctly Insulate a Rooflight Kerb 

The following process applies to both new installations and retrofits. The key principle throughout is continuity - the insulation must form a complete, unbroken wrap around the kerb, connecting to the main roof insulation with no gaps or compression. 

Step 1 - Select the correct insulation board Use rigid PIR or phenolic insulation board rated for external use. Minimum 50mm thickness is recommended; 75–100mm will achieve noticeably better Ψ-values, particularly on taller upstands. Avoid compressible mineral wool as the sole wrap material - it loses performance when compressed under clamping loads. 

Step 2 - Insulate all four faces of the kerb The most common error is insulating only the outer face. Heat escapes equally from all sides, so all four vertical faces of the upstand must be covered. Cut boards to fit snugly with no gaps at corners - use an L-shaped cut to wrap the corner in a single piece where possible. 

Step 3 - Connect to the main roof insulation layer The base of the kerb insulation must overlap with and connect directly to the flat roof insulation. A 50mm minimum overlap is recommended. This closes the thermal break at the most vulnerable junction - the upstand foot - where the greatest temperature differential exists. 

Step 4 -Tape all joints with foil-faced tape All board-to-board and board-to-structure joints must be sealed with foil-faced jointing tape. This prevents air movement through the insulation layer, which can significantly degrade thermal performance even in a correctly installed system. Do not use standard duct tape - it degrades with temperature cycling. 

Step 5 - Apply a breather membrane over the insulation Once the boards are taped, fix a vapour-permeable breather membrane over the insulated upstand faces. This protects the insulation from moisture during construction and in service, while allowing any trapped moisture vapour to escape outward. 

Step 6 - Install the rooflight with a thermally broken frame Even with a perfectly insulated kerb, a metal rooflight frame without a thermal break will introduce a further cold bridge at the glazing perimeter. Choose a rooflight unit with a polyamide or plastic thermal break in the frame profile. Our triple-glazed rooflights and flat rooflights are designed with thermally broken frames as standard. 

U-Values, Ψ-Values, and Building Regulations 

The thermal performance of a rooflight installation is assessed on two separate metrics. The centre-pane U-value of the glazing unit itself - measured in W/m²K - tells you how well the glass performs. The linear thermal transmittance (Ψ-value, pronounced "psi-value") - measured in W/mK tells you how much additional heat is lost per metre length of the upstand perimeter because of the thermal bridge. 

Under Approved Document L of the UK Building Regulations, new dwellings must limit overall heat loss including thermal bridges. The table below illustrates how dramatically upstand insulation affects the combined thermal performance of a rooflight installation. 

Signs That Your Rooflight Upstand Is Cold Bridging 

If you suspect an existing installation is suffering from cold bridging, look for these indicators. Condensation on or immediately around the internal face of the rooflight frame is the most obvious sign, particularly during cold mornings in winter. You may also notice damp patches on the ceiling or walls immediately adjacent to the rooflight - moisture tracking along the cold surface. In persistent cases, mould growth follows within weeks. A thermal imaging survey is the definitive diagnostic tool, clearly showing the cold ring around an uninsulated upstand against the warm background of the room. 

Choosing the Right Rooflight for Minimal Cold Bridging Risk 

Not all rooflights are equal when it comes to thermal bridging. The frame material and design play a significant role alongside the upstand insulation. Aluminium frames with no thermal break are the worst performers - aluminium has a thermal conductivity of around 160 W/mK, making it an exceptionally efficient heat conductor. A polyamide thermal break within the extrusion interrupts this path, typically improving the frame's linear conductance by a factor of four to six. 

Our range includes options specifically designed to work with correctly insulated upstands to achieve the best possible overall thermal performance. The triple-glazed self-clean rooflight starts from £163 and is available with same-week delivery, making it an accessible choice for both new builds and renovation projects.  

For a deeper understanding of how upstand design affects thermal performance, we recommend reading our companion guide on rooflight upstand basics — what it is and why it matters. 

Frequently Asked Questions