Getting insulation right at the eaves junction

19.10.2016

House builders today can face poor thermal performance at the eaves detail. This is where the roof insulation tapers out to next to nothing in the majority of trussed rafter roofs.

Mineral wool insulation is relatively inexpensive compared to other options. Generously deep layers of it are used at ceiling level, making up a significant element of the ‘whole house’ thermal calculations.

Unfortunately, the thickness of the insulation significantly diminishes at the eaves, causing potential cold spots.

The implications of having this cold spot at the junction of the roof and ceiling could result in mould growth where, for example, the cold spot meets moist, warm air from an upstairs bathroom. The homeowner will complain to the house builder and costly remedial action will be required.

How to cut down on heat loss

However, there’s a solution that is well worth considering according to Tim Tasker, Technical Manager at Pasquill. “Simply do away with this ‘pinch point’ by raising the rafter member.

"Using bobtail or stub-end trusses ensures that the correct thickness of mineral wool can be brought across at ceiling level to meet the insulation in the cavity wall. This avoids creating a cold bridge and minimises heat loss at this junction.”

Standard truss

Bobtail truss

By designing the roof at an early stage, suppliers like Pasquill can help maximise cost-effective sizing and configuration detailing before beginning the manufacturing process.

Because thermal bridging occurs at junctions in buildings where materials meet, a bobtail or stub-end truss solution can help house builders to achieve optimum thermal performance by removing the performance gap between design and build.

For more information on how to get this junction right, visit /labc-registered/construction-details.