Breaking the thermal mould

Heat lost through conventionally-wrapped parapets is a common complaint among architects, so an efficient space-saving alternative is welcome, says Schöck’s Chris Willett.

In a similar way to the thermal insulation issues manifested with balconies, in the case of parapet connections energy is seamlessly transferred through the thermal barrier. In addition, a link between the wall and ceiling can have a shape-dependent impact on the exterior corner, potentially affecting the exterior to interior surface ratio. Anchoring parapets onto a wall or ceiling also presents a material-related thermal bridge compara- ble to a balcony connection.

The conventional method of insulating parapets is to wrap the perimeter of the wall with an insulation barrier to make the parapet part of the heated building mass. As a result, the heat outflow is reduced; however, the loss of energy is still several times higher than when a structural thermal break is incorporated.

However when the parapet is thermally separated at ceiling level by a thermal break, it sits outside of the heated building mass. The two diagrams on the following page demonstrate heat loss from a parapet with insulation wrapped along its length and a parapet construction using a thermal break. They show that more heat is being lost through the wrapped insulation along the circumference, while in the case of the thermal break, barely any energy escapes through the load-bearing thermal insulation element.

Low U-values

Beyond being an effective insulant, thermal breaks offer more design flexibility as they can connect structurally at specific points. In fact, the levels of insulation achieved when a thermal break is employed are so significant that the solution is a viable choice on Passivhaus schemes.

Indeed, the thermal break is assessed as a ‘Certified Passivhaus Component’, provides BBA Certification, LABC Registration and NHBC approval and meets full compliance with UK Building Regulations. The temperature factor used to indicate condensation risk for occupants in residential or commercial buildings – the (fRsi) value that must be equal to, or greater than 0.75 or 0.50 respectively – is comfortably met by incorporating the solution.

In addition, the solution is compliant with the Government Standard Assessment Procedure, SAP 2012 concerning carbon emissions from buildings and respectively heat losses through non-repeating
thermal bridges.

Parapets incorporating thermal breaks are also designed for low maintenance, as opposed to wrapped components that are prone to damage and, inevitably, repair and maintenance outlay. This is particularly the case in areas where railings or covers pierce the insulating layer. With thermally-separated parapets however, railings and covers can be attached directly into concrete.

Another significant benefit is that extra useable terrace area is released because there is no thermal insulation applied on the internal surface. Thermally-separated parapets are also durable and water impermeable, minimising maintenance costs and the potential for expensive restorations due to waterproofing problems.

Cost savings

Recently, an independent cost analysis undertaken by Andrews Eades Chartered Surveyors involving two 12.5 metre-long parapet constructions: a conventional parapet construction detail and a second detail incorporating the structural thermal break with a spacing of 1.25 metres. Comparisons of construction costs show initial cost savings through the use of thermally-separated parapets of up to 10 per cent depending on the structure.

The reason for this is simplification of the formwork process, as well as the detailing of the rear of the parapet.

Chris Willett is managing director of Schöck