Energy efficiency is critical for new buildings as the New Zealand construction industry continues to take steps to deliver more environmentally friendly, energy-efficient builds. Under the revised H1 Building code, changes have been made to the roof insulation requirements for housing and small buildings under 300m² to meet the updated standard. When using the schedule method of compliance, all climate zones, must now meet R6.6 construction values in the roof. However, this can be a challenge for skillion roof insulation installations which are limited by rafter depth and thermal bridging.
Guy Manthel, Technical Product Manager, Knauf Insulation, said, “There are two common methods that are used to demonstrate compliance in a skillion roofs: the schedule method and calculation method. The schedule method specifies minimum R-values for building envelope components depending on construction type and climate zone. Comparatively, the calculation method calculates the buildings performance while comparing results with reference buildings. The calculation method provides greater flexibility and allows you to offset a lower performing roof with other higher performing sections of the building”.
Achieving the R6.6 construction value in a skillion roof can be a challenge due to the cavity depth provided by the rafters, and where thermal performance can be significantly affected by thermal bridging. For example, due to its construction, a skillion roof would require a 290mm rafter to accommodate an R7.4 glasswool insulation product and achieve compliance with the minimum R6.6 construction value within the existing rafter depth. The R6.6 construction value can also be achieved with a 240mm rafter and the addition of a 45mm R1.3 additional internal insulation layer. The additional layer has a positive impact through an additional thermal contribution and a reduction in thermal bridging.
Meanwhile, the calculation methods let lower roofing thermal values be offset by higher wall and underfloor values. New skillion roof insulation products maximise the thermal contribution in the given rafter depth. For example, an R7.4 product that’s suitable for a 290mm rafter achieves thermal values above R6.6 requirements, making it suitable for the schedule method and use in the calculation method.
Guy Manthel said, “Regardless of the compliance method, installers must consider key elements including roof ventilation, and the required separation between the membrane and insulation.”
To help overcome the challenges of insulating skillion roofs our technical experts have developed a range of H1-compliant skillion roof insulation products which are designed to maximise the thermal contribution in the given rafter depth, helping the New Zealand construction industry achieve compliance with ease.
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