Energy rating of insulated wall assemblies

CONSTRUCTION INNOVATION

Air leakage rates and thermal resistance values are important factors in the development of an energy code for buildings. Air leakage has a negative impact on the overall performance and durability of the building envelope. So how does the thermal performance of a wall system vary with the permitted air leakage?

Effect of air leakage on the WER number for an air leaky wall and an airtight wall with different thermal insulations and airtightness strategies. The better the airtightness strategy and R-value of the wall system, the higher the resulting WER number.

It is difficult to incorporate the effect of air leakage through the envelope on the overall thermal performance of the wall systems into building codes and related standards. There is a standard available for a window energy rating (CSA Standard A-440.2) but there is no similar standard for wall systems.

Researchers at the NRC Institute for Research in Construction (NRC-IRC) have developed a simple tool for determining the Wall Energy Rating (WER) of walls constructed according to field practices. In order to arrive at the WER number, two standard tests were performed on ten full-scale wall specimens: thermal resistance tests in the Guarded Hot Box (GHB) at zero air leakage and air leakage tests according to the NRC Canadian Construction Materials Centre Air Barrier Technical Guide 07272.

Walls with cavities filled with glass fibre insulation and a polyethylene-based air barrier were tested and used as reference walls. Then open cell and closed cell spray polyurethane foam insulations applied with old and new blowing agents were used. The polyurethane was sprayed in the cavity while other leakage paths in the wood-frame assembly were identified and sealed. The resulting WER tool accounts for simultaneous thermal conduction and air leakage heat losses through a full-scale wall system.

A 3D numerical representation of the wall specimens was then developed to combine the results of these tests to obtain an accurate prediction of thermal resistance (apparent R-value) under the influence of air leakage. The numerical simulations were conducted using the NRC-IRC hygrothermal model.

A new project utilizing 3D simulations for additional walls is being considered. Research will be conducted to refine the WER procedure through collaboration and partnerships with stakeholders. The WER procedure will eventually be proposed as an alternative compliance tool for future energy codes. The next step is to propose the development of national and international standards for that purpose.

For more information on the current studies and to discuss collaboration and partnering opportunities, contact Mike Swinton at 613-993-9708 or mike.swinton@nrc-cnrc.gc.ca.