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IGMA: February 2014

Good blocking is essential

February 7, 2014  By Margaret Webb

The use of setting blocks in an insulating glass unit addresses many
important issues and adds features that enhance the overall performance
of the IGU as well as the entire fenestration system.

The use of setting blocks in an insulating glass unit addresses many important issues and adds features that enhance the overall performance of the IGU as well as the entire fenestration system. The use of setting blocks is vital to successfully support IGUs and enhance their long-term performance. Compatibility, size, placement, hardness, profile, thickness, and material are all issues that should be carefully considered when using setting blocks.

Setting blocks serve many purposes. They cushion the bottom edge of the IGU and distribute the weight of the unit to the support frame or structure that holds the unit. Specific guidelines are given by IGMA regarding the placement of the setting blocks along with their size, material and hardness required to properly support the dead weight of the IGU.

Setting blocks also provide an exit path for any water that may infiltrate the glazing system by leading it via the sill member to the weep system or designed water drainage method. They provide uniform support to each lite of the IGU and assist in minimizing the potential for point pressures on the bottom edge of the glass that can lead to glass fracture or edge chipping. For multiple-cavity IGUs, the placement of the setting blocks must support each lite of glass.


Setting blocks is a critical element of the fenestration system’s water management system. Properly placed and sized correctly, they impede the capillary action of liquids (namely water) between surface finishes and IGU sealants. The thickness of the setting block should be considered for the prevention of trapped liquids. It is important to maintain a distance greater that that known to cause the capillary conditions between materials.

Compatibility is very important in the selection of the setting block material and ensures that the setting blocks will perform over the lifetime of the fenestration product. Incompatibility may lead to deterioration of the materials resulting in collapse of this crucial element of the weep system allowing water to become trapped in the glazing channel.

Adhering to the specific IGMA setting block guidelines will result in properly supported IGU and is an important element to successfully glazing the unit and ensuring long-term performance. IGMA recommends that all fenestration manufacturers consult with their IG fabricator to ensure that the setting blocks are sized correctly, are appropriately placed and that the setting block material used will be compatible both with the IG and glazing sealants.

Further information on setting blocks and other glazing recommendations may be found in IGMA’s TM-4000-90(07), North American Glazing Guidelines for Sealed Insulating Glass Units for Commercial and Residential Use. Please contact the IGMA office for further information on this and other industry publications.

Your input is important to the continuing evolution of IGMA guidelines like these. One great way to get involved is to attend our conferences, held twice each year in different locations in the U.S. and Canada. Our Annual Conference takes place March 4 through 7 in San Francisco, where attendees will hear from all our technical committees, attend the IGMA Education Seminar and have a chance to chime in with their opinions and feedback. But the conference is not all dry tech-talk. This year’s conference will include a night tour of Alcatraz’s famous island prison and a ‘60s-themed dinner where guests are invited to dress up in their best hippie or mod gear. If you cannot make it to our Annual Conference, never fear, we will hold another Technical Conference this August in Quebec City, date to be announced.

Margaret Webb is the executive director of the Insulating Glass Manufacturers Association.

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