Grout Cell Correction

Rebar and grout frequency can significantly alter the thermal performance of a wall system. Omni Block (with the assistance of the Masonry Institute of Michigan) has developed a short-cut to determine whether or not Omni Block will prescriptively meet the International Energy Conservation Code (IECC) per ASHRAE 90.1 (2013).

The tan fields contain value options in their drop-down menus. Select the options that meet the design criteria of your specific project.

Step 1: Enter the project’s climate zone or region per ASHRAE 90.1. The maximum allowable U-factor automatically entered in the cell below.

Step 2: Select one of the Omni Block products. Although the calculator includes the nominal 8″ high units, the thermal performance difference between the 8″ and 4″ high products is minimal so the calculator may be used for the 4″ high products.

Step 3: Select the expected concrete density. Lightweight concrete density starts at 85 pounds per cubic foot and ends at a heavyweight density of 135. The heavier the concrete the less the thermal performance. Most regions employ a concrete mix design between 115 and 120. A conservative entry would be 125.

Step 4: Enter the expected vertical rebar frequency. For this calculator the grout cell frequency matters, but the size of the rebar does not.

As long as the resulting Corrected U-Factor field is green, Omni Block passes the International Energy Conservation Code prescriptively.

The calculator accurately quantifies a typical wall area. It does not address horizontal bond beams, above opening masonry lintels, control joints, or in-line piers, which are areas that negatively affect total wall U-Factor. These items vary greatly from building-to-building and cannot be reasonably addressed in this type of tool.

Immediately below this calculator is an ASHRAE Climate Zone Map to assist in determining what Zone to input into the tool.

ASHRAE 90.1 Climate Zone Map

Grout Cell Correction Tables

The tables below provide the supporting R-Value and U-Factor corrections included in the calculator above and include the negative dynamic affect that vertical grout cells have on a typical wall assembly. Where there is grout there is not insulation.

As an example below, if the vertical rebar frequency, as determined by the structural engineer of record, is to be every 40″ on center then the R-Value is 18.2 and the U-Factor is 0.0549 at a concrete density of 115 pcf for the System 8.

The System 8 tables are immediately below followed by the System 12 tables. The Seismic 12 design is only affected by the concrete density and not the frequency of the grout, which is solid throughout.

The tables below provide R-Value and U-Factor corrections based upon the frequency dynamic of the vertical rebar and grout for the System 12 block. For example, if the vertical rebar frequency, as determined by the structural engineer of record, is to be every 40″ on center then the R-Value is 26.4 and the U-Factor is 0.0378 at a concrete density of 115 pcf.