Concrete masonry

In the thicknesses commonly used in buildings, concrete block walls (concrete masonry units -CMUs) provide approximately the same sound insulation as a solid concrete wall of the same weight per unit area - provided all cracks, openings and voids are properly sealed. Solid and hollow-core concrete blocks are manufactured in a variety of thicknesses, core sizes, aggregates, and densities. Representative values of sound transmission class (STC) for block walls are given in the table below. These values are only valid if the wall surfaces are properly sealed and the mortar joints are sound.


lightweight block normal weight block

block thickness
mm
weight/block
kg
STC weight/block
kg
STC

90 7 43 10 44
140 10 44 15 46
190 13 45 17 48
240 15 47 21 49
290 18 48 25 51

The sound insulation provided by a hollow-core block also depends on its porosity. The more porous the block, the more sound will leak through the block structure and reduce the sound insulation. To ensure that sound leakage through the pores of the block does not cause problems, the surfaces of the block should be sealed (for example, with block sealer, epoxy paint, or a skim coat of plaster). Improvements of 5 to 10 STC points are not uncommon after sealing bare blocks. A layer of gypsum board attached directly with screws or dabs of glue to the block surface is not an effective seal. It is enough to apply the sealant on one face of the block only. Even when gypsum board is to be used to finish a block or concrete wall, the wall should be sealed. Covering problematic block walls with gypsum board does not eliminate the problems.