How to Seal a Cinder Block

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Using concrete or cinder blocks for exterior basement walls is a typical construction practice. These porous blocks allow moisture to move through the small cavities in the block structure. Basements that use this type of exterior wall may exhibit seeping water through the blocks. Applying a sealant to the blocks is only one step in the sealing process, especially for an enclosed basement. You may also have to install a dehumidifier, rated for a basement location, to remove unwanted moisture before the process can begin.

Things You'll Need

  • Dehumidifier
  • Broom
  • High-grade concrete adhesive caulk
  • Caulk gun
  • Block sealant
  • Roller applicator
  • Paintbrush
  • Box fan (optional)
  • Install a dehumidifier in the enclosed basement space if applicable. The moisture-removing device will begin to pull residing water from the porous blocks. In most sealant applications, as provided by various manufacturers, the sealant must be applied to dry surfaces for best performance. Consult separate label products for slight variances to moisture-level application conditions.

  • Clean the block surface thoroughly with a broom. All dirt and debris must be removed from the block for the sealant to adhere properly.

  • Apply a high-grade concrete block adhesive caulk to all cracks and open joints in the block. Most roller- and brush-applied sealants cannot span large crevices. Cut the tip from the caulk cartridge to the width of the cracks. Install the cartridge in the caulk gun. Apply the caulk according to label directions. Allow the caulk to fully cure before covering with sealant. Curing times will vary depending on the type of caulk used. Place a solid bead of caulk along all seams between the block wall and the concrete floor if applicable.

  • Install the block sealant to the cinder blocks using a roller applicator. Use a paintbrush to work the sealant material into small crevices and the inside corners of the adjoining block walls. Follow the manufacturer's application rates for the cinder blocks. In most cases, the first coat will cover less area than subsequent coats. The porous cinder block surface will absorb less sealant as each extra coat is applied. Most cinder blocks will require up to three coats to fully seal the surface.

  • Run a box fan in the enclosed space to circulate the air if needed. This step is optional but needed if extra ventilation is required during the sealant's application. This air movement can accelerate the curing times of the sealant.

Tips & Warnings

  • If a basement space continuously seeps moisture, you may have to dig in a sump pump cistern to constantly remove water from the underground enclosed space.
  • Consult local building codes and covenants for any specific regulations concerning the modifications to foundations on your home.

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