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How do I Parge Over Board Insulation?

Marjorie Gilbert

Parging is the process of applying cement plaster to walls, such as foundation walls and rough masonry, as a waterproofing system. Parging serves as a vapor diffusion barrier that helps to prevent the penetration of water through the masonry into the building itself.

This inexpensive method of protection can help ensure that your building lasts for years to come.

  1. Attach insulation board to the foundation or substrate. Use 1-inch masonry fasteners. Do not drive the heads into the insulation board, as they can crack the surface. If you wish, you can use washers with the fasteners to keep the screw heads above the surface of the insulation board.

  2. Fill any cracks that are larger than 1/6 inch with spray foam. This is available at any hardware or building supply store.

  3. Rasp the wall with a file to make sure that the surface of the wall is within ΒΌ inch at any point. If you wish to add details such as joints or grooves, apply them now with a trowel.

  4. Mix 17 parts Portland cement to one part parge mix. Mix together with a stirrer attached to a cordless drill. Let stand for 10 minutes.

  5. Stir again, then add enough water for it to be at a workable consistency β€” not too runny and not too stiff.

  6. Spread the mix onto the wall or foundation. Do so evenly with easy strokes. Do not hurry.

  7. Press wire mesh into the parge coat. Apply more parge coat until the mesh is covered. The final thickness should be 3/32 inch.