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Best loose fill insulation for concrete block?

edkrause | Posted in Green Products and Materials on

The frost wall of our addition is concrete block. What is the best loose fill insulation for the hollow cores? Vermiculite? Perlite? EPS beads? (I understand they are made with the waste of EPS board, so this mitigates the environmental impact? Other ideas?


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  1. GBA Editor
    Martin Holladay | | #1

    Filling the hollow cores of concrete blocks with insulation has very little effect on the wall's R-value. It's not worth the trouble. To improve the wall's R-value, you need to install a continuous layer of insulation in the interior or exterior side of the concrete block wall.

    If you are insulating on the exterior, you can use closed-cell spray polyurethane foam, XPS, EPS, or mineral wool. If you are insulating on the interior, you can use spray foam or any type of rigid foam.

    The biggest problem with unfilled cores in a concrete block wall has nothing to do with the wall's R-value; it has to do with air movement. (The problem is discussed here.) It's important to fill the cores of the top course of blocks with concrete or mortar to stop air movement. This can be done by stuffing some fiberglass insulation in the cores before filling the cores with concrete or mortar.

  2. edkrause | | #2

    Thanks Martin,

    That is good news, as dealing with any loose fill is awkward (especially if there is wind). I had planned on both filling the cores with loose fill and 4" of foam on the interior of the block frost wall, with 4" under the slab on grade and at the slab edge.
    Interesting data on moisture and air movement in block walls, so fill the last block we will.


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