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Insulating Concrete Masonry Unit (CMU) Walls

johnarch1 | Posted in General Questions on

Hi Martin,

I have a question which may be quite basic.

We have an existing single family house with exterior above grade cmu walls
(8′-0″ +/- above grade) which are currently covered with Eifs.

The plan is to remove all the Eifs down to the face of the cmu and install new concrete board siding as follows:

concrete bd on
1″ or 1 1/2″ thk. pressure treated furring strips tapcon-ed to face of the existing cmu.

The house is currently insulated on the interior with batt insulation on wood framed wall just inside the cmu walls.


1- Is a liquid applied WRB on the cmu before installing the furring stripsĀ  required or can we just parge the cmu or just leave as is. What are pros and cones of each.

2- Should we install rigid insulation between the furing strips – if so should it be full depth or less to allow any water / moister to drain behind the concrete bd siding. What are pros and cones of each.

Thanks in advance for your repsonce.


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  1. Expert Member
    Akos | | #1

    Which climate zone you are in and how thick is the EIFS?

    Generally the best way to insulate CMU is with rigid insulation on the outside, insulating with batts only can cause moisture issues in colder climates. The rigid behind the EIFS is actually doing a big service for you by keeping the CMU warmer thus preventing most condensation issues.

    The better option is to keep most the the EIFS and only remove enough to handle the new window drainage details to the new siding plane. Most EIFS systems use EPS insulation which can easily support the strapping you want to use, there is no need to remove it, just screw through it with extra long masonry screw.

    Generally furring strips don't need to be pressure treated and most siding can be hung on 3/4" thick material. I normally use 1x4 SPF or strips of 3/4" CDX.

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