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Community and Q&A

Cement backerboard with stucco as exterior siding

ethant | Posted in Green Building Techniques on

In the comprehensive article ‘How to Insulate a Basement Wall’ ( one technique for finishing a concrete foundation wall with exterior foam insulation that is mentioned in the article but which isn’t discussed throughout the 160+ comments is ‘Cement backerboard, with or without a layer of stucco.’

I have a few questions about this suggested technique.

The first is: is this suggested technique possible without a drainage layer? So the foundation wall assembly from interior to exterior is: concrete–>dampproofing–>EPS–>cementboard?

Secondly, it is implied that cement backerboard can be left as the exposed finish at the exterior of the insulated foundation. Is this true? Are there particular cement backer board which are applicable in this situation?

Lastly, when suggesting ‘with our without stucco,’ is the assumption that this is an aesthetic decision or would the stucco serve a purpose. My concern is the stucco would tap moisture.

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  1. ethant | | #1

    A follow up to my own question ... I meant to ask the next question, which is whether backer board, if applicable as a finish over an insulated basement, can also be used as a substrate for stucco over a rain screen. The reason for asking is I'm trying to create a continuous finish where a wall transitions from concrete foundation retaining to framed above grade so that it looks continuous from the exterior. My thought is that stucco finish could be the answer, and maybe continuous backer board, of I can get the planes to sign, could make it work

  2. GBA Editor
    Martin Holladay | | #2

    A rainscreen gap on the exterior of a framed wall needs air intake holes at the bottom of the wall. These air intake holes (which also allow any liquid water to drain) are usually located right below the mudsill.

    In theory, you could have stucco above and stucco below. But ideally, these two stucco layers would not be co-planar. (In almost all cases, the stucco over the framed wall will be proud of the foundation.) You need to allow air into the rainscreen gap, and you need water to drain out.

    If you can come up with some type of vent that allows these two surfaces to be co-planar, you will, at a minimum, need horizontal Z-flashing or weep screed between the vent and the stucco that covers your foundation.

  3. GBA Editor
    Martin Holladay | | #3

    Old-fashioned cementitous backerboard (for example, National Gypsum Permabase) is made of Portland cement, sand, water, and some type of fiber. It's fairly impervious to water.

    I think that if you installed this backerboard to protect your rigid foam layer, the backerboard would last for many years. Most homeowners don't want to look at the fasteners and seams, however, so I would imagine that the usual practice (for aesthetic reasons) would be to cover the backerboard with stucco.

  4. ethant | | #4

    Martin, thanks for your reply. To clarify a point, the transition from concrete wall to wood wall I am working on occurs vertically, as the house is partially subterranean... Kind of like a walk out basement... So I'm proposing a stucco finish on the concrete portion, and then a stucco finish over the wood walls (which will be CLT). Therefore, I'm imagining the transition as a vertical joint. My concerns are as follows: Can the concrete backer board over EPS that you describe in the original article I cited function without a drainage plane? And furthermore, will concrete board (with rainscreen gap) over a wood find insulated CLT wall be too impervious to vapor... Or is this not even a concern in a rainscreen application?

  5. SwitchgrassFarmer | | #5


    Cement board stucco systems are rare, at least that was the conclusion I came to when I researched them for my home. I think one reason is that they are more labor intensive. As you deduced though, they provide an opportunity to match different types of surfaces, planes, etc.

    Here is a BASF page with info on cement board stucco:

    A key point to the above info is it assumes the use of synthetic stucco. Synthetic stucco is more impervious to moisture. That is good and bad, depending on how you want your wall to dry. The advantage of synthetic stucco is that a good EIFS contractor can obtain a consistency of finish throughout your project.

    As Martin pointed out, you need some sort of break to allow your rainscreen system to dry. In our case for all areas where we were covering foam, either on our ICF foundation or on exterior foam insulation covered stud wall, we held the cement board slightly above grade. This was for drying, but also to provide a means of access to the foam for any insect treatment that might be needed later. Where we covered block walls we ran the synthetic stucco below grade, but used copious amounts of gravel and drainage lines to move water quickly away from the wall.

    I have attached a picture showing a section of our home while we were doing the cement board stucco installation. You can see where the BASF synthetic stucco on the block retaining walls runs below grade, next to drainage. (Not so evident is the flashing and drip kerfs under the stone caps.) You can also see where on the ICF walls we are stopping it above grade utilizing a woven rainscreen backer in one area, and furred out channels with Cor-A-Vent ventilation in another.

  6. ethant | | #6

    Thanks all... So I'm seeing the drawback of this approach is in the fact that as far as I can tell the synthetic stucco products associated with backer boards seem not to be vapor permeable.

  7. GBA Editor
    Martin Holladay | | #7

    When choosing stucco products for installation over a poured concrete foundation, there aren't too many worries about vapor diffusion.

    When it comes to cross-laminated timber (CLT), I'm unable to provide experienced advice. I don't have the necessary experience or knowledge about vapor flows through CLT to advise you.

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