# plaster over CMU block



## TrapperL (Jul 23, 2011)

I would strongly suggest researching Autoclaved Aerated Concrete blocks. I worked a large apartment job with this stuff and I was impressed with the thermal values it brings. I actually put a small 2" thick piece of it in my hand and a torch on the block and my hand never even got warm. Amazing stuff. You can frame with steel studs and apply this like any masonry with masonry straps/ties. They have a building method for interior walls but we used drywall on this job so I have no experience with it in that application.
http://www.google.com/#hl=en&cp=11&...c.r_pw.&fp=5ebc4d519a3bd440&biw=2016&bih=1124


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## Maintenance 6 (Feb 26, 2008)

So if I have this right, you are going to build an 8" block wall. Fill the cores with grout, then plaster over it. The 4" wall is just for partitions. No insulation? No moisture barrier? You can plaster right to CMU. Without a moisture barrier of some kind it won't hold up. Any moisture that penetrates the block will disolve the plaster. Plus you'll have a whole host of condensation problems. Where you are located will determine which kind and how they'll show up. There was a time when a lot of houses were built entirely of cement block. They aren't much anymore (at least in my area). There is a reason. Because they are energy hogs. Masonry partitions tied to the outside walls will be like huge radiator fins sucking the heat to the outside walls or transferring any unwanted summertime heat to the inside. And that doesn't begin to address the residential insulation code you'll need to follow today.


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## AGWhitehouse (Jul 1, 2011)

Trapper L has a great point with AAC. I will assume you are sheathing the exterior in some kind of isulation system to quell Maintenance's concerns.

You can finish the interior faces with a stucco type of system. Skim coating of a cementitious material. Or, you can apply gwb to the partitions using metal z-furring, much like they do commercially. This will give you a chase for electrical routing without needing to chip the block.


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## stuart45 (Jun 20, 2009)

I'm building an extension at the moment using 4 inch concrete blocks on the external skin, 4 inch AAC on the internal skin, and a 4 inch cavity filled with insulation. This will have sand/cement stucco externally and plaster internally. This is a common type of construction here.


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## AGWhitehouse (Jul 1, 2011)

stuart45 said:


> I'm building an extension at the moment using 4 inch concrete blocks on the external skin, 4 inch AAC on the internal skin, and a 4 inch cavity filled with insulation. This will have sand/cement stucco externally and plaster internally. This is a common type of construction here.
> View attachment 36343


What kind of finish do you use on the exterior to keep moisture drive from entering the insulation space? Water and batts don't get along very well.


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## stuart45 (Jun 20, 2009)

It's got to have the same finish as the old cottage. You might be able to see it on the right side of the window.







Easy enough to do, just render the wall, don't use a straight edge and finish off with the sponge. Waterproofer added to the mix.
This type of cavity batt is designed not to be affected by moisture, as it's treated and the fibres all go the same way. It can be used where the outer skin is face brickwork.
There are other methods of achieving the required U values though. Another popular way is to part fill the cavity with rigid insulation boards.
There are also insulated plasterboards to fix to the interior walls instead of wet plaster. Another method is to use a 12 inch AAC block for a solid wall. The external is then protected with stucco, thin bricks etc.


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## jomama45 (Nov 13, 2008)

Stu

Tell your daughter to get away from the window, they're rioting just outside!!!!!!!!! 

In all seriousness, I think that's an interesting way to construct, especially since it takes advantage of the thermal mass of the concrete.

And I hope it' safe where you are and you're not near all the chaos there.


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## woodmeistro (Jul 7, 2011)

The main reason for the grout filled CMU is strength and longivity, I don't want a tornado tearing my house down. I am looking more into the AAC block. I am also looking into insulated concrete forms for the exterior and metal studs for the interior partitions. In the end, I want something that will be around for a long time and is very well insulated. Cost isn't as much of a factor as quality and longivity. 
For my plan B, my exterior I could use structural gauge stick framed metal studs with 1/2'' osb sheathing and 3'' of rigid foil faced iso and brick and 1 1/2'' or so of closed cell foam and topped off with blown in cellulose on the inside. 

thanks for the input and advice


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## stuart45 (Jun 20, 2009)

jomama45 said:


> Stu
> 
> Tell your daughter to get away from the window, they're rioting just outside!!!!!!!!!
> 
> ...


There isn't enough people living around us for a riot Joe:laughing:
A couple of sheep went missing last year which was the biggest crime here in living memory.
I think some of the teenagers in town tried to get one going on Facebook for 1pm yesterday in the town centre. The police read about it and sent extra officers but none of the kids turned up. That is typical Somerset, no one ever turns up on time here.


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