Does the gap between outer wall and inner wall = an air gap

Let's see if I can word this properly. I'm talking about a new construction with an outer "shell" of concrete blocks, with inner walls built on their own floated floors. What's the best (or indeed practical) way to approach the outer/inner wall construction. 1 - Should I consider treating the inner face of the concrete block walls 2 - Would the gap between the concrete block wall and the inner wall figure as the air gap 3 - If 2 is yes then, can I simply build the inner walls (those parallel to the outer skin) as a second leaf? The outer block wall being the first leaf. 4 - How much gap should I leave between inner and outer. I'm thinking of about 6" minimum 5 - Am I right in thinking that, if the outer wall = first leaf, then building internal double walls will introduce a second air gap and thus 3 leaves, violating the MASS - SPACE - MASS priniciple. Hope this all makes sense. :) Thanks in advance Colin

Actually Colin, if you're referring to the hollow concrete blocks like the ones with about 25mm concrete each side, and two large holes in the middle separated by 3 webs of concrete, then that by itself is effectively a double leaf wall - consider the webs as the studs in a framed wall, and the inner and outer concrete surfaces = inner and outer sheathing. Now, add a framed wall with gypsum on both sides, separated from the blocks by an air gap, and you have a 4-leaf wall (not nearly as lucky as a 4-leafed clover :=( For a hollow block wall to work well in sound construction, two ways to go would be 1 - fill the blocks with sand or grout, making it a single leaf of high mass, then add a frame that's isolated from the blocks, and put gypsum on ONE side of that frame only. Insulation filling the gap between gypsum and blocks of course. 2 - Render both sides of the blocks, and maybe strip-glue an extra layer or two of gypsum wallboard on the inside (be careful here, moisture can make this a really bad idea) In either case, if the outside of the wall isn't dry you need to clean it, and seal with a really good water barrier, as well as making sure it's properly drained AWAY from the wall. The following PDF file covers this VERY thoroughly - http://www.buildingscience.com/resource ... ystems.pdf Hope that helped... Steve

Steve, thanks for the quick reply. I'm actually refering to solid concrete blocks. They're used extensively here in the UK for building purposes, they measure approximately 15" x 8" x 5". Don't know the weight but they are equivalent to about 6 litres of water and extremely dense. Also they're fairly cheap (UK prices). So with that in mind, would walls made of these require treating and how does this affect the MASS - SPACE - MASS relationship to the inner walls, and could I then get away with my inner walls being single leaf. Cheers, Colin

I did a quick look at the thread that Steve linked, but at 4 pages didn't gp through all of them. I don't know if references this. The following is tests on concrete blaock walls with gypsum faces added. Should give a good idea, if it doesn't have exactly the construction you are thinking of. http://irc.nrc-cnrc.gc.ca/irc/fulltext/brn217/brn217.pdf Hope this helps. Andre

If those are the blocks I'm thinking of, they are fairly porous - if so, a render coat (or at least a couple of wash coats) over them would improve TL by several dB - still, basically they would (if no holes for weepage, etc) be a solid outer mass leaf, requiring only an air gap (larger the better) an inner frame and insulation, and a couple layers of gyp board as your inner leaf. Any acoustic treatment/traps, etc, inside that... steve

Thanks for that Steve and Andre. One final question on this point - rendering to be applied inside and out or outside only. Great link by the way - huge amount of testing done by those guys. Colin

Rendering - both sides gives better TL, but if you're adding any layer directly onto the blocks on the inside it wouldn't make as much difference - you get about 3 dB per side, as a ROUGH idea... Steve

A slght addition to what Steve wrote. At a minimum paint the blocks well to seal them. Adds a couple of dB. Andre