quick question about concrete block walls and cavity walls

Hi, Exterior Walls The existing structure where i am building my studio has cavity block walls on the exterior, by this I mean (from the outside in) 1/2" render - 4" solid concrete block - 4" cavity filled with styrofoam insulation - 4" solid concrete block - 4". The external and internal leafs of the cavity walls are connected by pins to strengthen the wall and i am sure lots of mortar fell into the cavity during construction. Question, can I treat these exterior walls as a single leaf due to the fact that the cavity is not air tight and the inner and outer block are connected... Approx, what would the STC of these walls be. Internal Walls The internal walls are all made of 4" solid concrete blocks and renders (plastered) on both sides... Question, Approx, what would the STC of these walls be. Any input much appreciated regards hally

Good question. In this case I'd say "yes", one wall (note that I didn't write "leaf"). The walls are connected so there's no mechanical separation which means sound will flank through as it was just one wall. But you will have 2 leafs in this case. Good thing those leafs are solid brick which means less change of disturbing resonating sound. Brick and concrete are way much harder to set in motion than gypsum or wood. I would add a real, separated, inner wall and leave a cavity in between those walls. But, I'm sure Rod (which I haven't seen here for a while) know exactly how to tread this. He's prolly gonna say something like "sometimes you've gotta work with what you have" :) cheers

I'm in a similar situation with my studio build. My existing walls are hollow core blocks. I asked about this as well because I was worried about having a 3 leaf wall system after framing my new walls inside the block walls. While the idea for optimum performance indeed is Mass-air-Mass (2 leafs), I've learned that cavity block walls are rigid enough that they basically act as one leaf. Also, in your case, it sounds like you said you have rigid insulation in the cavity of your wall which would actually turn your wall into one true leaf anyways. Even without the rigid insulation, it would still act as one leaf because of how rigid and massive it is.

Thanks Ro, I have started construction already on the studio and the first minute I get I will start my build diary here. I am currently trying to seal the outer structure of the rooms and i then intend to build the innner leafs, i won't go into here in detail but i will definately need some advise as I go along... warm regards hally

cyeazel wrote:

I'm in a similar situation with my studio build. My existing walls are hollow core blocks. I asked about this as well because I was worried about having a 3 leaf wall system after framing my new walls inside the block walls. While the idea for optimum performance indeed is Mass-air-Mass (2 leafs), I've learned that cavity block walls are rigid enough that they basically act as one leaf. Also, in your case, it sounds like you said you have rigid insulation in the cavity of your wall which would actually turn your wall into one true leaf anyways. Even without the rigid insulation, it would still act as one leaf because of how rigid and massive it is.

True, hollow core blocks are treated as one leaf; it's a cheese-wall (a wall with holes) :) Poring sand, cement in the cavities will sure strengthen your wall in all ways, but you'll gain much more weight.