Rockwool between studs in your two seperate stud frames?

Ok, Im across the whole two stud frame MAM wall design discussed here, or I thought I was. If you build two seperate frames with Drywall layers on ONE side of each frame and fill the air gap with the light fluffy stuff we are on the right path. But it just occured to me, is the space between the studs on each of these frames left empty or do you fill them with rockwool? Im hoping the answer is they are left empty otherwise thats add much much $$$. That being the case, I know what answer I have my money on.... :x Thanks.

You can fill the entire air space if you want, and if it is allowed by code in your area. Rod has pointed out that some fire codes might not allow that, so check. Entirely filling the gap buys you extra isolation. Partially filling an 8" gap gains you up to about 10 dB of isolation as compared to no fill. Complete filling can take that to 16 dB of isolation, as compared to no fill. So if you need extra isolation, can afford it, and code allows it, the go for it! If not, then don't! :) - Stuart -

Thanks Stuart. Whats the norm though? Is it normal then to use the high density stuff between the studs on each frame , then in the gap use the fluffy stuff? Or do you mean the entire gap including the gaps between studs in the fluffy stuff?

High density insulation is a bad choice: it is worse at dealing with low frequencies. Low density is better. If you are using fiberglass then you need a density of roughly 30 kg/m3. If you are using mineral wool then you need density of roughly 50 kg/m3. The insulation you use should be the same density throughout the entire air space, if possible. So you can use semi-rigid (such as 703) all over, or fluffy stuff all over, or mineral wool all over. As long as the overall density everywhere is roughly the same, you'll be fine. It should also not be "stuffed in" tight: just laid in so that it is not compressed. So the "norm" is whatever people happen to do! :) Some just do the stud spaces on one leaf (and get reasonable isolation). Some do the stud spaces on both leaves (and get much better isolation). Some do the entire air gap, including both stud spaces and the area between them (and they get the best possible isolation). By the way, don't get confused about the air gap and the insulation: it is the same. Insulation is mostly air anyway, so putting insulation in the "air gap" does not reduce the size of the air gap: it is still there, and still the same size, except that it now has insulation as well as air in it. In fact, from some points of view putting insulation into the air gap actually makes the air gap BIGGER! :) For example, when calculating MSM resonance, adding insulation to the air gap is theoretically the same as increasing the air gap by 140%... I don't think you are confused about that, by I added it just in case, and for future reference of others who might read this. - Stuart -

Thanks again Stuart. This place is all the better for your continuous input. I have noticed this through hours upon hours of searching and reading here over the last two months. Hopefully your still here when I get started. :D I think given the $$$, its loose fluffy stuff all the way between my walls. As you say, its really that simple in the end.

Thanks for the kind words! :oops: And don't worry: I don't plan on going anywhere else too soon! :) - Stuart -