quick question about air space between walls

Hi, The outter leaf walls in my under construction Main tracking room are cavity concrete block, the exact composition is from inside out render - 4" concrete block - air space - 4" concrete block - exterior render. The 2 walls are connected by steel block ties and lots of mortar that was dropped into the cavity during construction... In an earlier question I posted here the answer I got regarding this type of wall construction was that the wall would act as one leaf due to the coupling caused by the ties and mortar... So my question When building my inner leaf which will be inside out 2x4 studs with 3 layers of 15mm plasterboard, can I reduce the recommendeed minimun air space of 4" due the added mass of the outter concrete block wall. If so, what minimum air space would you recommend... Drums and loud amps will be recorded in this room so I need to maintain the intended isolation If any extra info is needed just ask warm regards hally

When building my inner leaf which will be inside out 2x4 studs with 3 layers of 15mm plasterboard, can I reduce the recommendeed minimun air space of 4" due the added mass of the outter concrete block wall. If so, what minimum air space would you recommend...

With that amount of mass, you'll probably be fine with 4 inches of air gap. I didn't do the calculations, but you have a LOT of mass in those walls, so I reckon you should be fine with 4". I reckon your weak point will be your doors and windows, not your walls! Are you able to keep the same mass and distance relationship for the ceiling? - Stuart -

Hi Stuart, The air gap for the ceiling can be anywhere up to 8" and the outer leaf of the ceiling is composed of 3 layers of plasterboard in between rafters. This is signifigantly less mass than is in the concrete walls. There are two windows into the room to let in some natural light. The Windows for both in side and outside leafs are composed of glass blocks. There is a total of 14 (19x19x8cm) glass blocks being used in each leaf and the density of each block is 48.63674 pcf. The doors will be double beefed up fire door construction. It has just dawned on me that the real week point of this room are the glass blocks, their density is approx. equivilent to 1 layers of plasterboard or am I missing something here. I remember reading somewhere on the forum that the vacume in the middle of glass blocks adds to there sound attenuation properties...Can u shed any light on this? Anyway, back to the walls, so 4" is the min. air space I should leave.

but you have a LOT of mass in those walls

Due to this added mass is there any way I could reduce the air space around the walls (not the ceiling) to 2"? regards hally

you could reduce it to 2" but then i'd use very light insulation in that cavity to damp it - probably 10kg/m3 max to allow air flow but still a lot of friction loss for LF. what will help you is the looseness of the air spring in that cavity so as much flow around the wall space will help. on the glass block, the mass still predominates the isolation of it so you need to adjust the outer windows as well (if you haven't already). in most cases where i use glass block, it try to minimize the size of the window on the exterior and the interior. say 300mm w x 700mm h max or even 150mm x 400mm - enough to let light in but also minimize the size of the penetration. use the heaviest glass block you can find.

Thanks Glenn, Would this knauf insulation be considered light insulation http://www.knaufinsulation.co.uk/PDF/Da ... oll-44.pdf They don't list density data, the 100mm roll - its termal conductivity is 0.044 (W/mK) and its termal resistance is 2.25 (m2K/W). Would it be acceptable to compress this rool by 50mm to fit into the 2" air gap, otherwise I could simply half its thickness by splitting the rool carefully... If this is not suitable can u recommend an insulation which would work and take into account I'm on the other side of the atlantic... regards hally

it looks about right based on the description and the flow resistivity but you'd have to split to 50mm for that space, you would not want to compress it.

thanks glenn