Garage recording studio conversion - Roof and Vapour barrier

3 Jan 2014
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United Kingdom
Hi All,

I’m currently in the process of converting my garage into a recording studio. This essentially means building another room with the garage completely decoupled from the existing structure. The garage is single brick build with a standard flat roof covered with felt. The new room with consist of 2x4 frame with 2x7’S for the ceiling. The walls and ceiling will be filled with 100mm Rockwool rwa45 and 2 layers of Plasterboard will only be added to the inside walls and ceiling layers only. I’ve been consulting with soundproofing specialists in various forums but have a few building questions.

The existing roof only has 2X5’s joists and currently no insulation. These also need to be completely filled with Rockwool. These will not be covered by plasterboard but left open with only netting to stop the Rockwool falling out.

1. Can I completely fill the joists with Rockwool? I understand in most flat roofs you need to leave a gap for ventilation but as the Rockwool will not be covered with plasterboard does this mean I don’t need the gap?
2. Do I need to add a vapour barrier to the inside of the new stud walls and new ceiling? Bearing in mind, as above, only the inside of the new walls and new ceiling with be covered with plasterboard not the outside.

Many thanks
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1) It's a tricky one, and unless you get really technical, nothing you can do is guaranteed to work 100% once you stray from the tried and trusted path of conventional building methods.
You may want to try rockwool cavity slab up there as it's fairly water repellent so if the worst happens you won't have a soggy mess of soaking rockwool to deal with.

2) Yes, use foil backed plasterboard as the first sheet.
You will also need a form of ventilation. If you are going with 'fans in boxes' ...then build a couple to vent the cavity too. Run them when you aren't using the space to record.

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