Is this a water vapor/condensation risk? Dwarf wall void in loft.

4 Jan 2010
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United Kingdom

I have been slowly upgrading a converted loft in my victorian semi.

A couple of years ago I replaced the insulation in the rafters with PIR board. 100mm in between the rafters, 25mm underneath the rafter. 50mm gap between insulation in the rafters and batten for ventilation. At the time, I thought I might build cupboards in the dwarf wall space, so I continued the PIR all the way towards the soffits. There is about 30cm of glasswool/mineral wool insulation on the ceiling area. Please see photo for detail.

I no longer want cupboards and would like to just seal up the space with a dwarf wall. For sound proofing reasons, I will use 2 x layers of acoustic plasterboard with Mass Loaded Vinyl (MLV) sandwiched between the plasterboard.

My questions are:

1. If I do this, am I going to get a potential condensation issue with vapor or warm air rising in to that sealed void with nowhere for it to go?

2. I understand that the MLV would create a vapor barrier on the dwarf wall. If I did not use the MLV and just used the plasterboard, would this reduce the risk? As I understand that plasterboard is vapor permeable.

Please note that I have had the boards up for a few months on the opposite wall (see other photo) and it looks OK if I poke my head in and look from one end but I am also aware that the area is not properly sealed off until the whole room is finished.

Any advice or comments gratefully received...

Happy Easter!



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I doubt there will be any condensation as the surfaces won't get cold enough.
Thanks catlad. That was my initial assumption too. It was when I was putting on the Mass Loaded Vinyl I started to think twice. Perhaps I am over-thinking it.

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