Insulating timber stud below window

[Mark Hindley]

Hi,

I have a window in our living room that is in a prepared-for French/patio door opening. However, as the opening just has an ordinary window in it, the wall below the window is timber stud, 100mm void, with painted timber cladding on the exterior. The piece of wall (1.2x0.7m) under the window is very cold in the winter. I am also going to move the radiator in that room to this piece of wall, and I don't want all the heat going outside through the void! So, before I decorate the room, I was planning on insulating it by placing 100mm mineral wool in the void.

The void itself, obviously, is not ventilated. If I put a polythene vapour barrier on the inside, will that be sufficient to avoid problems with condensation? Is there a better way of doing this?

Thanks

Mark

[Matt Beard]

Personally I would a) use something with an even higher termal resistance, such as Celotex or Kingspan, and b) ensure that there is an air gap between the insulation and the outer wall.

[Rick]

50mm kingspan = 100mm rockwool for insulation purposes, minimum recommend air gap = 50mm.

I'd use a "full fill" mineral wool, its cheeper. And then I'd clad the inside of the whole wall in 50mm (or more) of kingspan. But the cladding is maybe a bit more than you need.

Rick - who has IMM's views on insulation - more is better.

[Mark Hindley]

Thanks.

Just so I'm clear -- what is the 50mm gap for?

I thought the gap would need to be ventilated to prevent condensation forming. Or does it aid the insulation properties by avoiding bridging?

You didn't mention a vapour barrier. Is that just taken as read or not necessary?

Mark

[Rick]

Kingspan is not a "full fill" insulation, which means its not certified that it won't wick water, so you need a gap to maintain the cavity & the waterproofness.

The "full fill" mineral fibers dont wick water, but there is a difference between these, often sold as "cavity bats" and normal loft mineral wool - you need to make sure you specify "full fill" at the builders merchant.

I have no specific knowledge on vapor barriers in this application. I did spend a long time researching the most cost effective insulation, which is why I know this stuff.

Rick

[Hugo Nebula]

On Tue, 8 Nov 2005 13:46:07 +0000, a particular chimpanzee named Mark Hindley randomly hit the keyboard and produced:

From inside out: Plasterboard > vapour barrier (polythene) > ~140mm mineral fibre batts or 70mm Celotex between studs, or fully fill between the studs with mineral fibre batts and line internally with 25mm Celotex > timber sheathing (not necessarily plywood if not needed for structural reasons, indeed, the more vapour permeable the better) > vapour permeable membrane (Tyvek, etc) (to allow vapour through but keep rain and condensate out) > drained and ventilated cavity (to get rid of any moisture on the cold side of the timber frame) > cladding.