Insulating loft in 'house within a house'

11 Jan 2018
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United Kingdom
Hi all,
I'm looking to replace the insulation in my loft which has compacted and become old and nasty. There was maybe 100mm, but appears to have packed down to half that in areas. So, nice and easy, I'm aiming to put 100mm in between the ceiling joists, then add some loft stilts, and stick another 170mm perpendicular to the joists, bringing me up to 270mm.

My question is this, however: the house is VERY old (~1590), and has a thick (900mm) solid granite outer wall. Around the 60's or 70's, they effectively built a house within a house, with what I believe is a single block pinned to that outer wall, and goes up a floor to the loft/roof base level.

That void is pretty big, with chimneys on the gable ends, so the gap in between the chimneys and the inner wall is 2-3 inches, but nearly two feet in the other areas, going the whole height of the house.

My thoughts were to mount some joist hangers and put some woodwork down which I could then insulate over in order to 'cap' that big cavity, but my concern is that I'll be trapping moisture in, which could cause me major issues down the line. If the house if effectivly a single block, it would make sense to limit the amount of heat lost that way.

I hope I've described that reasonably - please see photos from inside the lofe. You can see the wooden flooring over the 'house within the house' area of the loft and the outer layer of 60's blockwork, followed by the *BIG* cavity that goes all the way down to ground level. There is a cavity like this in each corner. The second photos is the more narrow cavity in front of the chimney, looking all the way down to ground level.

Any thoughts welcome!!


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