Lath and plaster ceilings: insulation and downlights

13 Feb 2008
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United Kingdom
We recently moved into a 1930's house. On the 2nd floor are bedrooms built in to the roof space. Apart from a general makeover, I would like to add as much insulation as I reasonably can, and fit downlights.

The rooms have a horizontal ceiling in the centre that then meets a sloping section (parallel to the roof tiles). The sloping section then meets a short vertical section, leaving a crawl space in the roof area. All the ceilings and (internal) walls are lath and plaster.

Question 1 is whether it is worth taking down the lath and plaster and replacing with plasterboard so that while doing this I can get good access to insulate the sloping sections and also the horizontal section of ceiling. The alternative is to try to get kingspan or similar up the sloping section (roof timbers are 4inch thick, so I could only get 25-30mm insulation boards in there and still leave the 50mm ventilation gap; access quite tricky and the plaster sticks through the laths obviously making the poking more tricky); the horizontal section would require me to cut an access hole and work from there if I keep the lath. Replacing the lath / plaster would be a bit of a job I expect, but allow me to do a decent job with insulation (and make downlights much easier to install)? The lath and plaster otherwise seems in good condition though...

Question 2 - I am topping up the insulation in the crawl space around the edge of the rooms. Should I leave a ventilation gap where the horizontal timbers run under the top floor bedrooms? Also, I am using loose fill insulation - where the timbers go beyond the crawl space and under the bedroom floor, what is the best way to stop the loose fill simply running away - a piece of wood cut to size, or is there something made just for the job??

Many thanks for any advice - this post will be the first of many as I work my way through the house. If I don't post for more than a week, please send someone to help me back out of the crawl space...
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i would not hesitate to lose the ceilings, thus facilitating an easier insulation and electrical installation.

why are you using loose fill. it is not particularly user friendly. a ventilation gap will only be necessary whereby you are admitting air from outside via soffit or fascia vents.
Thanks for that. Having taken off a lot of the wallpaper now, there are a few good cracks in the plaster so that is a bit more reason to take it down.

Loose fill seemed like a good idea at the time - access into the crawl space is tight, and I felt that dragging bags along would be easier than strips of woolstuff. The gaps between the beams don't seem a standard size either (14 1/2 inch centre to centre). Not so sure now, since some areas I will need access to again for the lighting on the next floor down. It all seems to be leading to a 'why didn't we do it all before we moved in' situation, but I'll get through somehow or other...

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