Hi, I am looking at insulation of an attic bedroom in a victorian (1895) terrace house. Currently there are original slates, with no underfelt laid on rafter. There is a lathe and plaster ceiling under this. No insulation at all. The rafters depth is pretty standard (approx 12cm). The slates are starting to fail and I need a re-roof. And to install insulation.
Ideally I want to overboard the inside (as there is lots of headroom) and the walls too possibly, without removing the plaster to reduce the mess. If possible I would put say 50mm of cellotex style insulation above the original lathe and plaster between the rafter when re-slating, with a breathable felt , leaving an air gap. and then 100mm of celotex below the lath and plaster creating a sandwich. But I think this wouldn’t be recommended? Sandwiching the original lathe and plaster between insulation could cause ‘dew point’ damp? What about breather membranes??? Where would they go?.. or do I need to drop the victorian ceiling and start afresh?
Then I need to think about the attic spaces. Should I top up the fibreglass insulation laid on the floor in them 20 years ago, replace with something else, or fix insulation above to create a ‘warm’ attic? And the upstand walls of the attic room… do I insulate on the inside or outside of the vertical brick walls of the attic bedroom?
Basically, I’m not sure I know what I’m thinking? I don’t want to raise the slates as its a long terrace. I want to save on mess, and if possible do some of the work myself.
As you have a room in your roof it will be much simpler to have a warm roof with the room inside it. If the house hadn’t had a contiguous roof with its neighbours I would have recommended sarking board over the roof timbers but that would cause you problems.
I suggest that you use a breathable “roofing felt” such as Tyvek to allow the timbers between insulation and felt to breathe. Then a ventilation gap before filling the remaining space between timbers with rigid insulation, such as PIR or wood fibre, as your concerns and budget permit. Personally I would use PIR. It has a better U value and is unaffected by damp. Under the timbers fit more PIR or similar to the required total depth. Something like 200mm would be great. Tape all joins. Then use more insulation to board the party walls, unless your neighbours have warm roofs too.
To do the above you will need to remove your lathe ceiling and the tops of the internal walls. Or remove all the wall and enlarge the room.
Thanks very helpful. Enlarging the room by removing the walls not really viable as they only have non load bearing ceiling joists in the front and back attic, so would need new timbers to be able to walk into those areas, and head height already limited. But think you are saying continue the insulation across the whole roof? Rather than down the back of the existing vertical walls and then topping up ceiling insulation. Makes sense.
One elderly neighbour got a grant to insulate their attic (ceiling and walls internally), but think it was badly done by a cowboy firm. Not sure about other neighbours, but there is a separating wall for the staircase up to the top floor, so would need to insulate that party wall (without making the staircase too narrow…). Obvs lots of small technical details to think through, like what to do at eaves still to work out.
I notice that you have a skylight. Also known as “thermal bridge and draught black spot”. Unless you really need the light from it you should roof over and insulate, otherwise get a well insulated Velux window or similar.
Have you considered solar slates. They are more reliable than solar tiles and in keeping with your existing/neighbouring roofs.
Insulate down to the roof plate and then around the plate to ceiling height, remembering to tape the joins. You have insufficient eaves for external insulation, so no need to struggle to construct insulation to the soffit, unless it is easy enough to insulate both sides of the roof plate, which would be an ideal.
I’m considering rear external wall insulation as a next step, so will see about extending the roof rafters when re-slating to allow for that. Also getting rid of the small skylight, whilst retaining/upgrading the larger velux that is there (that provides essential light to the stairwell.) I have consider solar tiles, but concerned of the extra cost (and finding a contractor). Appreciate your advice on all this.