Hi All,
Roof structure is up using a joiner friend who seems to have done a good job. Man who measured for, and supplied roof came to have a look and said it's a good job.
We now need to decide how and where to insulate new structure before existing roof is stripped and all is felted and reslated, bearing in mind BC want U-value of 0.16 in new bit.
Photos attached:
View from front corner:
Angle showing how high ceiling joists are:
Inside existing roof - I've put a bit of 50mm celotex in against existing plasterboard slope to show depth we've got to play with.
The conflict we've got is: architect says seal the cavity at the head, put in 270mm Rockwool and use eaves ventilation system. Building control say don't seal cavity, put in your egg boxes and insulation and get your roof on (laid back). Since it is a raised tie roof we do not have 270mm of space where the ceiling slopes, just the 147mm depth of the rafter. From the reading we've done, if using breathable mebrane ventilation is not necessary but we can't have a 'well-sealed ceiling' since the old and new buildings share a loft and cavity is not sealed and damp as hell in the existing.
OK, so in new roof, the rafters are 147mm deep so we could put 100mm celotex in between rafters at the slope leaving ventilation gap. Do we "birdsmouth" the insulation so it 'wraps round' the wall plate and butts up to wall insulation then tape it as per wall insulation? To meet BR it would need to be 200mm though!! We could then put rockwool type stuff in remainder of ceiling even though there wouldn't be much space above it when there is 270mm in!! The Celotex alone would be ££££££££££. Oldun suggested insulated plasterboard on ceiling slopes inside but we were thinking maybe 35mm Celotex then foil backed plasterboard, just cos it might be easier to handle.
Do BC just let you get away with a bit in raised tie roofs as they seem to be problematic? He doesn't seem too concerned, which would be fine except we've had 12 years of mildew on bedroom ceilings so we want it right.
Any recommendations for eaves ventilation system? How do you stop Rockwool closing the ventilation gap? Is this the "egg boxes" that BC officer was talking about? What about eaves trays?
My husband is concerned about condensation in the loft. If it has at least 10mm ventilation at the eaves and 5mm at the ridge, plus breathable membrane, should we be concerned? Do we need to close the cavity to reduce chance of condensation? Can we use DPC under the last course of brick that needs to go on? - we would run it from above insulation and wall plate to under last brick course so it acts as a run-off, or does eaves tray do this?
What about the existing roof? We are thinking 50mm celotex in there since rafters are not as deep or will that not be good enough to prevent cold spots?
So many questions....
We need your knowledge and expertise - if you are able to help at all I would be very grateful.
Thanks and regards
Kirsten
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