Insulating an old loft room

[StevePvx]

Victorian house, Attic room (original as far as we can tell) plaster is falling off the attic room.

Room has walls up to about 1.8m then follows the slope of the roof with a section in the middle that is flat. Room runs from party wall to gable end.

Floor has been insulated with rockwool and hardboard loft flooring has been laid.

The eaves are accessible through hatch doors, no insulation behind the existing plasterboard walls. Hardboard floors are in place in the eaves as well again with rockwool insulation underneath/protruding everywhere. The eaves are only used to store old boxes, suit cases and such.

This kind of shape
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Walls of the room have been plaster boarded and are in decent condition. Plaster is falling off the ceiling (old lath and plaster) revealing 3x2 rafters and the felt membrane under the slate tiles. There's a couple of Velux fitted (north facing) so it has decent daylight.

During the winter the room gets stupid cold, we are planning on getting a radiator fitted which will help with that. But currently the room is unusable in the heat. So I'm hoping the insulation will help with that.

As I am re-plastering the ceiling and sloping sections I was intending to put some minimal insulation into the rafters (25mm polystyrene ?) then some insulated plasterboard (50mm ?) onto the sloping roof and flat roof section (can probably increase the polystyrene insulation on the flat "ceiling" section) I was also going to fit some insulation (75mm??) into the open stud behind the vertical plasterboard walls. Might have to staple some foil blanket to the hatch doors

Not sure what kind of insulation to fit and or if I am fitting stuff that is thick enough. So any advice would be appreciated? Am just at the stripping out stage, got to rip all the old plaster off this weekend or next.

 

[oldbutnotdead]

Sounds like a nice project. The tricky (expensive) bit is the limited headroom sloped section. Polystyrene is a less efficient insulator than PIR, to achieve compliance with current building regs you are aiming for the equivalent of 300mm of rockwool in the roof elements- this equates to about 150mm of PIR or a bit more of polystyrene (200mm I think but I hate polystyrene so never bothered with doing the sums on it).
Rockwool by volume is much less than half the price of PIR by volume, so where you have the space (in the eaves voids) you could use that (netted to the walls, don't compress it too much). If space is more important than budget then PIR will give you the best thermal performance.
With the sloped bit of the ceiling you may have to compromise a bit on thickness to keep the space usable- do keep the 50mm gap between underside of felt and top of insulation.

 

[StevePvx]

Sounds like PIR is a better way to go. I can afford to use those all round. 150mm of PIR will be a challenge. I could double up on the walls and probably get that but as you say the sloping roof sections at any more thickness than I’m currently planning will lose too much headroom. I will try and go for decent insulated plasterboard and see if I can get some 25mm PIR in between the joists in those areas

 

[StevePvx]

I did wonder if any of the hi tech foil blanket stuff might work better where I’ve got limited depth to put it in.

 

[oldbutnotdead]

The foils might work where you are space limited but check the installation guides carefully, most of them require a decent size static air gap either side of the foil to achieve the quoted performance.