Re-Roof and Insulation to Attic Room

[cube3d]

Hello,
A question if anyone can help.
A 2 bed terrace with attic room... in need of a re-roof at some point in the new few years... would it be better to add any insulation whilst the roof is being replaced rather than strip out the lath and plaster and insulate from with-in? is this even possible?

 

[freddiemercurystwin]

It should be possible theoretically but may be tricky getting the insulation down as far as possible as behind lathe and plaster it’s pretty messy/uneven, squirting any expanding foam in to seal up any gaps may blow the lathes off the rafters too I’d guess. But there may be other impracticalities to overcome, I’m no roofer. And you’ll likely be limited on what insulation you can add if the rafters are not very deep as even when a breathable membrane is fitted a 25mm air gap must be retained. Room in the roofs built today would typically have 75mm between the rafters and 50mm underneath. That said it’ll be one helluva mess bringing a lathe and plaster ceiling down. Pros & Cons to each.