Hi all
I’m just in the process of insulating some large areas of our roof which currently have no / minimal insulation. I’ve attached a pic of my plan. Any tips / advice gratefully received!

Plan is to deepen current 75mm rafters to 150mm and then install 100mm PIR in between, leaving the 50mm air gap behind.
I’ll then overboard with 25mm PIR, a vapour barrier (hopefully just taping the joints between the 25mm boards) and then a finish layer (plasterboard or cladding of some kind).

My plan is to fit 100mm PIR in between the floor joists over the top of the cavity wall so it joins nicely to the 100mm PIR in between the rafters.
We have condensation issues in the room below and in this loft room. I’m hoping that insulating over the top of the cavity wall as far as the rafters will help with this.

I’ve managed to come by a good amount of 100mm PIR - that’s why I’ve chosen that.

Does this look like a good plan?!

How far should I extend the 100mm in between the floor joists into the room? I’m thinking maybe 300mm from the inside of the cavity wall to ensure it’s cosy under there but not waste material?

Will I gain enough to justify overboarding with 50mm rather than 25mm?

Thanks!
Pete

Why are you planning to deepen the rafters? Are the present ones inadequate? If you didn't have to increase them you could put 50 mm between the rafters and 100 mm underneath.

What is the roof space used for?

It's awkward to fasten plasterboard through thick insulation (eg 100mm), and the fat long screws are thermal bypasses.

You could put the 25mm boards between the existing rafters, foamed as Peter described. Then run 100mm horizontal timbers criss-cross across the rafters, 100mm PIR between those, tape/membrane, plasterboard fixed to them w/ conventional screws. The only bridge is where the criss cross timbers touch the rafters, rather than the whole length of the rafters as in the OP plan.

If the existing 75mm rafters are old then they won't be spaced to match metric plasterboard, so space out the horizontal rails to suit the pb you are using. Beware that standard timber sizes can be frustratingly just less than 100mm, pack out the fixing point to the rafters if necessary.

Don't know if building regs are involved, you might need slightly more than 125mm to comply.

(Edit cross posted with Nick - are you fastening plasterboard to those blue horizontal timbers?)

@djh

Thanks to everyone for the replies - so handy! I'm a beginner here and really appreciate the tips and patience.

I was going to deepen the 75mm rafters to allow for the 100mm insulation. I had heard somehwere that it's recommended to have the bulk of the insulation between the rafters, but really happy for that not to be the case and to overboard with the 100mm on the inside.

The space is for clothes and occasional overflow sleeping. It has showed some signs of excess condensation, one of the reasons I'm trying to warm it up.

We also have condensation issues in the room below. I was advised that's most likely caused by cold air blowing over the top of the cavity wall - that's why i'm running the 100mm as far into the rafters over the top of the cavity wall as I can get away with and then joining to the roof insulation.

Cavity was filled 22 years ago - I'm waiting till it gets colder to get a thermal camera on it and see if that shows how well it's holding up.

Thanks!

Excess condensation - why the high humidity? Potential source? Supply ventilation? Mechanical extract?

@Nick Parsons

I was thinking the condensation in the loft area is mainly due to how cold the walls are compared to the rest of the house? Alot of that space has absolutley nothing between the plasterboard and the outside. It is a small room with a good size velux which is opened every day.

The downstairs room - again the walls are cold compared to the rest of the house and this room probably doesn't get heated as much. It's tucked in well away from any sun hitting it with a narrow dark external corridor down that side of the house. Our semi-detached neighbour has the same problem corner (but in reverse). We open the window and dehumidify in there but it's still not great. I was going to try insulating over the top of the cavity wall to reduce the wind blowing over it (and under our floors) and checking the cavity wall insulation for failure.. then apply thermal render to the outside and maybe heat with an infrared panel. Really open to any suggestions and advice though.. thanks!

I was wondering about the 50mm. Does this basically mean leave an air gap?! I've pulled out bits of celotex today that were pushed hard against the felt and had mould on. I have 75mm rafters.. as I understand I would need to bring these out 25mm to get 50mm of PIR in there..

Also, the PIR I'm using is second hand and some is a bit scratched and scuffed. I'm planning to tape over bits where the foil has ripped (and all joins) by way of a vapour barrier. Would anyone be doing different to that?

Thanks!

Posted By: Pete23I have 75mm rafters.

I have just done something similar for the purposes of reinstating an upstairs bathroom (with a vaulted ceiling). Our rafters were 70mm so I went with 40mm PIR between the rafters (foamed in) leaving a 30mm air gap, then 100mm underneath, fixed with roofing battens (because I had a load of used ones). Then Plasterboard & skim.

there's more info in this thread here:

http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=17634&page=1#Item_30

in my case it was about dividing up the boards evenly, and not all the rafter spacings were even, so one or two were tight fits but most were held in place with nails until foamed in, then nails removed, foamed trimmed off flush and all joints taped and across the rafters too. Then the 100mm layer was also taped.

The purlins on this roof were only small, same size as the rafters (you can see it in one of the pictures with electric cables clipped to it) the 40mm layer slid up behind them, the 100mm layer butted up to them, then foamed any gaps, trimmed off, and foil taped over it all.