Wed, 23 Sep 2026 17:37:03 +0100 https://www.greenbuildingforum.co.uk/newforum/ Lussumo Vanilla 1.0.3 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=17973&Focus=303545#Comment_303545 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=17973&Focus=303545#Comment_303545 Wed, 27 Sep 2023 09:31:28 +0100 Pete23 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]]> https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=17973&Focus=303546#Comment_303546 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=17973&Focus=303546#Comment_303546 Wed, 27 Sep 2023 10:03:20 +0100 Peter_in_Hungary
will you gain enough by overboarding by 50 mm? - well only you can decide if the extra is worth it. Do the sums, a calculator is here https://www.ubakus.de/en/r-value-calculator/? if you don't have a favourite. (use the demo version)]]> https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=17973&Focus=303548#Comment_303548 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=17973&Focus=303548#Comment_303548 Wed, 27 Sep 2023 10:19:41 +0100 djh
What is the roof space used for?]]> https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=17973&Focus=303549#Comment_303549 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=17973&Focus=303549#Comment_303549 Wed, 27 Sep 2023 10:45:01 +0100 Nick Parsons https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=17973&Focus=303550#Comment_303550 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=17973&Focus=303550#Comment_303550 Wed, 27 Sep 2023 10:52:51 +0100 WillInAberdeen
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?)]]> https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=17973&Focus=303559#Comment_303559 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=17973&Focus=303559#Comment_303559 Wed, 27 Sep 2023 16:24:57 +0100 GreenPaddy
- 50mm between, foam edges of all boards to rafters and each other
- 100mm over the whole lot ( a few temp fixings to hold them in place
- 22x100 treated off saw timber at 600mm centres , fixed with 150mm screws(can use 25x50, but it's easier to land the plasterboard edges on 100mm). This also gives a void for any cables, plus you get the benefit of the silver lowE face of the insulations due to the sealed void.
- plasterboard over the timbers

Really take care at the wall head, room corners etc, to join up to existing insulation, and leave no gaps - hello Mr Foam Gun - get all the joints, but don't over fill them.

New word of the day - Brandering - Scottish term for fitting those battens over the insulation/ceiling which the plasterboard is then fixed to, often to true up joists or ceiling ties.]]> https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=17973&Focus=303565#Comment_303565 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=17973&Focus=303565#Comment_303565 Wed, 27 Sep 2023 22:21:09 +0100 Pete23
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!]]> https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=17973&Focus=303567#Comment_303567 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=17973&Focus=303567#Comment_303567 Wed, 27 Sep 2023 22:22:14 +0100 Nick Parsons
Yes, 900 x 1800. I am too weak to lift 1200 x 2400!!]]> https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=17973&Focus=303568#Comment_303568 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=17973&Focus=303568#Comment_303568 Wed, 27 Sep 2023 22:24:10 +0100 Nick Parsons https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=17973&Focus=303569#Comment_303569 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=17973&Focus=303569#Comment_303569 Wed, 27 Sep 2023 22:24:42 +0100 Pete23
Thanks for this - great advice. No regs involved - just trying to get the house working better.. I like chopping firewood but I'm also happy to chop less!]]> https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=17973&Focus=303570#Comment_303570 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=17973&Focus=303570#Comment_303570 Wed, 27 Sep 2023 22:34:59 +0100 Pete23
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!]]> https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=17973&Focus=303571#Comment_303571 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=17973&Focus=303571#Comment_303571 Wed, 27 Sep 2023 22:37:30 +0100 Pete23 This all sounds great - am going to do this! Thanks..]]> https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=17973&Focus=303572#Comment_303572 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=17973&Focus=303572#Comment_303572 Wed, 27 Sep 2023 22:41:31 +0100 Pete23
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!]]> https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=17973&Focus=303574#Comment_303574 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=17973&Focus=303574#Comment_303574 Thu, 28 Sep 2023 09:22:48 +0100 GreenPaddy
to ensure you keep the 25mm void to the felt, I usually nail on rips of timber (split the 25x50 into 25mm strips, and nail to each side of the rafter, against the felt. Then you can push the 50mm insul board against those. Foam injected from the front will then tend not to expand backward into the void.

You could add a vapour barrier over the top of the 100mm insul, then battens (branders), but since it's a very low vapour room, low occupancy, I honestly wouldn't bother. Anything that gets through 2 layers of rigid insul has the void to vent away.

I don't think regs (PartC or section 3.15) talk about 50mm void to felt any more, but refer to BS - don't have access to BS. I'd go with 25mm. The potential for vapour to reach this zone is greatly reduced by what you're doing anyway. However, since you're removing what was a dehumidification source from your home, by greatly reducing vapour being vented from the house via the attic, you need to pay more attention to vapour removal in other ways.]]> https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=17973&Focus=303576#Comment_303576 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=17973&Focus=303576#Comment_303576 Thu, 28 Sep 2023 10:10:27 +0100 Dominic Cooney 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.]]> https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=17973&Focus=303591#Comment_303591 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=17973&Focus=303591#Comment_303591 Thu, 28 Sep 2023 22:01:47 +0100 Pete23
Brilliant - thanks. Will keep an eye on humidity for sure.. def looking forward to not having gales blowing through the house!

@ Dominic Cooney

Ah great. Were you cutting the PIR with a tolerance for the foam all round then rather than going for tight and foaming any tiny gaps?]]> https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=17973&Focus=303592#Comment_303592 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=17973&Focus=303592#Comment_303592 Thu, 28 Sep 2023 22:20:09 +0100 Dominic Cooney
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.]]> https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=17973&Focus=303606#Comment_303606 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=17973&Focus=303606#Comment_303606 Fri, 29 Sep 2023 12:52:51 +0100 Pete23
Did you have to go around any purlins? I have very limited space behind.. am wondering whether it's worth just sticking on some 25mm PIR to the back - keeping the air gap but get at least something on the outside of the purlins...

Thanks]]> https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=17973&Focus=303635#Comment_303635 https://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=17973&Focus=303635#Comment_303635 Mon, 02 Oct 2023 10:38:24 +0100 Dominic Cooney