Ben1984

Thanks ProDave. I was hoping to hear from someone with personal experience doing this. Do you happen to know what U-value this design achieved? I am not familar with wood fibre boards, is there a particular reason you chose these over PIR?

Thanks. I'm starting to think there is a good reason why warm pitched roofs are fairly uncommon!

It sounds like my original idea of PIR on top of the roof and 150mm mineral wool between the rafters could be problematic then. If I have a VCL immediately behind the plasterboard and then PIR ontop of the roof would this design fundamentally fail? Or are there ways I could make this work?

Thanks, that's useful insight. If I need to use PIR between the joists either way I would be more inclined to stick with a cold roof.

Considering options for roof insulation - having watched Robin Clevett do a warm pitched roof on his channel: I am seriously considering doing the same. He uses 100mm PIR on top of the roof and 100mm PIR between the rafters. I would be looking at 100mm PIR on top and ~150mm mineral wool between the rafters (mainly to avoid having to cut the PIR between the rafters) otherwise I would look to copy everything he did on the video. Does anyone have any experience of doing this? Part of me thinks it would actually be easier to cut the PIR between the rafters and go for a more standard cold roof. I can see pros and cons to both options.

It should have been specified on your planning drawings. Something like; Floor level 122.65. If your architect hasn't made any reference to the FFL on the plans it might be worth confirming with the planners (to avoid any potential problems in the future). You could assume that FFL would be 150mm above ground level (if you are building on a flat site) but this is not always the case.

I'm doing the blockwork (along with everything else) on our build which means there are no installation costs for the dritherml, just my time. I would have to pay a contractor to blow in the EPS beads so in my circumstances I suspect it would add to the overall cost of the structure. I have used cavity batts many times before and I know that I can install them well. I have no personal experience with EPS beads and while they could perform slightly better I perceive them as a risk so i'm reluctant to use them.

375 cavity? I presume you mean 375mm wall with 175mm cavity. The price difference between 32 and 36 dritherm is significant! This thread has convinced me that a wider cavity is the way forward for my project. If internal space really was a priority I might even consider PIR over 32 dritherm.

Interesting - what size cavity? I'm reluctant to use PIR in the cavity, but those blocks look like a good alternative to 'standard' dense concrete blocks.

I think I could hit the same 0.15 u-value by using a better quality mineral wool but it would pretty much double the cost of the insulation. I'm failry certain I would never get that £7000 back! I've re-done my calculations on ubakus.de calculator and they are coming out at 0.17 with 200mm of Knauf 37 - I'm more than happy with that. I've had a quick look at the aerobarrier - seems like a good idea. Have you got a quote for your project yet? I don't think I will need it with a wet plaster finish but i'd be interested to have a rough idea of what it might cost.

Thanks, that's seems much better than the calculator I had been using

I've done some calculations. I'll share them here incase they are of use to someone else (or someone can pick me up on any mistakes). I have about 420m2 of external cavity wall area which will sit on a total of 114m of foundations. I have compared the two following options: Option 1 - 20mm external render/100mm dense block/150mm 32 cavity slab/100mm dense block/20mm internal render+skim finish - which will give me a u-value of around 0.20 (not sure I trust the calculator I've used to reach this figure) Option 2 - 20mm external render/100mm dense block/200mm 37 cavity slab/100mm dense block/20mm internal render+skim finish - which gives me a u-value of around 0.18 Costings Option 1 - £11,500 420m2 of 150mm 32 cavity slab ~ £7000 37m2 of concrete in footings (based on 600mm wide and 550mm deep) ~£4000 1000 wall ties ~ £500 Option 2- £10,240 420m2 of 200mm 37 cavity slab ~ £4200 44m2 of concrete in footings (based on 700mm wide and 550mm deep) ~ £4840 1000 wall ties ~ £1200 (not sure about this figure, more research required) I can use concrete lintels on almost all the openings so there are no additional costs there. Unless I am missing something, it would appear that it is infact cheaper to go for the 200mm cavity option which also provides a better u-value.

Thanks. I will do some research into this. It's likely that I will be doing my own blockwork so there would be no installation cost (just my time) associated with the mineral wool. I would, presumably, have to pay someone else to install the beads so this might be a deciding factor for me.

Possibly! I had, until this morning, been looking at ways of making a 150mm mineral wool cavity work. Based on the responses i've recevied so far I am now inclined to favor a 200mm cavity. I don't know enough about the bonded poly beads to compare them with mineral wool in a 200mm cavity. I will need to look into this.

Thanks, another really helpful response. What software/calculator have you used to produce this? It appears to be vastly superior to what I have been trying to use. I have the space to accomodate a 200mm cavity, I was reluctant to go down this route initally because of the increased costs. Your post suggests that a 200mm cavity could, infact, be more cost efficient that a 150mm. I will need to look into this in more detail.