ianfish Posted January 22, 2021 Share Posted January 22, 2021 We had some good news this week, our planning which was submitted late Summer 20 has finally been granted!! I have gone over many elements of the build time and time again. I have done many elements of a build over the years but this will be the most comprehensive. We are adding a kitchen - dinner - living area to the back of the house, our initial design had to revised for the planning officer. The main part of the extension has a flat with fall roof. My thoughts have been for a warm roof going inside out 3mm Plaster 12.5mm Plaster Board i Joist (sent for quotes waiting firm details) thoughts are 600mm centres Trying also to get a comparison between metal frame and wood I beams 18 mm OSB Vapour Barrier Insulation 18mm OSB EDPM My thoughts having read a few places about insulation is to go for as much as is practical with a combination of a rockwool layer and a foam the foam I hope would have the fall in it negating the need for firrings. hoping for a U value of .10. I am hoping the mix will also go toward the warmest best sound proof I can realistically achieve, without going totally passive. I would be interested to hear to be from people about which VB they have used and why? there seem to be the so many onet the mark but I am getting a bit bogged down in trying to make any reasonable comparison. I get its purpose but is there any merit in using any particular . Again OSB wise a lot of illustrations show OSB on both inner and outer layers as it were, Then I went to SIP panels which do not seem to have a VP? am I missing something? They also all seem to come with a thinner layer of PLY/OSB. I understand the strength is not too involved in a material thickness but I can see the merits of using 18mm as it can be screwed well and glued. My thoughts on securing stuff would also be a mix of foam and screw to the appropriate length to get into the joist lower deck. Looking to start late March so a roof would be May June plenty of time to worry and overthink stuff. Ian Link to comment Share on other sites More sharing options...
Moonshine Posted January 22, 2021 Share Posted January 22, 2021 16 minutes ago, ianfish said: insulation is to go for as much as is practical with a combination of a rockwool layer and a foam I don't quite know what you mean buy rockwool and foam? I am looking at a flat roof myself and tbh i think that you are going to have to use furrings or some method to get the required 1:40 fall of the roof. I have been looking at the firestone EPDM product and there are two ways of fixing it, mechanically or adhesive, both of these methods you lay the EPDM straight onto PIR insulation without another layer of OSB. There is something very important about the EPDM with the PIR insulation you specify and the knock on effect on your U-value. If you choose to adhesive the EPDM straight onto the insulation you have to use tissue faced PIR, where as if its mechanically fixed you can use foil faced PIR, tissue faced PIR has a conductivity (W/mK) of around 0.024 (e.g. Thermaroof TR27) and foil faced PIR has a conductivity (W/mK) of around 0.022. For a u-value of 0.1 you will need 200mm of foil faced PIR (0.022) when mechanically fixing, and 220mm of tissue faced PIR (0.024) when sticking the EPDM down Link to comment Share on other sites More sharing options...
ianfish Posted January 22, 2021 Author Share Posted January 22, 2021 6 minutes ago, Moonshine said: I don't quite know what you mean buy rockwool and foam? I am looking at a flat roof myself and tbh i think that you are going to have to use furrings or some method to get the required 1:40 fall of the roof. The creamy insulation board is a blown pumped foam in its liquid state. Not an official name but thats what I meant I have looked at a few products with the fall built into them rather than having to cut firrings and having another layer to secure and attach, the less I have to attach I feel can only be a better end job. There are a few insulation products with built in fall. I have come across a rockwool product and a sheet manufacturer in Chesterfield which I have not recorded the full website for! Link to comment Share on other sites More sharing options...
Temp Posted January 22, 2021 Share Posted January 22, 2021 (edited) Spray foam has to be sprayed onto something. The exposed surface isn't flat. So it would be difficult to use on a warm roof. If you spray it on the underside of the top layer of OSB that OSB would need supporting. If you spray it on the top side of the lower OSB then the surface would be uneven and the top layer of OSB might not be supported evenly. I don't think Rockwool can be used for load bearing so again the top layer of OSB would need support that penetrates the insulation layer. The best solution I reckon is to: Install the joists. Put tapered firring strips on top to get the fall. OSB Vapour barrier PIR Insulation boards Squirty foam or tape any gaps in the PIR (shouldn't be any but if there are). OSB (Top and edges) EPDM Edited January 22, 2021 by Temp Link to comment Share on other sites More sharing options...
Moonshine Posted January 22, 2021 Share Posted January 22, 2021 22 minutes ago, ianfish said: rather than having to cut firrings FYI you can buy them, and i am sure a local sawmill could knock them up https://www.roofingsuperstore.co.uk/product/tapered-firring-for-flat-roofs.html 1 Link to comment Share on other sites More sharing options...
ianfish Posted January 22, 2021 Author Share Posted January 22, 2021 https://www.xtratherm.com/ Not saying you cant! Anout firrings but im thinking if that can be added into a product its one thing less to do. Link to comment Share on other sites More sharing options...
Temp Posted January 22, 2021 Share Posted January 22, 2021 (edited) My guess is firrings and fixing works out cheaper than tapered insulation or tapered i-joists or similar. Edited January 22, 2021 by Temp 1 Link to comment Share on other sites More sharing options...
ianfish Posted January 22, 2021 Author Share Posted January 22, 2021 1 hour ago, Temp said: Spray foam has to be sprayed onto something. The exposed surface isn't flat. So it would be difficult to use on a warm roof. If you spray it on the underside of the top layer of OSB that OSB would need supporting. 1 hour ago, Temp said: I meant how the board is manufactured over using as a foam! Link to comment Share on other sites More sharing options...
ianfish Posted January 22, 2021 Author Share Posted January 22, 2021 1 hour ago, Temp said: Spray foam has to be sprayed onto something. The exposed surface isn't flat. So it would be difficult to use on a warm roof. If you spray it on the underside of the top layer of OSB that OSB would need supporting. If you spray it on the top side of the lower OSB then the surface would be uneven and the top layer of OSB might not be supported evenly. I don't think Rockwool can be used for load bearing so again the top layer of OSB would need support that penetrates the insulation layer. The best solution I reckon is to: Install the joists. Put tapered firring strips on top to get the fall. OSB Vapour barrier PIR Insulation boards Squirty foam or tape any gaps in the PIR (shouldn't be any but if there are). OSB (Top and edges) EPDM Will digest this later thanks Link to comment Share on other sites More sharing options...
Temp Posted January 22, 2021 Share Posted January 22, 2021 3 minutes ago, ianfish said: I meant how the board is manufactured over using as a foam! Oh ok. So you can ignore my first paragaph ? 1 Link to comment Share on other sites More sharing options...
Temp Posted January 22, 2021 Share Posted January 22, 2021 Various companies make PIR/PUR foil covered sheet insulation - Celotex, Kingspan etc Its thermal properties are better than EPS. Link to comment Share on other sites More sharing options...
ianfish Posted January 22, 2021 Author Share Posted January 22, 2021 https://www.rockwool.co.uk/product-overview/flat-roof-solutions/hardrock-multi-fix-dd-tapered-en-gb/?selectedCat=downloads Link to comment Share on other sites More sharing options...
Moonshine Posted January 22, 2021 Share Posted January 22, 2021 27 minutes ago, ianfish said: https://www.rockwool.co.uk/product-overview/flat-roof-solutions/hardrock-multi-fix-dd-tapered-en-gb/?selectedCat=downloads if you want to use rockwool you are going to need circa 350mm thickness to get U-value of 0.1, as it has a thermal conductivity of 0.039 W/mK. Link to comment Share on other sites More sharing options...
ianfish Posted January 22, 2021 Author Share Posted January 22, 2021 5 hours ago, Moonshine said: if you want to use rockwool you are going to need circa 350mm thickness to get U-value of 0.1, as it has a thermal conductivity of 0.039 W/mK. its in the mix not 100% sold on it yet.. Link to comment Share on other sites More sharing options...
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