ashthekid Posted March 13, 2021 Share Posted March 13, 2021 In my conversion project there is going to be a large vaunted ceiling housing an open plan kitchen, dining and living room space approx 8m x 10m that I know will likely be expensive to heat and therefore retaining the heat will be very important. Currently the makeup of the roof will be standard slate tiling, then 100mm celotex between rafters, 25mm on top, then battening before having either a single or double layer of acoustic plasterboard. What I would like to know is if upgrading that 25mm to say 75mm or maybe even 100mm would make a huge difference to the thermal insulation of the room. Would essentially be 200mm in total. Also bare in mind that one whole wall inside this room will be double height full glazing which I intend to do in either an upgraded double glazing setup or triple glazing. Link to comment Share on other sites More sharing options...
nod Posted March 13, 2021 Share Posted March 13, 2021 (edited) It’s normal to fix 70 or mil insulated PB to the underside of the rafters You can add a layer of 15 SB board on top of that if you want Edited March 13, 2021 by nod Link to comment Share on other sites More sharing options...
Russell griffiths Posted March 14, 2021 Share Posted March 14, 2021 I have 70mm under mine, 25 is inadequate in my opinion. Dont forget vapour control layer, and don’t penetrate it with downlighting. 1 Link to comment Share on other sites More sharing options...
ProDave Posted March 14, 2021 Share Posted March 14, 2021 Have you thought about making this a warm roof? For that you put a layer of insulation on top of the rafters, i.e. between the rafters and the battens then tiles. Then you can fill the entire thickness of the rafters with insulation.. Much easier to detail, much easier to get more insulation in, much easier to detail the air tightness. Link to comment Share on other sites More sharing options...
PeterW Posted March 14, 2021 Share Posted March 14, 2021 23 hours ago, ashthekid said: What I would like to know is if upgrading that 25mm to say 75mm or maybe even 100mm would make a huge difference to the thermal insulation of the room. Would essentially be 200mm in total. Also bare in mind that one whole wall inside this room will be double height full glazing which I intend to do in either an upgraded double glazing setup or triple glazing. No point in spending the money. 125mm gives you bit better than building regs but the wall of glass completely undoes any benefit. Even using glazing with 0.8W/mK vs standard 1.0W/mK you are losing 10-12 times the heat through the glass than you are through the roof. To offset that wall you would need around 260mm of PIR in roof and walls from a quick fag packet calculation (purely elemental) What has the architect designed to meet building regs ..?? Link to comment Share on other sites More sharing options...
ashthekid Posted March 14, 2021 Author Share Posted March 14, 2021 260? Is that even possible? Architect has planned for 125-140mm total in the roof(100+25 or 40). Link to comment Share on other sites More sharing options...
ashthekid Posted March 14, 2021 Author Share Posted March 14, 2021 Link to comment Share on other sites More sharing options...
PeterW Posted March 14, 2021 Share Posted March 14, 2021 5 minutes ago, ashthekid said: 260? Is that even possible? Architect has planned for 125-140mm total in the roof(100+25 or 40). So bare regs ..? But not taken into consideration you needing to offset the glazing ..? And yes, 260 can be done in layers - how deep are the rafters..?? Link to comment Share on other sites More sharing options...
JamesP Posted March 14, 2021 Share Posted March 14, 2021 (edited) I have vaulted ceilings, a warm roof with 305mm of insulation, it was not difficult to build. 200mm Rockwool in the rafters, 70mm Rockwool above the rafters then 35mm Pavatherm T&G. Wall, floor,roof buildup.Sept 2017..pdf Edited March 14, 2021 by JamesP Link to comment Share on other sites More sharing options...
Temp Posted March 15, 2021 Share Posted March 15, 2021 (edited) On 13/03/2021 at 09:35, ashthekid said: What I would like to know is if upgrading that 25mm to say 75mm or maybe even 100mm would make a huge difference to the thermal insulation of the room. Would essentially be 200mm in total. Some care is needed because insulation on the inside effectively moves the rafters nearer to the cold side reducing their temperature and making interstitial condensation more likely to occur. Needs a good vapour barrier on the inside and ventilation on the outside. If there is an existing roof membrane? What is it and is it vapour permeable? If these issues can be addressed than yes I would go for it. A warm roof with the insulation above the rafters has some advantages including fewer gaps between sheets of insulation and the ability to leave rafters exposed on the inside if desired. Edited March 15, 2021 by Temp Link to comment Share on other sites More sharing options...
ashthekid Posted March 15, 2021 Author Share Posted March 15, 2021 There is a breathable felt membrane going directly into the rafters from the outside with the battening and slate tiles going on top of that. Rafter size is 150mm and architect spec is to put 100mm insulation celotex in between there. Is there any benefit in having a large ventilation gap of 50mm? Would adding another vapour barrier be suitable on the inside of the rafters but another ventilation gap then be required? From what I’ve gathered, putting 25mm layer straight under the rafters and then another 25-50mm layer directly over that in the other direction, both taped and sealed of course create a nice airtight 50-75mm total on top of the 100mm between the rafters so essentially a max of 150-175mm. With a 150mm rafter I’m assuming we could get 125mm in there so we’re almost at 200mm insulation total now. Can I do more easily? I’m planning on having 4 or 6 sunken spotlights within this ceiling, I’m assuming that’s a bad idea and I should avoid that for thermal leakages? Link to comment Share on other sites More sharing options...
Russell griffiths Posted March 15, 2021 Share Posted March 15, 2021 Definitely no sunken spot lights, there is no need with the amount of flat led lights available. 25-40mm service batten and you will get a multitude of lights to fit in that. Link to comment Share on other sites More sharing options...
ashthekid Posted March 16, 2021 Author Share Posted March 16, 2021 Ok I'll avoid sunken lights then. So going back to the insulation, what would a Passivhaus standard be for roof insulation thickness? Link to comment Share on other sites More sharing options...
Bitpipe Posted March 16, 2021 Share Posted March 16, 2021 1 hour ago, ashthekid said: Ok I'll avoid sunken lights then. So going back to the insulation, what would a Passivhaus standard be for roof insulation thickness? Thickness is not specified, but overall u value as it depends on the characteristics of the various materials used in the build up. Passive standard is u = 0.10 to 0.15 W/(m²K). To calculate u, add up R for each layer and then 1/R = u. https://www.uvalue-calculator.co.uk/calculator/ Link to comment Share on other sites More sharing options...
Gone West Posted March 16, 2021 Share Posted March 16, 2021 5 hours ago, Bitpipe said: Passive standard is u = 0.10 to 0.15 W/(m²K). Is that right? I didn't know there was a minimum U factor of 0.1W/(m²K) for roof construction in a PH. Link to comment Share on other sites More sharing options...
ashthekid Posted March 18, 2021 Author Share Posted March 18, 2021 Thank you, that is a useful link. Is there anyone I can speak to get a suggested upgrade on the layers from the current spec that is going in? I spoke to the contractor today who said if I upgrade the 50mm any more it's starting to get difficult to fix the larger 75mm rigid layer to the rafters and then 25mm battens before the final 15mm soundbloc plasterboard. he said he didn't think the extra 25mm would make a substantial difference to the U Value. What if I added the extra 25mm and then doubled the plasterboard later with a layer of thermal plasterboard as well as the soundbloc? Link to comment Share on other sites More sharing options...
ashthekid Posted May 20, 2021 Author Share Posted May 20, 2021 So to give an update, I’ve decided to upgrade the PIR underneath the rafters so now I have the 100mm in between the rafters, then 50mm on top and then another 50mm in the opposite direction to creat a better airtightness(I think) with foil taped joins. Do you think that 200mm total in the roof layers will be good enough to help with my thermal insulation protection? Link to comment Share on other sites More sharing options...
A_L Posted May 21, 2021 Share Posted May 21, 2021 9 hours ago, ashthekid said: Do you think that 200mm total in the roof layers will be good enough to help with my thermal insulation protection? That depends on your objective. The U will be 0.13, very good but not a 'no heat' value. Link to comment Share on other sites More sharing options...
ashthekid Posted May 21, 2021 Author Share Posted May 21, 2021 What do you mean by “no heat” value? Link to comment Share on other sites More sharing options...
A_L Posted May 21, 2021 Share Posted May 21, 2021 (edited) 1 hour ago, ashthekid said: What do you mean by “no heat” value? A house that at most only requires intermittent heating even in Dec/Jan. I am concerned that it will be subject to overheating in summer because of the low decrement delay of the structure. What is the compass bearing of the double height glass wall you mentioned? Edited May 21, 2021 by A_L minor typo Link to comment Share on other sites More sharing options...
Temp Posted May 21, 2021 Share Posted May 21, 2021 I think properly ventilating the 50mm void will help in summer. I'm surprised how cool my outbuilding is and I think its down to the well ventilated void behind the timber cladding. Its frequently cooler in there than in some bedrooms in our roof that have far more insulation. So I would look at decent sized eaves and ridge vents to get convection going. Link to comment Share on other sites More sharing options...
gc100 Posted May 22, 2021 Share Posted May 22, 2021 My house is all vaulted ceiling’s throughout and the roof buildup is about the same as yours thickness wise (except I have a zinc roof so extra void above the rafters). I have very large extensive glazing (tripled glazed). I paid great attention to the airtight membrane . The heating is hardly ever on. If you are careful I don’t think the extra cost of reducing the u value even lower in roof makes much difference compared to the heat lost through windows and not being air tight Link to comment Share on other sites More sharing options...
ashthekid Posted May 22, 2021 Author Share Posted May 22, 2021 @A_L the glazing wall is pretty much East facing. Do you think I have a particular glazing style? i.e. triple glazed? Link to comment Share on other sites More sharing options...
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