TFnovice Posted February 28, 2019 Share Posted February 28, 2019 I'm wondering whether anyone has real life experience of this... We are building a 1.5 storey house with rooms in the roof. These can be prone to overheating in summer and so I am planning to mitigate this as much as possible by keeping temperatures down during the day. I'm considering an in roof PV system on a small south facing roof which has bedrooms below. I've been quoted for a 2.7kwp Clearline system (12 panels). These are pretty low profile which is one of my criteria but I have read that the panels reach pretty high temperatures and need to be vented to perform properly (which seems to be at odds with designing them to be low profile). I am wondering whether there is a risk they will transfer some of their heat to the rooms below? Does anyone have a similar installation who could comment or other real world experience? Link to comment Share on other sites More sharing options...
Jeremy Harris Posted February 28, 2019 Share Posted February 28, 2019 In roof PV should actually reduces the amount of heat getting to the inside, when compared to just having any other roof covering, as around 16% or so of the energy hitting the panels gets taken away as electricity. The venting underneath is integrated into the in-roof system, but does work better if the roof is counter battened over any sarking and then battened, as this provide a nice clear path for air to flow up behind the panels. It also needs a ridge vents and eaves vent, but these are usually needed anyway. Link to comment Share on other sites More sharing options...
ProDave Posted February 28, 2019 Share Posted February 28, 2019 It is the job of the roof insulation to keep heat out as well as in. The amount and choice of type will sort out the details you need. Plain old roof tiles will also get very hot on a hot sunny summers day and will present just the same issue if you have got the roof insulation wrong. Link to comment Share on other sites More sharing options...
ToughButterCup Posted February 28, 2019 Share Posted February 28, 2019 43 minutes ago, TFnovice said: [...] I am wondering whether there is a risk they will transfer some of their heat to the rooms below? Does anyone have a similar installation who could comment or other real world experience? No sensible heat transfer : 280mm of in and under roof insulation. I'd have been well piddled off if there were any.... Link to comment Share on other sites More sharing options...
Jeremy Harris Posted February 28, 2019 Share Posted February 28, 2019 4 minutes ago, recoveringacademic said: No sensible heat transfer : 280mm of in and under roof insulation. I'd have been well piddled off if there were any.... Depends on time, though, hence the importance of decrement delay. If the insulation has a good U value, but a short decrement delay, say 3 hours, then heat is going to come through the roof after 3 hours. On the other hand, if the roof insulation has the same U value but a decrement delay of 8 hours then heat isn't going to come through the roof until after 8 hours. Makes a big difference on a very hot day. Link to comment Share on other sites More sharing options...
TFnovice Posted February 28, 2019 Author Share Posted February 28, 2019 Thanks all - very interesting and useful. 1 hour ago, ProDave said: It is the job of the roof insulation to keep heat out as well as in. The amount and choice of type will sort out the details you need. I'm planning on using 160mm of Kingspan Kooltherm K107 insulation between the rafters under Spanish slates. 31 minutes ago, JSHarris said: If the insulation has a good U value, but a short decrement delay, say 3 hours, then heat is going to come through the roof after 3 hours. I think the proposed insulation will offer a U value of about 0.15. No idea of the decrement delay or even how to find this out! Link to comment Share on other sites More sharing options...
Jeremy Harris Posted February 28, 2019 Share Posted February 28, 2019 1 hour ago, TFnovice said: I'm planning on using 160mm of Kingspan Kooltherm K107 insulation between the rafters under Spanish slates. I think the proposed insulation will offer a U value of about 0.15. No idea of the decrement delay or even how to find this out! The decrement delay will be short, perhaps 2 to 3 hours, with PIR foam, as it's heat capacity is pretty low. 1 Link to comment Share on other sites More sharing options...
PeterW Posted February 28, 2019 Share Posted February 28, 2019 Kooltherm is also flippin expensive ..! 160mm is best in 2 layers but there is also nothing stopping your thermal bridging of the rafters so why not improve things by going 140mm between and then 25mm across all the rafters ..? Could also consider 180mm Frametherm with a 55mm PIR over the rafters too as an alternative. Link to comment Share on other sites More sharing options...
ProDave Posted February 28, 2019 Share Posted February 28, 2019 26 minutes ago, PeterW said: Kooltherm is also flippin expensive ..! 160mm is best in 2 layers but there is also nothing stopping your thermal bridging of the rafters so why not improve things by going 140mm between and then 25mm across all the rafters ..? Could also consider 180mm Frametherm with a 55mm PIR over the rafters too as an alternative. I have 200mm of frametherm and 100mm of wood fibre and it has a decrement delay of something like 13 hours. I don't get any overheating issues. Link to comment Share on other sites More sharing options...
TFnovice Posted February 28, 2019 Author Share Posted February 28, 2019 34 minutes ago, PeterW said: Kooltherm is also flippin expensive ..! 160mm is best in 2 layers but there is also nothing stopping your thermal bridging of the rafters so why not improve things by going 140mm between and then 25mm across all the rafters ..? Could also consider 180mm Frametherm with a 55mm PIR over the rafters too as an alternative. OK, so would you put the 25mm over or under the rafters? Sorry for the dumb question. 8 minutes ago, ProDave said: I have 200mm of frametherm and 100mm of wood fibre and it has a decrement delay of something like 13 hours. I don't get any overheating issues. One of the reasons for having a 1.5 storey house is the height restriction, so increasing the height of the roof by 100mm might be restrictive. Alternatively reducing the internal height could cause issues in some areas eg showers in bathrooms where head height in certain places is particularly material. Link to comment Share on other sites More sharing options...
ProDave Posted February 28, 2019 Share Posted February 28, 2019 12 minutes ago, TFnovice said: One of the reasons for having a 1.5 storey house is the height restriction, so increasing the height of the roof by 100mm might be restrictive. Alternatively reducing the internal height could cause issues in some areas eg showers in bathrooms where head height in certain places is particularly material. Are the planners really going to refuse for being 100mm too high? Can you lower the floor level so you are starting from a lower point? Link to comment Share on other sites More sharing options...
Jeremy Harris Posted March 1, 2019 Share Posted March 1, 2019 10 hours ago, ProDave said: Are the planners really going to refuse for being 100mm too high? Can you lower the floor level so you are starting from a lower point? I had this discussion with the planning officer. The "rule of thumb" they seem to use is +/- 100mm on the nominal dimensions is considered to be a normal build tolerance. Link to comment Share on other sites More sharing options...
jack Posted March 1, 2019 Share Posted March 1, 2019 I've heard the 100mm rule too. Also, unless you actually have dimensions on your planning drawings shown relative to a specific datum, the only way of telling how tall a structure should be is by measuring off the drawing (even a thickish line is probably 10s of mm thick) and visually estimating relative to surrounding properties on whatever street scene was in the planning app. Link to comment Share on other sites More sharing options...
TFnovice Posted March 1, 2019 Author Share Posted March 1, 2019 The planning has already been granted. We pushed the height to get 1.5 storeys so are already at the limit. I'm a bit concerned about future flood risk (down the 'hill' but not far) and feel it would be foolish to be thinking in 10 years time under 100mm of water that I could have avoided it if only I hadn't lowered the floor level. I'm aware of the thickness of a line argument and we might well get away with it, but if you purposefully design in your 100mm tolerance then you no longer have a build tolerance! Can someone confirm whether it is preferable to put the 100mm (or whatever depth) above the rafters or below? TIA Link to comment Share on other sites More sharing options...
joe90 Posted March 1, 2019 Share Posted March 1, 2019 11 hours ago, TFnovice said: increasing the height of the roof by 100mm might be restrictive. Our planning specified total height (they have a thing about roof heights round here) and when we altered the roof design slightly found ourselves about 150mm above where we wanted, builder pointed out reducing the roof slope by a couple of degrees got over this, also they pointed out the planners had no way of measuring the total finished height anyway (no gables) and he had never been questioned about finished roof height in all his years building around here ? 1 Link to comment Share on other sites More sharing options...
ProDave Posted March 1, 2019 Share Posted March 1, 2019 2 hours ago, TFnovice said: The planning has already been granted. We pushed the height to get 1.5 storeys so are already at the limit. I'm a bit concerned about future flood risk (down the 'hill' but not far) and feel it would be foolish to be thinking in 10 years time under 100mm of water that I could have avoided it if only I hadn't lowered the floor level. I'm aware of the thickness of a line argument and we might well get away with it, but if you purposefully design in your 100mm tolerance then you no longer have a build tolerance! Can someone confirm whether it is preferable to put the 100mm (or whatever depth) above the rafters or below? TIA Someone mentioned adding extra insulation and getting walls an extra 100mm thick through as a Non Material Amendment. So now you have planning, try a NMA stating you are keeping the internal size of the house the same and floor levels the same but for reasons of energy efficiency you are making the roof structure 100mm thicker to add extra insulation so that will make the ridge line 100mm higher. If they pass that then you are sorted. Link to comment Share on other sites More sharing options...
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