DH202020 Posted May 23, 2017 Share Posted May 23, 2017 I need to cool down my roof space... Number one: it's warming my MHVR in-coming air (set on summer by pass) today at 2am the incoming air temperature at plenums was 23.3℃!! (Outside air temperature 12℃) I have blown in insulation to cold roof space where all MHVR ducting pipes are covered by at least 300mm. (Incoming have added wrap insulation, those to plenums are just buried). I have no natural air flow in roof space, just the usual good quality vapor control barrier (MBC constructed) What are my options? (I'm already thinking of turning off my MHVR and opening windows!!) Help Link to comment Share on other sites More sharing options...
Triassic Posted May 23, 2017 Share Posted May 23, 2017 Could you extract the hot air and replace it with cooler air? Link to comment Share on other sites More sharing options...
Crofter Posted May 23, 2017 Share Posted May 23, 2017 Sorry for my ignorance but where is your vapour barrier- in the roof, or the ceiling? Or one each? Link to comment Share on other sites More sharing options...
DH202020 Posted May 23, 2017 Author Share Posted May 23, 2017 45 minutes ago, Crofter said: Sorry for my ignorance but where is your vapour barrier- in the roof, or the ceiling? Or one each? Both...airtight build I guess getting shot of the extreme heat build up is priority... Does anyone else suffer with this problem? Has it effected your MHVR operation? Link to comment Share on other sites More sharing options...
Crofter Posted May 23, 2017 Share Posted May 23, 2017 Just wondering what the effect would be if you simply passively ventilated the cold space. It should really be either part of the heated envelope, or not. Maybe fit some kind of ventilation that can be opened or shut depending on needs. In my own house, my rather low tech solution is that I have a skylight that I can open or close. Link to comment Share on other sites More sharing options...
ToughButterCup Posted May 23, 2017 Share Posted May 23, 2017 4 hours ago, DH202020 said: I need to cool down my roof space... [...] What are my options? (I'm already thinking of turning off my MHVR and opening windows!!) Help As a first step, what's wrong with that? Then find out why the summer by-pass appears not to be working. Link to comment Share on other sites More sharing options...
ProDave Posted May 23, 2017 Share Posted May 23, 2017 How does this work if you have an air tight layer on the outside of the cold roof? Surely in winter that will be a recipe for condensation? Don't building regs demand ventilation of a cold roof, meaning your air tight layer must be at ceiling level and only there? Link to comment Share on other sites More sharing options...
DH202020 Posted May 23, 2017 Author Share Posted May 23, 2017 55 minutes ago, ProDave said: How does this work if you have an air tight layer on the outside of the cold roof? Surely in winter that will be a recipe for condensation? Don't building regs demand ventilation of a cold roof, meaning your air tight layer must be at ceiling level and only there? Air tight ceiling, vapour cold roof (sorry wasn't clear on that) Link to comment Share on other sites More sharing options...
DH202020 Posted May 23, 2017 Author Share Posted May 23, 2017 1 hour ago, recoveringacademic said: As a first step, what's wrong with that? Then find out why the summer by-pass appears not to be working. That's exactly what I'm doing...however the summer by pass is working fine (opening vents in MHVR and closing when I chang over)....and that's just it... The pipes are warming up, thus causing warm air to arrive at plenums. Its only happening with long spells of sunshine P.s I ran the evening purge for 6hrs, didn't lower the incoming air temperature by even .5° Link to comment Share on other sites More sharing options...
ToughButterCup Posted May 23, 2017 Share Posted May 23, 2017 Sounds like a job for @JSHarris level of analytical skill. He might not be feeling well today..... Link to comment Share on other sites More sharing options...
AliG Posted May 23, 2017 Share Posted May 23, 2017 (edited) I am assuming this, but is the inlet pipe to the MVHR inside the roof unusually long so that it has time to heat up the incoming air? Edited May 23, 2017 by AliG Link to comment Share on other sites More sharing options...
Crofter Posted May 23, 2017 Share Posted May 23, 2017 (edited) And is that inlet duct ventilated insulated? Edited May 23, 2017 by Crofter Ventilation, insulation, it's all the same innit Link to comment Share on other sites More sharing options...
Bitpipe Posted May 23, 2017 Share Posted May 23, 2017 Due to the low airflow rates (even in purge mode) and low heat carrying capacity of air, MVHR has pretty limited ability to change the temp of the internal air, its best viewed as fine tuning. For me, MVHR's main job is to deliver fresh air and extract stale/humid air while not allowing excess heat to escape in winter and (hopefully) avoiding unwanted heat being introduced in summer - although without any active cooling, not sure how effective summer bypass really is (as it's just turning off the heat exchanger when outside air is cooler than internal in summer). We have a couple of large velux in the roof over an atrium and these get opened regularly in the evening once the outdoor temp has dropped and this is proving very effective at cooling the house. Windows and doors are kept closed daytime to prevent warm air getting in, blinds are also adjusted to minimise solar gain to east and south. Link to comment Share on other sites More sharing options...
Jeremy Harris Posted May 23, 2017 Share Posted May 23, 2017 I'm guessing here that the loft space can get very hot in warm weather, because the roof isn't insulated and the loft isn't ventilated, and that also means it can get very cold in winter, What was the logic behind putting the VCL outside the ceiling insulation? It sounds a bit risky to me, as that means that vapour can enter the loft space from the house and then be subject to wide changes in temperature. Was a condensation risk analysis undertaken to see if the loft space could get below the local dew point? Normally, I would expect the VCL to be inside the insulation, with the "cold roof" section well ventilated. If the loft space is well ventilated, then, with all the ducts and the MVHR unit well-insulated, I'd have thought that this problem probably wouldn't occur. I strongly suspect that the loft space is getting very hot indeed, and that may well be the root cause. I was working in the loft space above my garage yesterday, and it was like an oven up there, as that has no roof insulation but a pretty good layer of ceiling insulation. The heat was enough to give me a stonking headache last night. Link to comment Share on other sites More sharing options...
Nickfromwales Posted May 23, 2017 Share Posted May 23, 2017 13 minutes ago, Crofter said: And is that inlet duct ventilated? You mean insulated ? Link to comment Share on other sites More sharing options...
DH202020 Posted May 23, 2017 Author Share Posted May 23, 2017 Hi all, just to clarify...under the tiles is a breather membrane, then roof void (cold roof), on top of ceiling 300mm blown insulation, in the insulation runs the main incoming pipe (4meters) for MHVR which also has its own wrapped insulation. I have Redland 50 double roman tiles with a bird protection brush, would these allow enough air under tiles? Would this be recognised as well vented?...or should there be additional vents inserted? Attached shows tiles and (small piece) breather membrane. Link to comment Share on other sites More sharing options...
Jeremy Harris Posted May 23, 2017 Share Posted May 23, 2017 The under-tile vents (and ridge tile vents, if fitted) are just there to keep the void between the tiles and the membrane ventilated, and won't really ventilate the loft space. I may have misread the earlier post, but from this quote: 6 hours ago, DH202020 said: Air tight ceiling, vapour cold roof (sorry wasn't clear on that) I thought you meant that there is a vapour control layer under the roof, which is not the best place for it. The VCL should be on the warm side of the insulation, and the void in the cold side of the insulation should be ventilated. I'm a bit confused by what you actually have. Link to comment Share on other sites More sharing options...
DH202020 Posted May 23, 2017 Author Share Posted May 23, 2017 2 hours ago, JSHarris said: The under-tile vents (and ridge tile vents, if fitted) are just there to keep the void between the tiles and the membrane ventilated, and won't really ventilate the loft space. I may have misread the earlier post, but from this quote: I thought you meant that there is a vapour control layer under the roof, which is not the best place for it. The VCL should be on the warm side of the insulation, and the void in the cold side of the insulation should be ventilated. I'm a bit confused by what you actually have. This may help... http://www.permavent.co.uk/cold-non-vented/ I'm thinking of adding additional ventilation via the gable end wall Link to comment Share on other sites More sharing options...
Jeremy Harris Posted May 23, 2017 Share Posted May 23, 2017 Is there any ridge ventilation, to let hot air out? I'd guess that if there isn't, then trying to find a way to allow hot air to escape from the very top of the loft space might be the best way to reduce the problem. The other thing is to try and improve the insulation around the ducts and MVHR unit that are in the loft. Ideally they need the same sort of insulation effectiveness as the ceiling. Link to comment Share on other sites More sharing options...
DH202020 Posted May 23, 2017 Author Share Posted May 23, 2017 24 minutes ago, JSHarris said: Is there any ridge ventilation, to let hot air out? I'd guess that if there isn't, then trying to find a way to allow hot air to escape from the very top of the loft space might be the best way to reduce the problem. The other thing is to try and improve the insulation around the ducts and MVHR unit that are in the loft. Ideally they need the same sort of insulation effectiveness as the ceiling. How does a dry ridge work? (That's what I've got) does it breath a little? Seems a little see through, would air be able to escape I wonder? In an hour I'm venturing up in the roof space, I will check the temperature of the ducting in the insulation Thanks Jeremy Link to comment Share on other sites More sharing options...
DH202020 Posted May 23, 2017 Author Share Posted May 23, 2017 OK...good news climbed up into roof space just as sun was setting... surprise...not as hot as I thought Second thing... checked ducting not warm at all!! Tucked my hand down through breather membrane overlap, touched tiles, plenty of air there Third thing...noticed one of the main ducting pipes did NOT have secondary insulation wrap (I think it is the incoming air flow) so going to add extra insulation in that area. So all in all I'm relieved....will check air temperature once added insulation. Thanks everyone. Link to comment Share on other sites More sharing options...
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