Warm Roof vs Cold Roof. What’s The Difference?

hello i'm roger brisby from skill builder and i just want to talk to you about the difference between warm roofs and cold roofs because this does confuse a lot of people and it's really very very simple now we're just doing this orangery here and our intention was to have a warm roof because a warm roof is a lot easier in situations like this but then we had a little word with the neighbor and the neighbor wasn't too happy about the height so we've reduced the height because everyone wants to get on with their neighbors and that means that when we reduced the height we had to take the insulation from off the top of the roof and put it on the inside of the roof thereby changing it from a wall roof to a cold roof here we have a flat roof these are the joists which you'll see going through here number two joist there so we've got a void and then we're going to have plasterboard here on the top we've got osb decking and on top of that we're going to have our waterproof cover all the way across there which in this case is grp now if we put the insulation on top of the roof insulation being over there eco therm we will put say 120 millimeters of insulation on top of that roof so we've got the insulation there 120 ml of insulation there we've got a sheet of osb going across the top of that and then we're going to put our grp on the top now as the water the moisture the airborne moisture in the house your family of four a lot of moisture coming out every day from the house and a lot of it is migrating up through the plasterboard into this void here now if you've got a warm roof you've got the insulation there it means that all this area in here is warm it's it's within the heated area of the house now that's the important thing so as your airborne moisture is coming up through here and it's hitting the underside of the insulation it's not condensing because the underside of the insulation is warm so most importantly what we put here is a vapor barrier and that vapor barrier goes all the way along the underside of the insulation so that's on top of our piece of decking that we've got here in this case osb so the vapor barrier goes over the top of the decking and the vapor barrier has to wrap up the sides and it has to be perfectly sealed all around so in the case here where we've got an opening we have to go around with the vapor barrier tape it up the sides make sure all the joints because any moisture that comes through here airborne moisture that's the vapor we don't want that vapor migrating through that board through the insulation and sweating on the top side of that insulation because if we do that we get rot we get rot on this piece of deck in here which is underneath the grp so if you're doing a warm roof you put the vapor barrier on the warm side of the insulation but we're not doing a warm roof because we have to lower it down and that's very often the case people haven't got unlimited height to to work with so they've got to put their insulation within the joists now this is great because we've got a 9x2 joist here we could pack that void full of insulation except for one problem we're still dealing with the same physics nothing's has changed we've got airborne moisture coming up here if it gets through the insulation as soon as it gets through the insulation it's going to be on the cold side of the insulation we haven't got this insulation that's gone it's on the cold side of the insulation because there's no insulation on top there's our insulation 120 mil say and therefore our dew point this is the point where the airborne vapor the moisture condenses into droplets of water our dew point is here our dew point is always on the cold side of the insulation wherever you see the cold side of insulation what you've got to think about is that is where the moisture is going to form so what we would end up with is a lot of moisture forming here trapped under that osb at the top trapped under that grp layer with nowhere to go so in that situation it is absolutely essential no two ways about it essential that you have ventilation all the way through that void there so if we've got say a 225 millimeter joist there and we're going to put 100 and say 220 millimeters in there you can see that we've still got ourselves a clear 100 millimeter space here to ventilate all the way through now we need a minimum of 50 millimeters really so 100 millimeters is good but we still need to get a through flow of air into that space and if we do that we can clear that air we can clear the the moisture as it comes in and condensing we've got a nice draught going through there and it will blow that moisture away and on a warm day that will evaporate as well so that will keep that nice and dry i have a friend who bought a house four years ago and he had a cold roof in this situation and they didn't ventilate the void and within four years the whole thing had rotted that includes the joists that includes the osb the whole thing had just turned to mush so the whole roof had to be stripped off a big canopy pulled over and the whole thing had to be stripped off and renewed and this time they put ventilation in so this is our insulation and in an ideal world we would have put that on top of the roof making a warm roof but we're going to have to cut it and put it in between these joists here and that means that we're going to finish up 100 millimeters of insulation between each joist and then we're going to have to ventilate across the front now we're going to put what they call a soffit ventilator in the front here which has got those tiny little holes in it so the air will come up and it will go through this void here it will go above the insulation here and we need that draft to come all the way through you can see that when we put our noggins up we've left a nice gap at the top for them and we need air to come out at this end we can do that in several ways we can build an up stand and put a little ventilated top on it what we call an over tile vent or in this case we're probably going to end up doing it with grp mushrooms which are going to be all the way along in every single joist so that we've got a nice through flow of air through there and out of there now when you look here you see another problem and this is joyce going the other way whichever way we had these if we carried these joists on all the way to there and put joist hangers in we still would have finished up with this dead spot here where although we've got a soffit ventilator at the front the air was coming through here and it couldn't go anywhere because it's on this trimmer so you get that with roof windows if you've got a roof window or in this case you've got a lantern going there and you've got these dead spots all the way around then you have to deal with that now some people would say okay just drill holes through the joists and therefore you've got a bit of ventilation going through which you can do but we don't really want to drill through these triple joists with a load of holes so in this case the grp guy again ben has got to come up with something really clever and he's probably going to end up putting ventilators all the way along here mushrooms just to ventilate this front piece and again at the back because we've got a dead spot at the back he's going to have to put mushrooms in there so this whole roof is going to be covered with all his little pots all these little mushrooms to ventilate it whereas what we could have done in an ideal world is a warm roof but that warm roof would have taken us up another 120 millimeters high and unfortunately in this case we can't do it so i hope you understand that the difference between a warm roof and a cold roof the warm roof has the insulation on the top the cold roof has the insulation underneath and you need ventilation if you've got a cold roof one more thing you must have even if you've got a cold roof is a vapor barrier under here so you put your insulation up you attach your vapor barrier or you can use what they call vapor check plasterboard which has a silver foil backing on it and you screw that plaster board up and that stops a lot of that moisture from migrating up into the roof so that's a very good thing because then it leaves the ventilation less of a job to do so any moisture you can stop migrating up through the roof you can then not have to bother about removing a lot of people will be very surprised to find how much moisture will migrate through a sheet of plasterboard it's a lot and then the worst thing is that people then go and put down lighters in there so if they've got a vapor barrier they go and cut loads and loads of holes in it put down lighters in the down lighters get warm they suck all the air up and they suck all the moisture up so you're back to square one so if you're going to have down lighters make sure that a kind where you can put the vapor checked barrier over the top of the down lighter so you formed a complete seal there there are those kind of down lighters around all you've got to do is look for them so i hope you found that interesting bit of a whistle stop tour but come back and watch this project because it will be coming up on school builder soon we're going to be doing this whole orangery you'll see it from start to finish in all its glory [Music]

Channel: Skill Builder

Views: 405,134

Keywords: flat roof insulation, warm roof insulation, warm flat roof, roof ventilation, cold roof, vapour barrier, insulating a flat roof, vapour control layer installation, flat roof problem, flat roof construction, warm roof construction, new flat roof, glass fiber grp, sweating roof, warm roof vs cold roof, flat roofing, how to build a flat roof, sweating flat roof, cold roof insulation, cold roof vs warm roof, cold roof construction, cold roof ventilation

Id: i41oW85zPm8

Length: 9min 30sec (570 seconds)

Published: Wed Aug 18 2021