Insulating between and under the rafters for a loft conversion

for a lot of conversion we're going to want a nice plasterboarded finish something like this and the issue that we've got with insulation is that once we put the plasterboard up against the existing rafters we only have a depth of about let's say four inches 100 millimeters between the plasterboard and the buttons which have the tiles on so that's not enough space to put insulation in and to still have an air gap because what we're after is for there to be a air gap flowing here from the eaves all the way up and over to the other side and out again at these there so what's often done is for insulation to be put between the rafters and then also under the rafters but this insulation needs to have a 50 millimeter that's two inch air gap so if we take a look here at the installation that's been installed in between the rafters we can see that here we've got sufficient air gap this insulation is flush then there is only going to be a possibility of about 50 millimeters of insulation and in order to get the u values that we need we're going to have to have more than that but there's also now a structural issue here which is that if we add insulation to the existing roof and the existing Rafters and we plasterboard underneath it's going to increase the weight so actually what we need to do is to put additional Rafters in which are thicker than the original ones so here we've got something like five or six inch Rafters put alongside the existing now obviously we wouldn't be able to view from this side because this kind of Loft conversion is done from within the Loft and the whole idea is that we don't need to take any of the roof tiles up or have any scaffolding so we're trying to do all the work from within the existing Loft space so this is the sort of view we might have from within the Loft space and you that's the sarking felt over the top of the existing Rafters and we've now put the 50 millimeters of insulation which as I said is probably not going to be enough Lush with the existing Rafters so now that we've put the new Rafters in we can increase the thickness of this and so we've now got this additional insulation in there now I've left a gap there because obviously this same rafter detail needs to be copied over here so the thickness of insulation we're able to get here now is potentially about 100 millimeters so if we have a look as a section through here now now this is what we've got so we've got the original Rafters here and here we've got the new Rafters up against them and screwed into them here and here we've got the 50 millimeter Gap and then we've got here we've got the rest of the roof build up which is actually a bit big for what it would actually be in real life so I'll modify that in a bit so having put that insulation in there we do have a issue that there's this Gap here that now needs to be filled so that could be filled with Timber it could also be filled in if we were to do this in a two-stage process so if we reduce that one back down to the 50 millimeter or so that it was so to bring it flush with that rafter we could then put a second amount of that insulation in here so looking at the section now we've got 50 millimeters here we've got 50 millimeters there and then we've still got the option of which will probably have to be done let's just close this up here then we've got the option of putting the rest of the insulation over the top of the new Rafters here and this is insulation with plasterboard bonded to it you can either buy these that come like this albeit this has been cut in half or you can make this up yourself by buying the plasterboard and the installation separately and gluing it together this is then fixed up into each of the new Rafters and the specifications for that are on the installation website so this is just showing an indicative amount of insulation and this will depend on the area that you live in and whether it's renovation or whether it's new build so the the U value that's required by building regulations depends on whether you're in England Ireland or Wales so if we move the section now we can see that we've got roughly 50 millimeters here and 50 and 50. so 150 millimeters of the PIR insulation which is the high spec material so with this you don't need the full 270 millimeters of rock wall that's generally specified for lofts because this has a better U value so using two sheets of material in here is quite a laborious way of going about it although it probably is the most airtight method what we probably really want to do is have the thicker material in here so say a hundred millimeters in there and then potentially cut some extra to put in over that existing rafter and I'd be interested in your comments below to see how you deal with this if you've done this kind of Loft conversion before I don't know whether it's acceptable potentially to put rockwool in that Gap here and presumably that's better than leaving it entirely open like this so structurally speaking this needs to be put again in a brick bond pattern so the next sheet would go here and so we would build that up we would probably put that in line with the rafters so let's just move these Rafters along and we would line these up to the center of the rafters so that we can get the right fixings in there and get the fixings into each panel as we put them in so there's going to be one next to that so here we can now get a we can get the fixings in on the edge of all of those insulated plasterboard panels and this is classed as a cold roof because the rafters are exposed to the cold air so on this side of the roof the cold side we've got the air running over the top of of the insulation between the insulation and the tiles which then means that the timber is cold so it's a cold roof because we've got the insulation underneath the rafters here so if you were making a warm roof which is preferable you would have the insulation layer over the top of the rafters beneath the tiles so that the Timbers are actually on the warm side So structurally speaking you're going to need the additional Rafters within your Loft space you're going to need most probably the ceiling ties as we've got them here and that then also gives you the benefit of having the extra depth to put the insulation in

Channel: Robin de Jongh

Views: 74,996

Keywords: cold roof insulation, how to insulate a loft conversion, insulating a loft conversion roof, insulating between & under rafters, insulating between and under rafters, insulation between and under rafters, loft conversion insulation, insulating between rafters, diy loft conversion, how to fit insulation board, insulate between rafters, rafter insulation, victorian house loft conversion, how to insulate a loft room

Id: Ub_YcDtKuF0

Length: 8min 12sec (492 seconds)

Published: Fri Feb 17 2023