Jilly Posted June 11, 2021 Share Posted June 11, 2021 (edited) We are chopping PIR insulation to fit between the rafters, it's quite PITA and there are some gaps and a lot of broken bits. Whats the best way to tape this? I've just seen Gapotape used on all the cut PIR PIR edges to seal all the bits of insulation before pushing into the gaps. Is this a good idea or is stuffing the gaps full of foam the done thing and then taping over this on to the rafters? Can anyone steer me towards a resource on making the building airtight (a bit at least), it hasn't been mentioned, I think because we are a conversion. I'm not sure I (or any of the builders) fully understand the principles, nor how it relates to the vapour control layer. Thanks! Edited June 11, 2021 by Jilly Link to comment Share on other sites More sharing options...
George Posted June 11, 2021 Share Posted June 11, 2021 What does the Building Regs drawing show? Can you post it? # If it's a cold ventilated roof you'd normally have another sheet of PIR below the rafters. That can be taped or another (!) layer of PE can be the vcl. 1 Link to comment Share on other sites More sharing options...
Jilly Posted June 11, 2021 Author Share Posted June 11, 2021 (edited) Ahh ok, we've had to change this cos of bats, so yes, it's now a cold ventilated roof. Yes going to put 25mm PIR plasterboard. Thank you Edited June 11, 2021 by Jilly Link to comment Share on other sites More sharing options...
wozza Posted June 11, 2021 Share Posted June 11, 2021 When I did mine, I used small pieces of timber fixed to the sides of the joists / rafters to stop the PIR being pushed too far - this is especially helpful where you have a long length of PIR butted up against another piece of PIR. I then foam filled (expanding foam - use a proper gun) any small gaps. I then taped over all joints with foil tape straight onto the rafters / adjoining PIR. I would not tape any cut sides that are being pushed between timbers as it would not grip as well. Link to comment Share on other sites More sharing options...
Cpd Posted June 11, 2021 Share Posted June 11, 2021 (edited) You need to buy an expandable foam gun and gun grade expandable foam. make sure that all the insulation is flush with the face of the wood, check with a long straight edge over a few at a time as it will make a BIG difference to how well the 25mm stuff goes on.. are you having a service void ? Either way I would be filling ALL gaps with foam, cut it back and silver foil tape over all joints before starting the over boarding. if you use 100mm tape it should bridge right the timber - it will stick to timber but better to foil faced boards. there are numerous threads on how to do this… you should read them before starting the detailing. Edited June 11, 2021 by Cpd 2 1 Link to comment Share on other sites More sharing options...
TonyT Posted June 11, 2021 Share Posted June 11, 2021 Did the same as CPD. Link to comment Share on other sites More sharing options...
Faz Posted June 11, 2021 Share Posted June 11, 2021 (edited) I am doing this at the moment and yours looks like a proper pita with that bracing! As said above get gun grade foam and fill in any small gaps. Do you need to leave a void of 25 - 30mm to the membrane? Generally specified to avoid condensation. I bought this tape - Gocableties Aluminium Foil Adhesive Insulation Tape 50metres x 100mm (4") Silver Premium Metal Roll: Amazon.co.uk: DIY & Tools My technique is to cut it to size and a bit and use a bit of stud and a lump hammer to smack it into place while swearing at the chippie with each hammer blow for not propping the trusses true plumb. Buy some insulation saws (I got the Bosch jigsaw blades) as it cuts down dust. My other finding is that it is worth taking the time to keep your work area clear of scrap - it will pile up and get in the way otherwise. Edited June 11, 2021 by Faz 1 Link to comment Share on other sites More sharing options...
Faz Posted June 12, 2021 Share Posted June 12, 2021 The end is in sight for me thankfully - what a horrible job to do! Then I have to insulate the garage ceiling..... Far easier though as noggins rather than bracing although 2 sheets of PIR need to go in it. 2 Link to comment Share on other sites More sharing options...
Jilly Posted June 13, 2021 Author Share Posted June 13, 2021 Can I clarify, this taping of the insulation here isn't the vapour control layer, it's just to make airtight? There will be another layer of foil backed plasterboard? Link to comment Share on other sites More sharing options...
Cpd Posted June 13, 2021 Share Posted June 13, 2021 (edited) If the taping is continuous and goes from sheet to sheet it will act as a VCL (vapour control layer ) and airtight layer all in one at this point and would not need another. Some people like to add in a layer of polythene as a vcl just to be 100% sure. adding another layer of inundated plasterboard does not act as a vcl ( I think) but will stand corrected if need be Edited June 13, 2021 by Cpd 1 Link to comment Share on other sites More sharing options...
crispy_wafer Posted June 13, 2021 Share Posted June 13, 2021 @Faz Loving your pictures and progress, you are a way ahead of me and your pictures are really helping me visualise what I'll be needing to do later on. Your insulation between the rafters, are you insulating right down the attic truss void aswell? Curious, as I've not decided as yet. Good work sir. 1 Link to comment Share on other sites More sharing options...
Faz Posted June 14, 2021 Share Posted June 14, 2021 12 hours ago, Jilly said: Can I clarify, this taping of the insulation here isn't the vapour control layer, it's just to make airtight? There will be another layer of foil backed plasterboard? 25mm insulated plasterboard to go on top. Taping is belt and braces. Link to comment Share on other sites More sharing options...
Faz Posted June 14, 2021 Share Posted June 14, 2021 10 hours ago, crispy_wafer said: @Faz Loving your pictures and progress, you are a way ahead of me and your pictures are really helping me visualise what I'll be needing to do later on. Your insulation between the rafters, are you insulating right down the attic truss void aswell? Curious, as I've not decided as yet. Good work sir. No point - I am putting the PIR in the upstands, roof and in the garage ceiling below. 1 Link to comment Share on other sites More sharing options...
SimonD Posted June 14, 2021 Share Posted June 14, 2021 On 11/06/2021 at 14:02, Jilly said: I've just seen Gapotape Personally I'd be taking a serious look at the tape. Once you've used tapes instead of foams/sealants, you'll not likely want to go back. Also, when you have very small gaps between insulation and rafters, it's very difficult to get the foam gun in there to fill and you can end up with several mm gap that only gets taped. Whether it's still the case, PIR has been known to shrink with age so any small gaps left over may expand. 1 Link to comment Share on other sites More sharing options...
JFDIY Posted June 14, 2021 Share Posted June 14, 2021 2 hours ago, SimonD said: PIR has been known to shrink The timber will shrink even if the boards don't 1 Link to comment Share on other sites More sharing options...
Jilly Posted June 14, 2021 Author Share Posted June 14, 2021 Have just spoken to a roofer who advocates putting tile vents in (as Peter suggested) and making a little cold roof, but will have to explore proper insulation of the steel (and other steels) as it will be outside the thermal envelope if we do that. Link to comment Share on other sites More sharing options...
Dave Jones Posted June 15, 2021 Share Posted June 15, 2021 On 13/06/2021 at 18:42, Cpd said: If the taping is continuous and goes from sheet to sheet it will act as a VCL (vapour control layer ) and airtight layer all in one at this point and would not need another. Some people like to add in a layer of polythene as a vcl just to be 100% sure. adding another layer of inundated plasterboard does not act as a vcl ( I think) but will stand corrected if need be no it wont. Link to comment Share on other sites More sharing options...
Faz Posted June 15, 2021 Share Posted June 15, 2021 On 14/06/2021 at 14:53, Jilly said: Have just spoken to a roofer who advocates putting tile vents in (as Peter suggested) and making a little cold roof, but will have to explore proper insulation of the steel (and other steels) as it will be outside the thermal envelope if we do that. If your trusses allow a 25 - 30 mm void after the install of the PIR you should be fine without vents depending on your breather membrane. You just need to be careful (if such a thing is possible) that when you smack the PIR in place it stays level with the truss. Half the trick is to pull out the PIR so it doesn't get too close to the membrane as you smack it in - too late once it in place coz you can't get behind it to pull it back out. Link to comment Share on other sites More sharing options...
Faz Posted June 15, 2021 Share Posted June 15, 2021 Be under no illusions - it is a horrible job and the mess is unreal. Stay on top of the mess but keep potentially useful offcuts although these pile up. I have found that even 5mm offcuts have been useful elsewhere in the house so it is the real mess that need to go. Link to comment Share on other sites More sharing options...
Jilly Posted June 16, 2021 Author Share Posted June 16, 2021 Thank you! Yup, it's horrible. The problem is that it's NOT breathable membrane, its F1 bitumen because of bats. it's caused loads of problems and has to have 50mm gap and good ventilation. Link to comment Share on other sites More sharing options...
Faz Posted June 24, 2021 Share Posted June 24, 2021 Finally finished that attic room today - the en-suite took 2 days! Too many fiddly areas. Now for the garage ceiling..... Lots of long runs there so shouldn't be so bad apart from the fact that so much gear is in the way on the floor. Link to comment Share on other sites More sharing options...
Cpd Posted June 25, 2021 Share Posted June 25, 2021 9 hours ago, Faz said: Finally finished that attic room today - the en-suite took 2 days! Too many fiddly areas. Give us a photo then ! Link to comment Share on other sites More sharing options...
Faz Posted June 25, 2021 Share Posted June 25, 2021 I'll take one on Monday - first Saturday off tomorrow for a month! Link to comment Share on other sites More sharing options...
Mr Punter Posted June 25, 2021 Share Posted June 25, 2021 19 hours ago, Faz said: Now for the garage ceiling..... Lots of long runs there Glory work at last! Not a favourite job for anyone. 1 Link to comment Share on other sites More sharing options...
Thorfun Posted June 25, 2021 Share Posted June 25, 2021 we're having attic trusses and after reading about everyones thoughts on cutting PIR for between the rafters on this and many other threads I've decided to ditch the architects plans of 50mm ventilation gap, 150mm phenolic between the rafters and 60mm phenolic under the rafters and I'm fully filling the rafters with 200mm of mineral wool and 100mm PIR below which should give similar U-values by my calculations but result in a lot fewer PIR cuts and is also a LOT cheaper than the Kingspan phenolic specified. I've checked with BC and she's ok with fully filled rafters with Roofshield membrane, battens and counter battens above the rafters. should be a lot easier to install and as it's just storage space in the loft the extra thickness of PIR is not an issue with reduced room space. Link to comment Share on other sites More sharing options...
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