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Struggling and need some advice please


Thorfun
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Hi all,

the PIR insulation below the rafters in our loft is a real pain and I'm struggling and looking for some advice. the problem I have is that there are lots of 4"x1" bracing across the trusses. I'm struggling with how to cover those in PIR.

 

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I started by trying to 'channel' out of the PIR where the braces are and put the PIR on top but getting that in any way accurate to fit just wasn't working and I failed miserably. I then tried to cut pieces to fit around the bracing and then put thinner PIR on top of the bracing to bring it to the same level. i.e. we're using 100mm PIR so I was thinking of adding 80mm PIR to the bracing which will bring it to a couple of mm over the 100mm. But, the problem with this is it's very fiddly and hard to measure and also there are places where I don't have trusses to screw the PIR in to!

 

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and that's where I've left it for the day but I seriously don't have time to be mucking around with cutting all those angles and fiddling about trying to cut strips of 80mm PIR to fit on the braces. it'll take weeks and destroy my will to live. I can't believe that a builder would take the time to do it this way.

 

am I being stupid here and precious about making it perfect? should I just slap a sheet of PIR on top of it all and screw it to the joists and not worry about the PIR 'bending' over the bracing nor the small areas of air gap that will be behind the PIR  and the mineral wool as it 'bends' over the braces? that area is not going to be plasterboarded. I will simply cover the PIR in AVCL and continue that up into the attic space and will board down the attic trusses leaving the eaves unboarded. so, I guess the area in the eaves doesn't need to be level PIR?

 

please can someone point me in the right direction as this is starting to get me down. I know that I sometimes get too hung up on trying to make it 'perfect' so please tell me if this is one of those situations where it doesn't need to be and bending it over the bracings is not an issue.

 

if anything above doesn't make sense then please let me know and I'll clarify it with photos or diagrams etc if I can.

 

many thanks in advance as always to the very helpful folk on this forum.

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4 minutes ago, Thorfun said:

am I being stupid here and precious about making it perfect? should I just slap a sheet of PIR on top of it all and screw it to the joists and not worry about the PIR 'bending' over the bracing nor the small areas of air gap that will be behind the PIR  and the mineral wool as it 'bends' over the braces? that area is not going to be plasterboarded. I will simply cover the PIR in AVCL and continue that up into the attic space and will board down the attic trusses leaving the eaves unboarded. so, I guess the area in the eaves doesn't need to be level PIR?

 

 

Exactly this, I could see a real issue when thinking about plasterboard over the top however when you don't have this issue you'll be fine. Maybe if you're feeling particular a few strips of expanding foam on the back of the pir to enclose any air gaps but I'd crack on. otherwise. 

 

 

 

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20 minutes ago, Iceverge said:

 

 

Exactly this, I could see a real issue when thinking about plasterboard over the top however when you don't have this issue you'll be fine. Maybe if you're feeling particular a few strips of expanding foam on the back of the pir to enclose any air gaps but I'd crack on. otherwise. 

 

 

 

thank you! sometimes you get so involved in something that it's hard to take a step back to see a better way. I'm glad I stopped for the day and had a beer and a think and then posted here rather than struggling on and spiralling further and further down the road to despair! 🙂 

 

I like the idea of the expanding foam but I'd need to spray it on to the mineral wool either side of the bracing. but, then again, I may not bother! 😂

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I've been wondering about this exact same issue for a pending job of mine. I've been contemplating just adding more bracing to create a sufficiently supportive surface for the PIR and then sheet over it all. The air gap left under the PIR is only the thickness of the bracing and, as I understand it, there's a slight benefit in having it there with regards to the conductivity and emissivity of the reflective foil layer.

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Posted (edited)
6 minutes ago, Radian said:

I've been wondering about this exact same issue for a pending job of mine. I've been contemplating just adding more bracing to create a sufficiently supportive surface for the PIR and then sheet over it all. The air gap left under the PIR is only the thickness of the bracing and, as I understand it, there's a slight benefit in having it there with regards to the conductivity and emissivity of the reflective foil layer.

glad this post has been a benefit for you too! 🙂 

 

I do have lots of leftover 4"x1" bracing that the TF company left here. so I could, in theory add more to make the PIR more level so that's also something I haven't thought of, thanks. but I thought the benefit of the emissivity of the reflective foil layer was on the inside of the PIR and not in the air gap to the outside? 

Edited by Thorfun
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Went the channelling route myself, I'm stingy with space and didnt want to lose the inch, It was me measuring the missus was marking it out on the pir, we trimmed the full boards down to 4 by 4 or whichever was closest to the nearest rafter so could get a screw and washer to fix the board down.  We had 70mm of the pink kooltherm stuff on the inside, which I found was easier to mould  Mark the board cut the lines to 20mm or so, then run some cut lines then run the saw horizontal and ping bits out of the channel, use a bit of 4 * 1 to check before fitting.

 

Probably why it took me so long.  Couldnt imagine a contractor going to the trouble and I guess that fact, made me feel a bit better about the PITA job. 

 

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44 minutes ago, crispy_wafer said:

Went the channelling route myself, I'm stingy with space and didnt want to lose the inch, It was me measuring the missus was marking it out on the pir, we trimmed the full boards down to 4 by 4 or whichever was closest to the nearest rafter so could get a screw and washer to fix the board down.  We had 70mm of the pink kooltherm stuff on the inside, which I found was easier to mould  Mark the board cut the lines to 20mm or so, then run some cut lines then run the saw horizontal and ping bits out of the channel, use a bit of 4 * 1 to check before fitting.

 

Probably why it took me so long.  Couldnt imagine a contractor going to the trouble and I guess that fact, made me feel a bit better about the PITA job. 

 

wow! that's impressive. I really struggled with channelling but it was me on my lonesome. maybe with the wife's brains on board it might've gone better. I have the plumber starting in 5 weeks or so and have a lot of insulating, airtightness and service cavities to do to get ready for first fix so really can't spend the sort of time I'd need on it to do the channelling and so the 'slapping of the PIR over the top' will commence tomorrow morning. 😂

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12 hours ago, Thorfun said:

glad this post has been a benefit for you too! 🙂 

 

I do have lots of leftover 4"x1" bracing that the TF company left here. so I could, in theory add more to make the PIR more level so that's also something I haven't thought of, thanks. but I thought the benefit of the emissivity of the reflective foil layer was on the inside of the PIR and not in the air gap to the outside? 

Works both ways. If you point an IR thermometer or camera at a shiny surface it looks colder than a matte surface at the same temperature - hence the shiny surface not radiating away as much heat. Conduction is cancelled in those areas where the air gap remains so double win. Also this all helps in the summer when roofs get stupidly hot.

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