PIR over PIR Timber kit

[Steve450]

Hi all,

 

I have a single brick external leaf rendered with a 50mm cavity then a timber kit with paper, ply wood and 70mm studs.

 

I have jumped the gun a bit and purchased 70mm PIR.  My plan was to fit tightly inside the 70mm studs then fc foil vcl then battern 2x1 for a service void then plasterboard or soundbloc, I’m aware that would only give me roughly .28 U value going on the average figures I’ve got back from online calculators.

 

my question is, if I was to add another sheet of PIR say 30mm would I install it directly over the stud face and 70mm PIR then install battens and plasterboard or am I better installing the battens over the 70mm PIR then installing a 30mm PIR or 40mm insulated plasterboard and if so on either option to gain a better U value of approx .22 where would the VCL go?

 

thanks in advance

[Steve450]

Hope that wasn’t too confusing, basically to add to my single 70mm PIR external insulation between studs, do I add the 30mm PIR over the 70mm then can I batten over the 30mm with long screw for a service void and then plasterboard? or do I batten then add insulated plasterboard?

 

[Roundtuit]

You may struggle to get 70mm pir into 70mm studs as there is probably a tolerance of +/- up to 5mm on the pir boards.  70mm studs seem pretty light tbh; I think I'd be considering filling the studs with mineral wool insulation, and fixing the 70mm pir on the inside of that with battens.  Someone clever will hopefully be along shortly to advise on vcl and condensation risk.

[Steve450]

Thanks for your reply, yeah I’m hoping for some more views and advice, I was shocked when I cut inspection holes to find the 70mm stud.  I’m guessing I wouldn’t be able to extend up with it but I’m no where near that point l, I have no kids and a big bungalow to try and insulate lol I just want to get it the best I can without losing too much floor space.  The 70mm studs currently have material insulation in a brown paper bag which I’d guess is no great than 50mm and it’s extremely breezy when I poke my hand in the void, maybe I could strip the plasterboard and old wool then buy new wool then PIR, maybe should have planned it out better 

[Steve450]

I have already purchased the 70mm PIR but if someone could advise a good spec I’m sure the merchants would exchange it

[Iceverge]

@Steve450 

 

Welcome and seasons greetings. 

 

Do you have any photos to share please, they always help. 

 

Sounds like fiber glass batts in the wall as is. If they're ok I'd leave well enough alone. Put the 70mm PIR cross the studs then VCL and then service cavity and plasterboard. 

 

[Steve450]

Hi thanks for the warm welcome and thank you for your reply and time to work out the calcs.  Impressed!

I can take some photos in the morning and upload them, now that I have Googled it it does seem like fiber glass batts in the brown bags, on concern is they don’t feel 70mm deep but if I strip all the plasterboard I guess I can inspect closer.  I’m guessing the original plasterboard covered in wood chip wallpaper can’t stay on and PIR over?