Internal wall insulation questionable installation method

[Jonny pop rivet]

Hi all, new here, and I was hoping o get some advice regarding a contractor who has quoted on carrying out internal wall insulation of the external walls. 
when he first came he said he would remove the plaster board and insulate behind with PIR board, then re-plaster, seems relatively legit? Now he’s saying so not to have to remove radiators in some rooms, they would cut out the middle section of plaster board, i.e. half way up the height of the wall and slide the PIR board down behind the plaster! Now, this to me seems, well, ridiculous.
I am no expert by any means, but apart from the obvious difficulty in sliding boards into that gap, shouldn’t there be an air gap to the stone external wall, and should there be any damp proofing membrane, the inability to create tight seals between the PIR boards etc?

Like I say I’m no expert, and that’s why I’m here, I don’t want to pay for what sounds to me like, a bad job waiting to happen, but equally, if it is legit I would like to be proven wrong. 

thanks 

[Hastings]

You are right that the PIR boards must be joined to each other or to something else in an airtight way, to make the thermal barrier continuous. Otherwise it won't do much good. You will get thermal bypass.

 

PIR sheets should be at the very least taped together and where butting up to walls or timber studs.

 

Something like Gapotape can be attached to the edges and is designed to ensure a perfect fit.

 

Your contractor is very typical in my experience - it's very hard to find one that understands how insulation works.

 

You might also need to check you won't create damp problems. I just spotted this, which looks useful:

 

[Jonny pop rivet]

6 minutes ago, Hastings said:

You are right that the PIR boards must be joined to each other or to something else in an airtight way, to make the thermal barrier continuous. Otherwise it won't do much good. You will get thermal bypass.

 

PIR sheets should be at the very least taped together and where butting up to walls or timber studs.

 

Something like Gapotape can be attached to the edges and is designed to ensure a perfect fit.

 

Your contractor is very typical in my experience - it's very hard to find one that understands how insulation works.

 

You might also need to check you won't create damp problems. I just spotted this, which looks useful:

 

Yeah thought as much. Thanks for the advise. I’ll give those guys a call about the condensation risks.  

[Temp]

What @Hastings said. 

 

You also need a plan for the wiring. For example if the house is old this might be best time to do a rewire. That could be done by insulating then fitting battens and plasterboard to form a service void for wiring and sockets so these dont penetrate the vapour barrier on the insulation.

 

In some cases if the wiring is in good condition sockets and switches can be raised up the wall to provide enough extra wire to go through the insulation. But if you do this I'd want them to tell me (ideally without asking) that they will seal the back of back boxes where wires enter to ensure they are air tight. Eg to stop water vapour getting through into the cold cavity behind the insulation.

[Temp]

PS Some of the insulation manufacturers like Kingspan and Celotex probably have guides on their web site on how to do IWI on solid walls. For example last time I looked they had a guide on how to do a garage conversion when it's a single brick wall. They might even provide  condensation risk analysis free if you give them details of the proposed wall construction with thickness dimensions.

[Hastings]

Be aware regarding manufacturers' installation guides - they don't always comply with building regulations.

 

Eg. Medite 18mm OSB T&G flooring is more than strong enough for a domestic floor (given the correct support) as recommended and tested by the manufacturer but is not dense enough to comply with the regulation on prevention of sound transmission (in Scotland at least).