Retrofit insulation no vapour barrier

[WillKen]

I’m currently insulating the first floor of my 70s chalet bungalow. There is one wall which is essentially a stud wall which opens up to the loft above and eaves at the back. I can fully access these walls from behind, and am planning not to disturb the plasterboard. I’ll instead insert 100mm PIR from behind.

 

The issue I have is that there’s no vapour barrier on the current plasterboard/wall. It’s ordinary paper backed board. Am I safe to add PIR behind?

 

My instinct is that foil taping the PIR to the studs at the rear is a bad idea as it traps any moisture that forms on the inside of the wall.

 

Should I add the PIR and foam gaps, but not tape? Or should I try to retrofit some kind of vapour barrier inside the studs?

[TonyT]

2 coats of emulsion is a vapour barrier

[JohnMo]

I would be tempted to leave it vapour open, and use a mineral wool, something like Rock wool Flexi or similar. It's nice and stiff and will self support and you could over fill with something like 150mm. To get similar U value as 100mm PIR.

[WillKen]

9 hours ago, TonyT said:

2 coats of emulsion is a vapour barrier

So then why fit a vapour barrier ever?

 

8 hours ago, JohnMo said:

I would be tempted to leave it vapour open, and use a mineral wool, something like Rock wool Flexi or similar. It's nice and stiff and will self support and you could over fill with something like 150mm. To get similar U value as 100mm PIR.

I like the thinking. My only issue is I have a dozen sheets of PIR on the drive already! Can you think of a way to use it?

 

What about foil taping the stud to the plasterboard then installing the PIR? Or even foil taping the whole rear face of the plasterboard…?

 

i may just have to cut my losses and try to sell the PIR. 

[Mike]

8 hours ago, WillKen said:

So then why fit a vapour barrier ever?

You wouldn't generally fit a vapour barrier ever.

 

However there's a difference between a vapour barrier (stops moisture vapour movement), a vapour control layer (allows moisture vapour to move through it in a controlled way - a.k.a. a 'breathable' membrane), and an air barrier (stops air moving through it).

 

Thinking has changed over recent decades and now, in most circumstances, the advice is generally to include an air barrier - to stop the movement of air - but not to stop the transfer of moisture vapour (to allow any moisture within the structure an escape route). In some circumstances that may involve fitting a vapour control membrane.

 

In this case I'd just cut the PIR board a little loose and foam it in place using a flexible breathable (moisture-permiable) foam such as Ilbruck FM330. Or choose an alternative insulation. And a vapour-perimable emulsion paint; if you use a standard vinyl one then the wall is only going to breath towards the outside.

[MikeGrahamT21]

Definitely use the PIR be a shame to sell it and lose money, it’s perfect for the job. Ensure the eaves are well ventilated as they will experience a big temperature drop in winter once the habitable space is insulated 

[WillKen]

1 hour ago, Mike said:

You wouldn't generally fit a vapour barrier ever.

 

However there's a difference between a vapour barrier (stops moisture vapour movement), a vapour control layer (allows moisture vapour to move through it in a controlled way - a.k.a. a 'breathable' membrane), and an air barrier (stops air moving through it).

 

Thinking has changed over recent decades and now, in most circumstances, the advice is generally to include an air barrier - to stop the movement of air - but not to stop the transfer of moisture vapour (to allow any moisture within the structure an escape route). In some circumstances that may involve fitting a vapour control membrane.

 

In this case I'd just cut the PIR board a little loose and foam it in place using a flexible breathable (moisture-permiable) foam such as Ilbruck FM330. Or choose an alternative insulation. And a vapour-perimable emulsion paint; if you use a standard vinyl one then the wall is only going to breath towards the outside.

Thank you for the thorough explanation. Wall is already painted. Not sure on paint type but will check.
 

1 hour ago, MikeGrahamT21 said:

Definitely use the PIR be a shame to sell it and lose money, it’s perfect for the job. Ensure the eaves are well ventilated as they will experience a big temperature drop in winter once the habitable space is insulated 

 

Thanks. It’s pretty well ventilated due to a lot of gaps everywhere! But when I have the soffits done, I’ll get them to add some vents, as well as close up the gaps that shouldn’t be there…