Insulation over walls and ceiling, vapour barrier

[revelation]

I have had my roof re-done and also a dormer added with a cold roof.

 

I have 120mm kingspan between the rafters of pitch and the flat roof of the dormer.  We have 80mm kingspan in the walls of the dormer.

 

I plan to add another 25mm kingspan in large 1.2 x 2.4 sheets over the ceilings and walls and tape all the joins.   The plasterboard.

 

The question I have is whether I need to have a vapour barrier added anywhere, if so where and what kind would be best?

[tonyshouse]

The foil and foil tape is a vapour barrier, there is a question about how long it will last. 
 

how will you join it to the vapour barriers in the adjoining elements?  Wall under the floor, wall, window, access hatch linings etc? 
 

then where is the air tightness layer? Is this hermetically sealed to adjoining elements? 
 

I always used a sheet of thin poly, omit the tape and join the poly to adjoining elements 

[nod]

It will depend on what is specked on your drawings 

Some require a vapor barrier Most don’t 

 

[revelation]

We have had a loft conversation with a dormer which has a staircase going up so no hatch.  

 

I hadn't thought of a vapour barrier until one of my builders asked if I am putting one in while I was taping the insulation.  I wasn't sure I assumed the foil tape would suffice.  

 

I am not sure what you mean by airtightness layer and whether it is hermetically sealed.

 

I have attached images which show the set up (this was before finishing expanding foam and foil taping).  So I was going to just add the large insulation sheets screwing them to the timber work.  Then the insulation on top.

[Mr Punter]

After you have fixed the top sheets you can either use foil tape over the joins or polythene over the lot.  I would use foil tape as it is not easy to staple the polythene onto the 25mm Kingspan.

[revelation]

I have foiled taped what’s in the picture. So add another 25mm layer. Then foil tape again. Then plasterboard. 
 

(when foil taping the join between insulation and timber does the whole timber need to be covered or just the joint?)
 

[TonyT]

I covered the timbers, may not have needed but I thought it was more belt and braces and less likely for it come away from the timber

[revelation]

Thanks Tony, I have a fair bit off tape so I will tape the timber too.  Looks like a separate vapour barrier doesn't seem that necessary.

[JohnB]

Yes i would use foil tape, make sure no gaps are left but in the long run i would use polythene as it will be a piece of mind job and you won't have to redo if problems occurred in years to come.

[revelation]

1 hour ago, JohnB said:

Yes i would use foil tape, make sure no gaps are left but in the long run i would use polythene as it will be a piece of mind job and you won't have to redo if problems occurred in years to come.

 

Just standard polythene or is there a specific material for use as a vapour barrier.

[TonyT]

Standard is fine, you want a thickness that is easy to handle but not too thin that it tears when you look at it!

 

 

[Mr Punter]

125 Micron (mu) / 500 Gauge. Transparent is better so you can mark the joists on the insulation with a sharpie and see where they are.

[revelation]

2 minutes ago, Mr Punter said:

125 Micron (mu) / 500 Gauge. Transparent is better so you can mark the joists on the insulation with a sharpie and see where they are.

 

Would it be best to secure the VCL with staples? Or to use some kind of adhesive?  Especially considering it will be pierced when screwing the 25mm insulation boards and the plaster boards

[Mr Punter]

I would fix the insulation boards with screws and penny washers, then foam and gaps, secure the polythene with staples, then plasterboard.

[revelation]

From what was being said I thought that the vapour barrier was going on top (of the 120mm insulation between the rafter) before the 25mm insulation.  I have plastic washers that I was going to use for securing the insulation.

 

So I get this right, the vapour barrier is on the last layer before the plasterboard goes on?

 

[Mr Punter]

17 minutes ago, revelation said:

So I get this right, the vapour barrier is on the last layer before the plasterboard goes on?

 

Yes, the vapour control layer should go on the warm side of the insulation.  It is not crucial in your case as the final layer of insulation is thin and is not bridged by timber etc.  It will be much easier to staple onto the studs than the Kingspan.  That was why I suggested just alu tape, but it really does not make much odds either way.

 

It would be a much bigger deal if you have mineral wool type insulation fit the vapour control layer outside of it.

Edited February 26, 2021 by Mr Punter

[revelation]

2 minutes ago, Mr Punter said:

 

Yes, the vapour control layer should go on the warm side of the insulation.  It is not crucial in your case as the final layer of insulation is thin and is not bridged by timber etc.  It will be much easier to staple onto the studs than the Kingspan.  That was why I suggested just alu tape, but it really does not make much odds either way.

 

I have taped most of it already but will add the VCL too to be extra safe. Appreciate the help.