Correct method of vapour barrier installation

[djcdan]

Hi all!

 

I'll be installing the roof insulation this week. Everything feels clear RE: insulation/battening, but wanted to check from those who know that 1) I understand the primary role of the vapour barrier and 2) the correct way to install.

 

For clarifications of build details:
- Our roof makeup is shown in the below architect drawing. 170mm insulation between the trusses. Then vapour barrier stapled to the trusses. On top of this, boards of 25mm insulation secured in place with 35mm service battens nailed at 600mm centres in to the trusses through the 25mm insulation.

- It is worth noting that we will have MVHR and understand one of the results of a correctly installed vapour barrier is increased airtightness.

 

With regards to the vapour barrier, am I understanding correct that the primary objective of the vapour barrier isn't as a final barrier of keeping water out (in the event of a loose tile etc combined with breakage in roof membrane), but to keep condensation (formed from the potential meeting of warm internal air and cool external air and uninsulated spaces) from infiltrating the trusses and insulation?

 

I am expecting to secure the VB to the trusses with staples, but my questions are really focused on:

 

- Does the vapour barrier completely encircle the upstairs ceiling as shown on the red line I have drawn on the plan below?

- Does the VB run down the walls beneath the coombs? This would seem to make sense as the eaves are uninsulated, so provides the barrier between heated and unheated space. If so, where and how does this terminate?

- In the roof space, we have six velux's. What is the correct way to lap and joint the VB in to the velux spaces?

- Am I correct in saying the polythene VB I am installing is jointed to the preexisting silver faced VB on the internal gable installed by the kit manufacturer using foil tape?

 

I feel this is quite crucial in order to achieve our air tightness goal, so any assistance would be appreciated.

TIA

 

[nod]

You are correct the VB needs to return down the walls 

It’s to keep you airtight 

I definitely wouldn’t use polythene 

Ive stripped so many ceilings out that have sweated because of using Polythene 

I always use DuPont Airguard Control Unless instructed otherwise 

More expensive at £100 a roll 

 

 

[Hopeful]

6 minutes ago, nod said:

You are correct the VB needs to return down the walls 

It’s to keep you airtight 

I definitely wouldn’t use polythene 

Ive stripped so many ceilings out that have sweated because of using Polythene 

I always use DuPont Airguard Control Unless instructed otherwise 

More expensive at £100 a roll 

 

 

If it comes up the slopes and 4-600mm each side onto the flat ceiling cross beams, do you think its necessary to do the whole ceiling when theres 300mm of glass wool there anyway, and so many holes needed for light cables etc? Thats if the ceiling isn't brandered down right enough. 

[djcdan]

1 hour ago, nod said:

You are correct the VB needs to return down the walls 

 

Once at ground level of the first floor, does it just terminate at the joists, or continue somewhere?

 

2 hours ago, nod said:

I definitely wouldn’t use polythene 

Polythene is what is supplied via the kit manufacturer, Scotframe. Is it worth purchasing my own?

Any idea on the correct way to lap around the Velux's?

[nod]

1 hour ago, djcdan said:

Once at ground level of the first floor, does it just terminate at the joists, or continue somewhere?

 

Polythene is what is supplied via the kit manufacturer, Scotframe. Is it worth purchasing my own?

Any idea on the correct way to lap around the Velux's?

Yep 

We normally go over the Velux windows

Then slit it and fold it in 

Staple it down PB over

the reveal and leave the excess to be cut off prior to skim or tape 

Building inspectors like us to leave a flap sticking out of the PB 

The picture is of 1 of ten top floor  apartments we did two years ago 

Brown patches have appeared (sweating)

I drafted an email at the time warning of this 

This was common in the 80s as there was little alternative There loads of excellent products on the market 

But none as cheap as £20 for a 100m2 roll

of polythene 

 

[djcdan]

20 hours ago, nod said:

Yep 

We normally go over the Velux windows

Then slit it and fold it in 

Staple it down PB over

the reveal and leave the excess to be cut off prior to skim or tape 

Building inspectors like us to leave a flap sticking out of the PB 

The picture is of 1 of ten top floor  apartments we did two years ago 

Brown patches have appeared (sweating)

I drafted an email at the time warning of this 

This was common in the 80s as there was little alternative There loads of excellent products on the market 

But none as cheap as £20 for a 100m2 roll

of polythene 

Cheers @nod. I have purchased a few rolls of the Airguard Control VB based on your recommentation, so thanks for that.

 

Quick question, can you clarify the abbreviation of PB?

On the ceiling and walls beneath the coombs, the plasterboard will be applied directly to the timber of the trusses/roof frame. I presume they'd be no issue with laying the Airguard right beneath plasterboard with no air gap?

 

 

[tonyshouse]

Correct

[nod]

On 17/08/2021 at 14:54, djcdan said:

Cheers @nod. I have purchased a few rolls of the Airguard Control VB based on your recommentation, so thanks for that.

 

Quick question, can you clarify the abbreviation of PB?

On the ceiling and walls beneath the coombs, the plasterboard will be applied directly to the timber of the trusses/roof frame. I presume they'd be no issue with laying the Airguard right beneath plasterboard with no air gap?

 

 

?