Vapour Barrier in Loft Conversion

[firapara]

Hi Guys,

I am having a loft conversion done.
The insulation is going in between the 170mm rafters. 150mm Celotex and 20mm gap for breathing.

Do I need a Vapour Control Layer on the inside? Some polyethylene sheet before attaching the plasterboard?

Thank you

 

[noseall]

Do I need a Vapour Control Layer on the inside? Some polyethylene sheet before attaching the plasterboard?

Yes, but there are ways to (robustly) achieve this.

You will also need an insulation under-draw too.

Fit your first layer of foil sandwich Celotex between rafters. Fill any large gaps with ex-foam. Foil tape (75mm) all joints.

Fit your under-draw layer of Celotex. Fill any gaps (less likely with under-draw) with ex-foam and foil tape all joints.

Fit Duplex plasterboards.

Vaulty2

• noseall
• 15 May 2022

Vaulty1

• noseall
• 15 May 2022

Foil1

• noseall
• 2 Apr 2019

Foil3

• noseall
• 2 Apr 2019

 

[firapara]

Yes, but there are ways to (robustly) achieve this.

You will also need an insulation under-draw too.

Fit your first layer of foil sandwich Celotex between rafters. Fill any large gaps with ex-foam. Foil tape (75mm) all joints.

Fit your under-draw layer of Celotex. Fill any gaps (less likely with under-draw) with ex-foam and foil tape all joints.

Fit Duplex plasterboards.

Thank you,

The 150 is already inside between the rafters and the builders said the BCO is happy with this.

Do I need some layer of vapour control, or do I just tell them to tape everything, including the rafters?

 

[noseall]

Thank you,

The 150 is already inside between the rafters and the builders said the BCO is happy with this.

Do I need some layer of vapour control, or do I just tell them to tape everything, including the rafters?

We tape. It's a good method.

How are you dealing with cold bridge shadowing? All the Celotex (between) in the world, won't counter that.

 

[^woody^]

The 150 is already inside between the rafters and the builders said the BCO is happy with this.

I would check that. And if the BCO is "happy", I'd check that too. There should be no thermal bridge via the rafters.

Foil on insulation board is the vapour control layer. All joints and perimeter should be foil taped and gaps around the insulation filled.

 

[firapara]

Thank you guys

Have asked the builders to add a 25mm under-draw layer and tape it