Wind proof above fibreglass

[Huckleberrys]

Hi. I am soon to be undertaking a small barn conversion. I am just in the process of sorting some bits out. In the loft I will be having fibreglass insulation...would it be good practice to provide a windproof membrane on the underside of the the rafters so I don't have wind blowing up and over the top of the insulation? 

[Iceverge]

What is your roof construction as is? 

 

It's easier to tape a suitable breathable roofing membrane above the rafters. 

[joe90]

3 hours ago, Huckleberrys said:

Hi. I am soon to be undertaking a small barn conversion. I am just in the process of sorting some bits out. In the loft I will be having fibreglass insulation...would it be good practice to provide a windproof membrane on the underside of the the rafters so I don't have wind blowing up and over the top of the insulation? 

I once read (but not seen since) that paper or similar laid between layers of fibreglass insulation stops wind/air washing between the layers giving better insulation performance .

[Iceverge]

https://www.eco-home-essentials.co.uk/how-to-install-insulation.html

 

There's some reference to this method from this company. 

 

 

[Huckleberrys]

It had a new roof put on 12 years ago. Slate, baton, membrane, rafter, purlin, a frame. I was thinking about putting some membrame under the rafter so the rafter depth would be a ventilated void but then wind proof the area insulated then air tight under insulation.

Edited April 22, 2023 by Huckleberrys
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[Radian]

28 minutes ago, Huckleberrys said:

It had a new roof put on 12 years ago. Slate, baton, membrane, rafter, purlin, a frame. I was thinking about putting some membrame under the rafter so the rafter depth would be a ventilated void but then wind proof the area insulated then air tight under insulation.

It's still a gamble if you get condensation or not. If this happens the main loft space won't be adequately vented.

[Iceverge]

How good is the vapour control layer below the insulation going to be? 

 

There's a risk with this approach if you don't get it 100%. 

 

We have a sealed cold attic as per the TYVEK detail and the attic has zero condensation but I did the airtight/vapour control membrane myself and got it (and tested it) to zero leaks. 

 

 

 

[Huckleberrys]

Vapour control layer should be good...I will be doing it myself. Will also have mvhr.

[Iceverge]

Use a suitable breathable membrane then and go for it I say. Don't forget about the airtight attic hatch.