Hi, we are currently self building an extension with standard block cavity walls on the ground floor and timber frame on the first floor.

We have an overhang over an alleyway that is supported on blocks one side and steel the other.

 

Do I need a VCL under the overhang section? The internal walls will be taped 30mm PIR 

 

This is a crude sketch of the detail I was thinking. Inner skin will continue to top of joists. Insulation in overhang floor will tie in with cavity insulation. 
 

I’ll also attach a photo for reference
 

Thanks

 

 

 

is that a cold roof or a warm roof ?

Hi Dave

 

The joists pictured are for the first floor. 
In effect the section under the overhang will be a cold roof upside down? Just don’t know if VCL should go below the joists or above.

 

The pitched vaulted roof above first floor will have 165mm PIR insulation with 30mm PIR taped to form VCL 

 

Don’t know if this will help? 

What's your buildup for the overhang as is? 

 

Have you considered batt material instead of the PIR between the rafters. Easier to fit, cheaper and no thermal bypass or gaps with shrinking timber. 

 

 

Thanks @Iceverge will consider that.

At the moment it’s just the full fill cavity insulation. Wanted to check the VCL detail before insulating. 

I have building control coming out Wednesday so will run both options past them but the non-shrinkage makes sense as this could be a big thermal bridge. 

 

 

If you use something like caberdeck flooring over the joists and are careful to glue all the joints with foaming glue this will do fine as a VCL. Use a flexible mastic at the perimeter. 

 

If you want to be belt and braces make an insulated service under the caberdeck and then a taped VCL. 

 

 

Thanks @Iceverge I appreciate your help.
We will be using caberdeck and the d4 adhesive.

VCL or equivalent above the insulation so the deck would work fine. What is closing off the walkway below .?? Cement boards ..??

Thanks @PeterW yeah will be clad with cement board underneath. 

I’m probably overthinking it, anything will be better than the solid uninsulated walls of the existing structure. 

 

 

I'm guessing at the joist spacing here and sizes. 

 

If you run 50mm battens at 90deg under the main joists (can't show it here) and then some 50mm insulation in-between it'll eliminate most of the repeating bridging.  Make sure the membrane is well taped as this will be a windy corner and you don't want wind blowing into your floor. 

 

22mm airgap and then cement boards. It'll be a good spec. If you use denser batts like Rockwool it'll make a difference with noise for the room above too. 

This is amazing, Thank you  @Icevergeyou were spot on with the joist size and spacing too. 

On 17/02/2023 at 22:22, Iceverge said:

If you use something like caberdeck flooring over the joists and are careful to glue all the joints with foaming glue this will do fine as a VCL. Use a flexible mastic at the perimeter. 

 

If you want to be belt and braces make an insulated service under the caberdeck and then a taped VCL. 

 

 

 

sigh. no this isnt a VCL. These bodges are all too common.

Minimum risk however as it is on the warm side of the insulation and everything is breathable outboard. 

 

Most damage occurs anyway I thought when vapour is carried through air leaks rather than diffusion.