Builder has not put in VCL in my flat roof, what can be done?

[Cormac Foley]

Hi all,

 

I'm having a 4m x 5m garden office built at the moment (cavity wall construction).

 

My builder hasn't constructed my warm roof as per the detail I provided him (LABCwarranty.co.uk warm ventilated roof detail).

 

He hasn't completed the construction of the roof yet, is there any way of fixing it without having to strip off battens, insulation and the breather membrane he's put in the wrong place?

 

What he has done so far is (from underneath up):

• OSB3 deck.
• Breather membrane.
• Foil backed 100mm rigid insulation (joints are not taped).
• 1st layer of battens fixed through insulation into rafters.

 

There's 225mm x 50mm rafters underneath the OSB3 deck, could the VCL be installed here although I'll have spotlights and electrics running in this void.

 

The finish will be an EPDM membrane on top of the higher OSB3 layer.

[Cormac Foley]

I was considering 2 options:

 

A: run the vapour barrier along the underneath of the joist, up the side of the joist, across the underside of the OSB, down the side of the next joist, under the joist etc. Then stick it to the block wall. I could notch the joists where required in advance for electrics. Then use a butyl tape on the underneath of the joists when installing the plasterboard.

 

 

 

 

 

B: add further insulation between the joists and underneath the OSB. Then install the VCL under the joists, up a bit of the sides of the joists to leave room for spotlights and services, across the underneath of the insulation, down the side of the next joist, etc. Again using butyl tape on the underneath of the joists when slabbing and sticking the VCL to the walls.

 

 

 

 

 

 

 

[Temp]

Yes just add a vcl below the rafters and before plasterboarding.

 

Perhaps seal any gaps between roof and walls before fitting the VCL.

Edited November 3, 2023 by Temp

[Mr Punter]

The battens and 25mm airflow path means that it is not essential to have the VCL.  The insulation is not bridged by timbers so all looks good.  150mm insulation would have been better, but what you have looks OK.

[Cormac Foley]

thanks for the responses guys.
 

In relation to going with additional insulation, do you see any issues with adding additional insulation under the lower OSB deck and above the ceiling/potential VCL layer, so that I would have a sandwich of insulation-OSB-breather membrane-insulation, like I have shown in (insulation shown in orange but not annotated!) in the second image in my last post?

[andyscotland]

That would turn it into a hybrid roof, which can be risky. You would need to do a u-value calculation & condensation risk analysis to make sure that the joists & OSB stay warm enough to avoid any condensation within them.

 

A small bit of extra insulation may be ok, but I'd want to be confident I had a good undamaged VCL below the joists was not punctured.

 

If you were doing that and therefore adding a VCL below joists, I'd go with your idea B but you probably don't need to fuss about taking it up the joists a bit and the extra staple holes that will create. Just let it drape down when fixing so you have enough slack to push it upwards to sit on services/downlights where needed. Assuming you pre-cut downlight holes before the plasterboards go up and use LEDs / a cap to prevent heat damage to the membrane.

 

If you go that route, just take services up and down the joist gaps and go across at the top of the wall if needed, to avoid having to notch anything.

[MJNewton]

With regards to the design detail why is there ventilation? You wouldn't normally have it in a warm roof construction. 

[Temp]

+1 Hybrid roof has risks because it moves the structural wood towards the cold side.

 

Ideally you would need a condensation risk analysis done.

 

I have heard recommendations that no more than 1/3rd of the total insulation should be below.