Warm Roof, No VCL

[Lhes30]

Hi,

Currently having a flat roof extension built on to the back of the house which has a "warm roof" on top.

The roof is finished to GRP level, The build up was completed by the main builder and the GRP applied by a separate roofing contractor.

Whilst finishing the GRP it was pointed out that the build up was incorrect and VCL layer had been left out on top of the OSB3 below the insulation level, the current build up is as follows (inside out) -

1. Joists
2. Furrings
3. OSB3
4. Celotex foil backed insulation
5. Kingspan foil backed insulation
6. OSB3
7. GRP

The builder missed the VCL between 3 and 4, As its impractical to remove the full roof this close to Christmas (which I'd prefer!) we're trying to come up with options to add in a vapour barrier below number 3 (internally).

The builder suggested adding 1 inch kingspan between the joists and taping but from reading up this would cause issues creating a hybrid roof system, The only plausible option offered by my architect other than to remove and re do seems to be to add a VCL (Visqueen or similar) continuously across the underside of the joists, Add 50mm battens or similar along the lengths of the joists to hold the VCL in place, attached foil back plasterboard to the battens to create a secondary VCL which creates a void to run cables and downlights without damaging the new VCL layer.

With this method the build up would be as follows -

1. Foil backed plasterboard
2. 50mm batten
2. VCL (Visqueen or similar)
3. Joists
4. Furrings
5. OSB3
6. Celotex foil backed insulation
7. Kingspan foil backed insulation.
8. OSB3
9. GRP

Would this be a correct way to preserve the new roof? Or can anyone suggest another option, Although not ideal it seems like a best case scenario currently.

Can anyone advise if the VCL being relocated from between the OSB and insulation to below the joists is likely to cause issues within this area trapping moisture or causing rot, mould or damp etc to form anywhere above or below it?

Thanks in advance, I am not a builder so apologies if my terminology is wrong on anything above.

[Canski]

18 minutes ago, Lhes30 said:

Whilst finishing the GRP it was pointed out that the build up was incorrect and VCL layer had been left out on top of the OSB3 below the insulation level

Isn’t it just great when someone tells you when it’s too late.  🤯my plumber told me that the flush pipe he needed was inside the concealed cistern behind the tiled access panel just after I had just grouted the tiles.  

[G and J]

[ALERT - THEORY BASED RESPONSE SO NO DIRECT EXPERIENCE AVAILABLE - CORRELATE WITH ANSWERS FROM GROWN UPS!]
 

I’ve been researching flat roofs for ours.  My understanding is that you are creating a wet pocket, and that will rot your roof timber.  
 

If it’s a temporary measure to reduce how damp it gets over Christmas before stripping the roof maybe it’s a good idea, but I’m not sure.  
 

I wonder if your roof would be better left so moisture can evaporate into the building, as I wouldn’t think it would get out via the GRP at all.  Better in this sense means taking longer to rot.  
 

If you throw a few bodies at it will it really take that long to remove and replace?  If not why not forget the plasterboard inside, etc. and cover the joists with baubles and tinsel then give your renewed roof a happy new year?

[Mr Punter]

Paint on the ceiling and no holes for downlights will help prevent moist air making contact with the OSB.

[Lhes30]

Hi all,

 

Thanks for the comments so far, It is great when someone tells you something after its been completed ha!

 

@G and J, What you're saying does make sense but I've been assured that the method of using a continuous barrier under the joists / rafters (unsure of the terminology in this case) Would prevent any moisture from the building entering this space OR the roof, So subsequently no wet pocket to cause rot, Unless the moisture you're referring to is coming from somewhere else that I am over looking?

 

The idea behind what the architect has come up with is the same method used when insulating a "warm" pitched roof, the VCL layer is mounted underneath the rafters / joists in this scenario with plasterboard finishing going directly over this layer, There is no issues with rotten rafters / joists when this method is used so unsure why there would be on my flat roof? I hope that makes sense its confusing me but trying to get the best scenario for the situation ha. 

 

Cheers 

[Lhes30]

@Mr Punter, There will be holes for downlights which is why they're creating a 50mm void after the plasterboard before the continuous vapour barrier (plastic sheet), This will allow me to put the downlights in the plasterboard and for the electrician to run cabling without damaging or otherwise puncturing the vapour barrier layer. 

[Mr Punter]

To put your mind at rest you could do a condensation risk analysis.  I don't think you will have any issues as there are 2 layers of foil backed insulation, so as long as they are snug round the edges you should be fine.

[Lhes30]

@Mr Punter Thanks for the reassurance, That is the case with the insulation, If you were in my position would you be inclined to not fit the vapour barrier below the joists and just plasterboard over the top leaving the warm roof without a VCL?

 

I keep getting contradicting information from different sources, When asked the question Kingspan have emailed to say no VCL would be needed at all due to the foil backed insulation, Whilst my architect says that we must install one somewhere (hence putting a layer under the joists) Building control are no help we've had one chap say you definitely need one, and another one say that they're happy with just the two layers of foil backed insulation. 

[Mr Punter]

I don't think there will be any issue.

 

[Canski]

Rats !! I could have saved that Alutrix money. 🙂

[Lhes30]

@Mr Punter Thanks for that, Bit reassuring ha!

 

I've told my architect what you've shown me, He has said it might be wise to still install the VCL between the 50mm batten and the joists as it will do no harm but will act as a back up just in case. They're planning on fitting this tomorrow.

 

Since you're obviously way more clued up than me, Can you clarify if this plastic sheet whilst actually being redundant according to your software, would cause and harm by trapping moisture above the plasterboard and below the OSB3 sheet (in the unventilated joist area). 

[Mr Punter]

I don't think it will make much difference either way.  The condensation point is nowhere near the joists.  The aluminium on the insulation will help reduce vapour.