Cold Flat roof - use liquid DPM underside of decking as a VCL?

[oranjeboom]

Due to circumstances I have ended up with a cold roof for front hallway/porch. During renovation of the old house, I noticed mildew on underneath of the decking due to condensation. There was quite a bit of humidity at the time when the old internal slab of the house was being dug up. Roof didn't have any insulation or VCL at this time so I can see how condensation occured between cold/warm surface of the roof deck. I'm now at the stage to insulate and plasterboard the flat roof and want to minimise any future risk.

 

I will have a 50mm ventilation gap between insulation layer and also a VCL beneath insulation layer as in this pic:

However, still a bit concerned about condensation underneath the deck itself, so was either going to stick some VCL/polysheeting directly under deck or use some liquid DPM instead:

 

 

 

 Painting on the DPM would be a lot easier and a bit safer as over time the spray adhesive with the VCL could fail causing the sheeting to come unstuck and block any airflow.  Any cons for me for using a liquid DPM instead?

 

I'd end up with 2 VCLs - good if the main VCL is punctured at some point (electrician / downlights).

 

[Ian]

@oranjeboom

generally speaking it's not a good idea to do what you've suggested. Normal practice is to have just one VCL which here in the UK is always located on the warm side of the construction layers.

 

The problem you might get if you install a second VCL in the location that you showed in red on the drawing is that it could trap any condensation that might form on the u/s of the roof deck. It's best to let the ventilation do it's thing in getting rid of any minor condensation that forms. BTW, any timber you use should be treated.

[Nickfromwales]

I would put full sheets of 25mm PIR under ( so on top of ) the joists to stop the cold bridge, and use foil tape to cover the joins and create your VCL. Two birds, one stone, and a far better job IMO. 

[oranjeboom]

56 minutes ago, Nickfromwales said:

I would put full sheets of 25mm PIR under ( so on top of ) the joists to stop the cold bridge, and use foil tape to cover the joins and create your VCL. Two birds, one stone, and a far better job IMO. 

Joists are already in place and so is deck. Didn't have that option as the joists had to be a specific size and the whole structure had to be higher than the lintel on front door and also fit under the sloping roof of the bungalow. But I will also have PIR under the joists which I will have taped up to act as another VCL.

 

1 hour ago, Ian said:

@oranjeboom

The problem you might get if you install a second VCL in the location that you showed in red on the drawing is that it could trap any condensation that might form on the u/s of the roof deck. It's best to let the ventilation do it's thing in getting rid of any minor condensation that forms. BTW, any timber you use should be treated.

I see that could happen with a standard VCL in the form of a sheet, where it may not stick to the underside of the deck. However, if I use a painted DPM instead, then surely no condensation can occur between the layer of painted DPM and the deck?

[Ian]

5 minutes ago, oranjeboom said:

I see that could happen with a standard VCL in the form of a sheet, where it may not stick to the underside of the deck. However, if I use a painted DPM instead, then surely no condensation can occur between the layer of painted DPM and the deck?

I still think you would be best not painting a DPM onto the u/s of the timber deck. IMO it would be best to allow the timber deck to 'breathe' by exposing it to your 50mm ventilation zone. 

 

My worry about painting the u/s of the deck with a DPM is that you'll be trapping any condensation that forms in the layers above it. I've seen the results of trapped condensation in roofs like that - when trapped water vapour in a flat roof gets hot it can exert enormous pressure which would be enough to blister your DPM and weathering layer.

[Nickfromwales]

1 hour ago, oranjeboom said:

But I will also have PIR under the joists which I will have taped up to act as another VCL.

That's what I meant ?

[oranjeboom]

On 31/05/2018 at 12:01, Ian said:

My worry about painting the u/s of the deck with a DPM is that you'll be trapping any condensation that forms in the layers above it. I've seen the results of trapped condensation in roofs like that - when trapped water vapour in a flat roof gets hot it can exert enormous pressure which would be enough to blister your DPM and weathering layer.

 Thanks Ian - I will proceed in that manner then. Thanks for the advice.