water on underside of osb

[redtop]

6 hours ago, Dave Jones said:

 
pretty much all wrong.

 

the vcl is there to stop moist air from inside migrating into the insulation. How you going to do that without lapping it down the walls?

 

you could have a parapet and lap it up that but don’t see many sheds with a parapet.

 

the whole point of a warm roof is ventilation is not required as it’s a sealed sandwich by the vcl from below and the covering above.

But you don't seal anything that can rot, ideally as you will get damp from condensation, and wood always has moisture in. Anyhow its our roof was specified and I went online and looked at a load of others and I didn't see any done like you recomend, is there anything you can point to that shows the above approach?

[Dave Jones]

Warm roof rot lol

 

 

[Nickfromwales]

17 hours ago, Gordo said:

I wouldn’t recommend timber between insulation and EPDM. It will get damp from condensation and be vulnerable to decay.

That's how most SIP's roofs are done. There needs to be a solid deck to walk on to be able to EPDM a large flat / sloping roof, as the compressive ( point load ) strength of the rigid insulation would be insufficient to take a mans weight ( foot / knee etc ). Also a requisite when installing PV with Nickelson plates etc as you would also not want to introduce cold bridging through 250mm+ long mechanical fixings. 

The detail there needs to be; a compressed sandwich with layers bonded ( where there can be no ventilation to ridge etc, eg no gap between the insulation and the deck board + EPDM ) or counter-battens and a gap for ventilation and then the deck board + EPDM.  

[redtop]

13 minutes ago, Nickfromwales said:

That's how most SIP's roofs are done. There needs to be a solid deck to walk on to be able to EPDM a large flat / sloping roof, as the compressive ( point load ) strength of the rigid insulation would be insufficient to take a mans weight ( foot / knee etc ). Also a requisite when installing PV with Nickelson plates etc as you would also not want to introduce cold bridging through 250mm+ long mechanical fixings. 

The detail there needs to be; a compressed sandwich with layers bonded ( where there can be no ventilation to ridge etc, eg no gap between the insulation and the deck board + EPDM ) or counter-battens and a gap for ventilation and then the deck board + EPDM.  

Nick, I get that approach, makes sense. But I have never seen an approach where you fix the vapour barrier to the underneath of the joists! It seems to me you would end up with a large area on the warm side with no ventilation that you cant seal properly and then how would you get the plasterboard fixed without making a thousand small holes. And what about running cables etc through the joists making more holes in the membrane. And, as its a shed so maybe not being plaseterboarded you would probably want to use the ability to hang stuff of the joists. Cant get my head around it

[Nickfromwales]

1 hour ago, redtop said:

Nick, I get that approach, makes sense. But I have never seen an approach where you fix the vapour barrier to the underneath of the joists! It seems to me you would end up with a large area on the warm side with no ventilation that you cant seal properly and then how would you get the plasterboard fixed without making a thousand small holes. And what about running cables etc through the joists making more holes in the membrane. And, as its a shed so maybe not being plaseterboarded you would probably want to use the ability to hang stuff of the joists. Cant get my head around it

Yes, totally agree. 
I’ve made no reference to VCL, as I’d need to know about detail of the rising walls too to be able to comment. 
If it’s a draughty shed, like my home office / man cave, I’d just deal with the worst of the cold bridging and open a beer. Adventitious airflow will do the rest. If folk don’t allow air through in winter then the ? will hit the fan regardless. 
I’ve gone with metal uninsulated roof panels, one piece 6.4m lengths, over 5x2’s @400mm centres. I used roof membrane 1500mm wide and tripled it ( overlapping every 500mm ) as I couldn’t get the metal in time. Bone dry atm, with joists ( treated ) exposed and painted with the grey floor paint. 
As it’s Baltic in there, during the warmer weather I will install 75mm in between the joists, with a 25mm timber ( like a doorstop ) to allow me to push the insulation up and prevent it touching the membrane. I purposely kept the membranes very taut during fitting in case this was necessary.
I’ll then install 30mm PIR in 8x4 sheets over the joists and infill PIR to kill the cold bridge of the joists, and then foil tape, and prob then just paint white or leave as foil ( as both will reflect light ). Joints between 8x4’s of PIR will be foil taped for the hell of it. That will be my lot for the roof. 
I will then pull off the external membrane wrapped around the outside of the stud frame / walls, I’ve not yet installed the 150mm feather edge thankfully, and fill those voids with cheap rock wool and then reinstall the membrane and fit the cladding at the same time so it stays dry.

 

“It’s just a shed, Rodney”. 

Edited January 27, 2022 by Nickfromwales

[Gordo]

On 27/01/2022 at 11:29, Nickfromwales said:

The detail there needs to be; a compressed sandwich with layers bonded ( where there can be no ventilation to ridge etc, eg no gap between the insulation and the deck board + EPDM ) or counter-battens and a gap for ventilation and then the deck board + EPDM.  

The cold timber deck between EPDM and insulation will be vulnerable to decay there is no doubt. To do this properly their should be a ventilated void between the deck and the insulation in your second option. You have been warned ? impermeable weatherproofing layers are usually laid direct onto rigid insulation from my experience. No issue with walking on insulation doing this?

Edited January 28, 2022 by Gordo

[Dave Jones]

26 minutes ago, Gordo said:

The cold timber deck between EPDM and insulation will be vulnerable to decay there is no doubt. To do this properly their should be a ventilated void between the deck and the insulation in your second option. You have been warned ? impermeable weatherproofing layers are usually laid direct onto rigid insulation from my experience. No issue with walking on insulation doing this?

 

this is not a warm roof. 

[Gordo]

13 minutes ago, Dave Jones said:

 

this is not a warm roof. 

Oh ? I thought he mentioned SIP panels roof under EPDM and your last post here discussed a warm roof. In that case and it’s a cold deck roof ignore all I said ? But advice stands if it’s a wam deck roof.

Edited January 28, 2022 by Gordo