I'm designing a timber frame utility building with a flat green roof, probably cold roof:

- green roof
- waterproofing
- 18 mm plywood structural deck
- firrings to provide ventilated void
- breather membrane
- 18 mm OSB sub-deck
- 225 mm open web joists
- insulation of some sort between joists
- probably plasterboard ceiling

It will house inverters, batteries, meters so nothing will produce moisture but the building is to be insulated to mitigate heat/cold and the electrical equipment will presumably produce a little heat.

So question is, on the basis that it will be warmer inside, should we be fitting a VCL to studs and to underside of open web joists, even though no moisture is produced internally?

Tony, it'd be helpful if you explained why.

The indoor air will presumably usually be slightly warmer than the outdoor air so slightly lower density but with the same pressure and specific humidity as the outdoor air and somewhat lower relative humidity. Whatever, there won't be much to push water vapour one way or the other through the wall so it's difficult to see what function a VCL would perform.

I'd wonder about water vapour coming up from the ground, though.

Roofs get warm when the sun shines and then they dry towards the inside. Leave it all vapour open, for me.

Especially for outbuildings without the regular heating and ventilation cycles you get in a house.

What does the breather membrane do for you?

Edit: Any reason for the cold roof - why not combine the OSB and ply layers, membrane on top, polystyrene, soil and plants on top of that? The underside of the ply and joists vapour-open to the room, warm and dry.

If you have a concrete floor, where will the construction moisture dry out to?

Posted By: WillInAberdeenRoofs get warm when the sun shines and then they dry towards the inside. Leave it all vapour open, for me.

That's my inclination too. But maybe a VCL below the floor deck. Did wonder if it should have a wall vent or two too.

Inside the building the temperature will vary from day to night

The moisture in the air can condense on cold things. The roof and walls can get very cold on the cold side of the insulation. And reach dewpoint.

Once this happens any moisture in the air in the building will be literally sucked or siphoned to those places, without a vapour barrier it cant be stopped. I call it pumping.

No longer relevant how much moisture there is in the air or the RH or partial vapour pressure it will head for dew point places.

Posted By: ShevekI switched to a warm roof detail, but how best to link a VCL in the wall with the VCL to the warm roof deck? The timber frame manufacturer/installer wants to use 120 mm PIR off cuts to insulate the wall.

I presume that's the first of your diagrams? And that there should be a red line (which I presume to be the VCL?) as in the second diagram? How are they going to use rigid insulation offcuts to insulate in and among open web joists? I could believe quilts and I could believe blow-in insulation such as warmcel or EPS beads or blow-in foam. The VCL will be whatever is used to hold the blow-in insulation in place, or on top of it. It'll be a royal pain to build I would have thought, with lots of cutting and taping to be done.

Joist hangers sound like a sane alternative, or add a ledger beam below the joists.

They also want a sub-deck below the firrings so that they can use it to safely install the firrings

Why can't they safely install the firrings from a step ladder or a scaffold tower?

Posted By: Shevek

Why can't they safely install the firrings from a step ladder or a scaffold tower?

It's a bit weird right? It's only 2.5 m high. I think I'll send them this and see what they say.

It's just occurred to me that there's no reason in principle why they can't attach the firrings to the top of the joists on the ground before they install the joists (assuming that's where they're planned to be fitted).

Posted By: tonyLooks like too little fall on roof and not fail-safe edge detail

What fall do you think is correct, Tony? What detail would you suggest?

Why don’t joists sit on the walls

Which diagram are you looking at and have you read the messages?

Posted By: djhIf it's a new building then I'd expect building regs for floors would prevent this. Our garage has a DPM for example.

Depends on size of building if regs apply.

To give you an idea I have taken over an outside WC constructed of cavity wall block work 50 mm insulation on a concrete base with DPM to house similar equipment has mono pitched slate roof. The temperature in there in the summer can get quite warm even thouhg the building is in the shade as the inverter when working hard gets to 40C and will control the solar input so it does not exceed this but different makes will be different I guess Currently this time of year typically tops at about 25 to 30C. The battery about 25C in summer currently about 15. I was concerned at first it might get too cold in winter so have a heater in there as well. The main thing to be wary of is RH so have a window I can open if need to control that. The inverter will be happy to 95% RH and battery to 80% so I would check the spec of the equipment that will go in your building and make sure you can meet that requirement. On a damp day last week I checked the RH and it was 60 to 65% at temp of 14C.

I should have mentioned that there may be a requirement for the equipment to be within a certain distance of the connection to the grid to avoid an unacceptable voltage drop to the inverter. Inverter VD specs can be more demanding than "regular" equipment where 5% VD may be acceptable inverters need 1.5% or better depending on manufacturer. Some will not give a spec one model i was considering just stated as close as possible but would not give a specific spec.