Insulating Garage - DPM or VCL for underside of joists?

[LancsRick]

The floor is in, the material is here for the walls, and I'm now wondering if I'm about to make life harder for myself with the ceiling than necessary...

Floor - concrete base (with DPM), 50mm studs with 50mm PIR, DPM over the top, 22mm chipboard.

Walls - Single brick, VCL, 50mm studs with 50mm PIR, DPM over the top, 18mm OSB

Ceiling - 150mm rockwool between joists, then some sort of membrane, plasterboard.

The question is the membrane on the ceiling. I was originally going to put up DPM, but I hadn't appreciated how heavy and unwieldy the stuff is, plus I can't help thinking VCL would do the job I need it to (it's an apex roof, I'm working on the horizontal joists). There seems to be a real lack of information on the net about doing this aspect so I'd be grateful for any views or experiences!

 

[MikeG.]

Whoa.....hang on a minute. Can you go through your wall build-up again, carefully, in order, starting from the inside and working out, and including cavities.

For the ceiling, foil backed plasterboard is fine as a vapour barrier. Lighter weight polythene is also OK (600 gauge, or less), but the heavier one (1200 gauge) would be pretty near impossible to handle for that task.

 

[LancsRick]

Yea I've got 1000gauge which is going to be a real pain! The plasterboard isn't foil backed unfortunately.

Walls are.

Inside ---- Outside

18mm OSB, DPM, 50mm studs with 50mm foil backed PIR between, VCL, single brick.

 

[MikeG.]

Can't you swap the plasterboard? You'll really struggle with that plastic.

Or else, put an OSB ceiling up, as that's what you've got on the walls anyway. With OSB you don't need a vapour barrier....and that applies to your walls as well. The only thing that bothers me a little about your walls is that I would have preferred a gap between the bricks and the studs. Even 15mm would be better than nothing. As I said, omit the vapour barrier from the walls because of the OSB. If you haven't bought the OSB yet, you don't need it 18mm thick. If the studs are reasonably well fixed (ie it isn't floppy), then 10mm is fine. And if you are going to do the ceiling in OSB as I suggest, then you certainly don't want to be trying to fit 18mm thick stuff up there, unless you have a board lifter.

Sounds like you are lining out an existing garage. Is that right?

 

[LancsRick]

Thanks Mike.

I'll probably grab some thinner plastic as I agree it's probably going to be a pain. I definitely need DPM up there and not just VCL?

I debated that but reading around I decided I should be ok - it's treated timber and the VCL should further add to the robustness.

18mm is purely so I can hang stuff off the OSB wherever I choose to without risking any fixings pulling through.

Yea, existing double garage, enjoying doing it but dreading the ceiling!

 

[MikeG.]

If you aren't in there much, you have a ventilated roofspace, and you aren't heating the workshop hugely for long periods of time, you don't need anything.

There is some terminological confusion going on here, too. What do you mean by vapour control layer?

 

[LancsRick]

I definitely want a ceiling in there.

VCL is the membrane that will allow vapour out but not allow water ingress. As far as I'm aware it's a standard term?

 

[MikeG.]

VCL is indeed a standard term, but as I suspected, you aren't using it correctly. You are applying it to a breather membrane (Tyvek Housewrap, for example). A VCL is not the same thing. The VCL is sort-of an updated version of a vapour barrier..........it goes on the inside of the wall, where a vapour barrier always went. As you are proposing a good old-fashioned vapour barrier in the walls , then you have already done the VCL's job. You just need the breather membrane on the outside.

As I said, you don't need a vapour barrier if you are using OSB because OSB is very nearly vapour-impermeable with all the glue involved in its construction. You need the breather membrane on the outside of the walls to keep the studs/ battens dry from contact with the back of the bricks in driving rain......but you wouldn't really even need this if you left a gap between the bricks and the timber, albeit you are probably better off having it in place..