A moisture barrier under the iron is essential. 

you can actually see the run marks down the timber from the condensation. i have the same issue.

Ideally you need to take the iron off and start from scratch.

larknz:

 

Ideally you need to take the iron off and start from scratch.

 

Yeah that's not happening lol.

jonherries: If it were me I would line it (edit: the ceiling) - not with cardboard (that is insane), and then hold the insulation in with the lining.

I would avoid the roof purlins for the moisture issues (essentially the same as the crying windows problem). If the roof is ventilated, it should then be ok in the short term.

I wouldn’t underestimate how hard lining it might be - in the photo that roof tie looks like it will get in the way, you might need to batten it out a bit and pack it out to get it straight. Basic 7mm ply is $65 retail per sheet - and much cheaper with a farmlands or csc card which are easy to get (like 20-50% off for timber).

Jon

won't work. condensation will drip down onto the ceiling. roof purlins or ceiling doesn't matter, same issue.

Lias:

 

larknz:

 

Ideally you need to take the iron off and start from scratch.

 

 

Yeah that's not happening lol.

 

its the only way. can't pocket install the roofing underlay. you would need to pull the roofing nails then slide it under.

thats why i've put up with mine for so long. i will replace the roof and do it then.

One thing they do on many commercial sites is have two layers of roofing, with the insulation between them. I'm not sure on if it's an option for retrofit but it's an interesting thought. 

It's still a decent amount of work. 

Basically, a roof without at minimum building paper was never designed or built to be a heated or habitable space; it's there to keep the rain and wind off things. 

Scott Brown has a video on pulling a garage roof, laying paper, re-laying the roof, and repainting it. It's not a terrible amount of work but it's not trivial. 

So if I take the roof iron off, then put new tar paper or whatever between the purlins and the iron, will I then be able to put polystyrene/batts between the purlins? 

Polystyrene not recommended, batts yes. Use plastic strap and staples to give the batts the right amount of sag so that they're just clear of the paper. 

SomeoneSomewhere:

 

Basically, a roof without at minimum building paper was never designed or built to be a heated or habitable space; it's there to keep the rain and wind off things. 

 

It does have tar paper in maybe 3/4 of the roof, but a bunch of it is missing (I'd say it has been ripped down in the ~35 years since it was built)

If you are going to leave the roof space open after insulating I'd use polyester rather than fibreglass batts and definitely not polystrene.

Polyester isn't going to have fibres break if it gets a knock like glass batts and polystyrene is the devils insulation. It sucks to install and inevitably you'll have gaps. If you are going between the purlins use straps as advised above.

Lift the iron, put down some barrier and re-lay the iron is the best option. You can correct the fasteners at the same time.

Lias:

 

So if I take the roof iron off, then put new tar paper or whatever between the purlins and the iron, will I then be able to put polystyrene/batts between the purlins? 

 

i would look at polyester (not polystyrene). you can staple that to the purlins. that gives you the gap. or even under the top of the truss as you can get polyester in long rolls. as mentioned it won't come apart like batts does.