I want to insulate a garage roof that I had covered with fibreglass onto board, attached to rafters I put up. I want to
add insulation between rafters 500 mm with gap between it and the roof. Currnetly the roof does not condensate but slightly worried if I add foli backed insulation it may. Any thoughts very welcome.

Also use foil back plaster board or other vcl before plaster boarding.

If you insulate the roof will be slightly colder and so more likely to gather condensation this can be drawn from indoor or outdoor air and ventilation is there so that it can disperse. A vapour barrier will reduce but not eliminate for cold roofs. Consider doing warm roof on top.

I live the idea of 500mm insulation - that is what I have in my loft

Posted By: tonyaThe fibreglass has been fitted (not be me-i Traded it for doing his website!). So I can't remove it. It overlaps the facia board and hangs into or just above gutter so I can't really modify that side with an air vent.

Not sure who you're replying to, but I don't think anybody has been suggesting removing the fibreglass. I certainly wasn't and I don't think Tony was either. Insulation on top of the waterproof layer is called an 'inverted roof' and is what I was talking about. Yes, the insulation gets wet, but it doesn't care.

Since I wanted toi insulate inbetween all the joists as opposed to running across joists (merely for aesthetics as I want to see the joists)

Doesn't sound like a good reason to me. If the particular aesthetics are of such a concern to you, then do the insulation first and add extra 'joists' afterwards where they will be visible.

Maybe to be safe I could just insulate one area between two joists and wait and observe-just screw the plasterboard and take down and have a look.

I'm not sure that will tell you much as it may be difficult to guarantee that the conditions inside your sample are the same as will be if you did the whole job.

Tonya,

you write: ''I want to
add insulation between rafters 500 mm with gap between it and the roof. ''

Unless you have among the fattest rafters I have ever seen, do you perhaps mean 50mm? 500 + an air gap would be *two feet*. Is that really what you mean?

If you have a 'warm roof' you don't have to worry about ventilation. ventilation is to whisk away water vapour before it can condense on the underside of the wooden deck. If you have a 'warm roof' - the insulation above, rather than below, the deck - the deck is in the warm zone and 'safe'. You don't need a gap. A true (not 'hybrid') 'warm roof' has no insulation under the deck. Does that make sense?

<blockquote><cite>Posted By: Nick Parsons</cite>Tonya,

you write: ''I want to
add insulation between rafters 500 mm with gap between it and the roof. ''

Unless you have among the fattest rafters I have ever seen, do you perhaps mean 50mm? 500 + an air gap would be *two feet*. Is that really what you mean?

If you have a 'warm roof' you don't have to worry about ventilation. ventilation is to whisk away water vapour before it can condense on the underside of the wooden deck. If you have a 'warm roof' - the insulation above, rather than below, the deck - the deck is in the warm zone and 'safe'. You don't need a gap. A true (not 'hybrid') 'warm roof' has no insulation under the deck. Does that make sense?</blockquote>

Yes this makes sense entirely and yes 50mm.
Its too late adding insulation above 'the deck' as the fibreglass has been added.
My feeling is that if I insulate below the rafters- I have some rolls of multi foil that I could use-and then plasterboard over the fil it will not condensate. I used similar method in a annex in an old building in Dorset I owned and there was no problem with condensation. I think the fibregalass together with the substarte boarding won't get so cold as to cause an issue. My experience is that concrete block work causes issues of condensation especially around window reveals. Given that I have the foil more than enough- it won't cost ma naything to do the work and will only take a day max. I can then observe the outcome.

Is this garage heated, any source of moisture inside?

If not, then no problems with condensation on the OSB deck even on starry nights, it won't mind.

If so, 50mm of multifoil will not be enough insulation value. Try laying insulation boards on top of the fibreglass, and clad over them with something just to protect from wind and rain.

How were you thinking of ventilating the gap between the rafters?

<blockquote><cite>Posted By: WillInAberdeen</cite>Is this garage heated, any source of moisture inside?

If not, then no problems with condensation on the OSB deck even on starry nights, it won't mind.

If so, 50mm of multifoil will not be enough insulation value. Try laying insulation boards on top of the fibreglass, and clad over them with something just to protect from wind and rain.

How were you thinking of ventilating the gap between the rafters?</blockquote>

Garage is not heated. There are two 'garages'. One well built and this one adjoining it, not so well built. It had dodgy roof without any support so i took it all off and layed rafters. Its all supported properly.
I cut a hole between the two garages to open it up. There are signs of damp getting throught the wall of the one I am working on. There is only one wall of blockwork and no damp proof course. I could easily install an internal wall using timber and drop it down from the rafters (not fix to block work) I could insulate in between rafters and puts some vents in because water will build up in the gap between the two.

In answer to your query, there has been no condensation under the roof on the OSB. However if i were to increase insulkation it could create an issue.
I may use this 'gagare' as secondary and put a door in the section I cut out between the two and then develop the other area to a better standard.