Insulation of lean to garage/utility roof question

[BuggBairn]

Or more accurately a ventilation question about insulating the roof.

We have a single brick garage/utility room built on the side of our 60's bungalow.

My intention is to insulate the utility part of the structure with PIR board between the joists.

Having previously used this style of insulation fitting in another part of the house I'm aware that a ventilated 50mm gap between the PIR board and the timber roof is required to deal with any condensation created should warm air meet the cold.

I'm a bit tight space wise to cut holes and fit vents so I'm open to ideas/solutions/suggestions.

I've hopefully attached a couple of images to illustrate just how tight things are inside and outside the structure for vent installation.

 

[^woody^]

Stick the insulation on the bottom of the deck

 

[BuggBairn]

Stick the insulation on the bottom of the deck

Without a ventilated gap ? Would that not be required ?

 

[^woody^]

You wont get any through ventilation on that roof - you have not got abutment ventilation at the bungalow wall have you?

The idea of venting a roof is to allow moisture from air that that hits a cold surface to evaporate. If you remove the cold surface you remove the condensation risk. Insulation on the underside deck removes the cold surface.

 

[foxhole]

If the gap is full of insulation there is no air , cold or warm.

 

[BuggBairn]

Interesting, so those joists are 120mm deep. If I used 120mm PIR in those spaces how would I finish the ceilings ? Plain old plasterboard?

Also how does this differ from a cold deck style flat dormer roof ? I have insulated 3 rooms upstairs that I assumed needed ventilation so went about creating a 50mm ventilated void above the insulation.

 

[noseall]

Also how does this differ from a cold deck style flat dormer roof

It's considered a 'hybrid' and a bit of a cheat and is frowned upon by Celotex but accepted by Building Control.

We have just done the exact same thing and added another 40mm across the joists prior to adding foil back (Duplex) plasterboards. We foil tape all of our joints, creating a robust seal.