Flat roof insulation.

[Mickfb]

Hi all
I am in the process of lining out my garage. I have been reading that if you have a cold flat roof you have to leave ventilation between the roof deck and the insulation of 50mm. Now this is not a problem, but they say you have to ventilate the roof at both ends to the outside air. I can't do this as the garage is attached to the house on one side. Also I would need a vent between each joist. Has anyone done this without any ventilation. If yes have you had any problem with condensation.

Many thanks
Mick

 

[MMUK]

I did something similar on a garage conversion a few years ago, just using 100mm Celotex instead of 50mm. The joists were 6x2s and I laid the Celotex in flush with the bottom edges, leaving a 50mm gap to the ply. The wall plate end was attached to the house so in this 50mm void I cut four 20mm holes about 150mm apart into each joist to allow air circulation. At the eaves end I fitted a vented soffit board.

 

[Mickfb]

Thanks MMUK. Looking closer I can't even put vent holes in the fascia. I think I shall leave it as it is at the moment. The existing felt has been their for at least 15 years, so can't be far of needing replacing. So look into having a warm roof installed.

 

[MMUK]

If there's nothing majorly wrong with the roof, have you considered having a fibreglass resin overlay? You can deal with the insulation whenever suits afterwards.

 

[Mickfb]

Would that still leave with the problem of condensation between the deck and insulation.