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How best to insulate a flat roof of a brick built shed?


I have a large brick built shed in the garden with an OSB/Felt flat roof with exposed rafters below. I am currently converting the shed into a workshop and would like to insulate the roof as cost effectively (yet efficiently) as possible.

My initial plan was to shove some celotex type boards inbetween the rafters and then plasterboard over it. But after some research I learned that you need to leave an air cavity of at least 50mm which needs to be cross ventilated.

The problem is, the rafters don't have any ventilation so I'm worried about condensation rotting the roof timbers.

What's the best solution that will be relatively inexpensive - after all, it's only a shed / workshop not a permanent living space. My current thinking is:

1. Use a multifoil, stapled to the underside of the rafters, batten across and then plasterboard - But should I still have ventilation in the rafters?
2. Drill holes in the fascia and install the small round soffit vents with an insect mesh. Then fit celotex or something with a 50mm air cavity, followed by a membrane and plasterboard.

Are there any other options? I'm not keen on creating a warm roof solution - seems too much work and too expensive for a space that isn't used all the time.



1 Answer from a MyBuilder Insulation Installer

the flat roof part is easy,because you have exposed rafters below,use 150mm knauf earthwool loft insulation ,put it between the the rafters,and use a insulation netting ,stapled to the rafters to hold insulation in place,then plasterboard the ceiling.
Then for the inside of your garage ie walls you could do internal wall insulation... fix battons to the walls,wooden or metal,then once you have attached battons,fix 25mm P.I.R board to battons ,then plaster board over top of insulation board and screw through plaster board in to battons to hold in place...hope this may be help full to you. kind regards Chris (outwood insulation ) wakefield.


Answered 22nd Nov 2016

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