insulation

[Anonymous]

i have removed the plasterboard ceilings from our 1970`s flat roof single storey extension. my builder says i shouldnt insulate as the joists need airflow but my electrician says insulate to stop heat escaping. who`s right please?

 

[Anonymous]

The builder could be right but for the wrong reason
The electrician could be right but forgot to say if you keep the heat in you've got to keep water vapour in as well

It's not airflow per se that your joists need, just a totally dry environment in which to survive. Insulating your flat roof WILL result in damp joists due to condensation but you can alleviate the problem by covering the entire ceiling with a good sound unbroken thick polythene sheet before you nail the plasterboard back. Avoid ANY breaks, tape up any joints, take care at the edge, don't fit any lights which will create holes in the barrier. Nail holes for the plasterboard are just about ok in this scenario.

Ideally you would fit fibreglass an inch or so less deep than the joists - leave a gap on the cold side - against the underside of the flat roof deck - and ventilate it to the outside somehow. You can get little round push in soffit vents that would do the job.

For the avoidance of doubt, then, it's plasterboard (foil-backed would be best), vapour barrier (that blue polythene), insulation, air gap (preferably ventilated but if the vapour barrier is good you should be ok without), roof deck, sky.

 

[Anonymous]

Wind proofing is vital if you insulate.

depending on the type of insulation you use it is the only way to keep the moisture in the air from condensating in the insulation, (when the air goes through the insulation it gets colder and can only hold part of the humidity, which then condensates.)
The problem is that insulation takes humidity badly, insulating much less, and then attracting even more condensation from passing warm air.

Vapor proofing: not bigger the better.
the higher the vapour proofing is, the bigger the problems from holes are. Edges and points where the beams pass the insulation are critical points, if the vapor proofing film can't move (a bit), it can tear if the wood or panelling expandes or contracts due to humidity.
Ideal are adaptable vapor films, they let any vapor out into the interior during the summer. (Isover Varionorm for instance).

So it is decisive to have an experienced workman, architect check the details.
Greetings Andrew



Vapor control layer