Insulating A Garage Flat Roof.

[longshot]

I like to do odd jobs on the car in the garage throughout the winter but even with a small fan heater, it can get very cold.

I would quite like to insulate the roof to hold the heat in better but I know there can be condensation problems if not done properly.

It is your typical flat roofed garage.
3 single course brick walls, door on the end and a roof like this....



...but without the small bits of wood running down the middle.

Money is tight so I'm trying to keep this nice and cheap if I can.

What I was planning on doing is fitting loft insulation into the gaps between the joists and holding it on position in some way and then sealing the whole underside of the roof with a large sheet of heavy duty polythene, holding it in place with heavy duty staples.
I know it's not going to be pretty.

What I wondered is would this method cause any condensation issues or would it work correctly?

Many Thanks.

[wolfracesonic]

That's the right way you've described for a 'cold deck roof', vapour barrier on the warm side of the insulation: if you can get some ventilating airflow between the top of the insulation and underside of the flat roof all the better(a 50mm gap is usually asked for), though you're not going to have as much moisture build up in garage as you would in your house i.e no showers, baths, cooking etc.

[longshot]

wolfracesonic said:

That's the right way you've described for a 'cold deck roof', vapour barrier on the warm side of the insulation: if you can get some ventilating airflow between the top of the insulation and underside of the flat roof all the better(a 50mm gap is usually asked for), though you're not going to have as much moisture build up in garage as you would in your house i.e no showers, baths, cooking etc.

Thanks for the reply.
I'm pleased to hear my method is along the right lines.

Ventilation would be difficult to create so I was hoping that my sealing method would not require it. One side of the garage is very much hemmed in by next door's house being practically next to it and (forgot to say) the other side is our house. The garage is built onto the side of our house.

Thanks

[V8RX7]

My only issue is if there is any chance of the polythene catching fire.

Hence I'd like to see a plasterboard fitted over it (cheapest at Wickes)

[longshot]

V8RX7 said:

My only issue is if there is any chance of the polythene catching fire.

Hence I'd like to see a plasterboard fitted over it (cheapest at Wickes)

Thanks for the reply.

I hear you and that may ultimately happen but in the meantime this way lets me get on with it and also confirms or dismisses any suspicions I have about condensation as the polythene will let me visually see what's going on up there.