Flat Roof insulation

[paulontour]

Hi
After some guidance if anyone would be so kind.

I have a flat roof section on my garage, about 15sqm at the back. The front of the garage has a pitched roof.

The interior section underneath the flat roof is split into 2 rooms, seperated by a double door and a breeze block wall. I want to make the first bit (which is the middle of the garage) into a play room, and the rear section into a utility room.

It is an attached garage, with one side against the house.

I am about to get the flat roof redone with a 3 layer felt system but we cannot afford to make into a warm roof.

My question is - can i just FULL FILL the insulation between the new plasterboard ceiling (not done yet) and the roof? If so, what insulation is best (and cheapest!)

Do i need a vapour barrier? Or do I need to leave a gap and vent the roof space somehow? Problem with venting is I have no soffits.

Thanks!

 

[tony1851]

You can't full-fill the gap between the joists and then just plasterboard it without creating a condensation risk.

You need to leave a sufficient gap above the insulation to allow any vapour to be carried away.

Not easy with a flat roof abutting a wall, but there are soffit- and abutment vent systems.

The alternative is to try and keep vapour out of the roof space in the first place, eg with a poly. sheet stapled to underside of joists, and foil-backed plasterboard, but what about penetrations such as lighting wiring?

 

[paulontour]

Thanks Tony.

What vent systems are available then? I am about to get the roof replaced so would be interested.

So would even the slightest gap in the poly sheet cause a condensation risk? I only have the lights to worry about, nothing else coming down from the ceiling. And with the poly sheet do you still leave the gap above the insulation?

 

[tony1851]

This is the sort of thing you should be looking at;

http://www.cavitytrays.co.uk/ventilation/view/2/89/1/type-rav-fl-roof-abutment-ventilators

Yes, you still need the (2") gap above the insulation to allow cross-ventilation.

You will also need some ventilation on the opposit (gutter?) side of the roof, though if you have an overhang, this is easier as you can fit the little circular soffit vents.

 

[paulontour]

Untitled

• paulontour
• 29 Oct 2013
• 1

Thanks Tony.

So this is a very rough drawing of my setup!

The wall closest is attached to the house. The middle section is the new play room I want to creat, with the joist running as shown. The left section is the back of the garage and will be a utility room.

Can I vent on the right hand side into the pitched roof section or does it need to be outside? Also are there any other products available as I do have access to 3 sides of the garage, just a shame there are no soffits.

 

[limerockconstruction]

You need to vent it outside and you need to provide cross ventilation, this is the most important part as the air needs to move through the roof taking the moisture with it.[/i]