Roof insulation.. Airgap or no?

[TwoFeetShorter]

Will be doing my roof in the next few days.... I will be using 2x2 spars to span across. When I get it ready for insulation foam sheets do I double up my sheets to completely fill the space between roof and ceiling, or do I use one sheet and have an air gap?

My house construction knowledge says air gap for roofs, but that requires ventilation, which is a no go on this camper.

So, air gap or no?

Any input appreciated!

Neil

[bobhenry]

I filled mine full for the support of the inner and outer skin.

[noseoil]

At work we use a simple rule for attic space. If it's insulated at the diaphragm (at the roof deck itself) there are no vents. If the insulation is on the ceiling, it needs a vent to move the air out above the insulation. If it was a vented area, the rule of thumb is the square footage (floor area) divided by 150 for vented area in the building. In my tear (5' X 9') that would be about 1/3 of a square foot or about a 10" X 5" vent. Since the spars take up space and block airflow, the insulation needs to be about full at the roof. If you can find some foil-faced insulation, put the foil facing outwards (walls & roof). I'm planning on two layers of 3/4" polystyrene with expanding foam to seal the edges which don't fit perfectly. I will use the foil on one face to the inside and one to the outside. Our biggest problem where I live is the heat in summer.

Short answer, what Bob said!

[TwoFeetShorter]

Thanks for the input guys. Looks like I will be filling it full with no air gap. I will be using screws/glue of course, but with all my insulation I will also be taping off the edges with foil tape.

Neil