I think its the cheapest solution. Another very important thing is to get rid of the stagnate hot air neat the highest paint of the roof. it helps a lot.

This may help, a little.http://forums.pelicanparts.com/uploa...1532418564.JPG

Update. I pressure washed and scrubbed the roof in prep for 10 gallons of elastomeric white paint. Before this my inside ceiling temps closely matched outside temps (100F+). Now my inside temps are maxing out at 88F - so that is decent progress. Next step is to install the rigid foam between the beams inside the home.

I found a 2" product at HD (Rmax Thermasheath-3 2 in. x 4 ft. x 8 ft. R-13.1 Polyisocyanurate Rigid Foam Insulation Board) and calculate I will need 8 sheets to complete the inside coverage (again wedged in between the support beams). This product has a foil coating for reflective property.

Question 1, does the foam need to make contact with the inside ceiling or would it be better to leave a 1" space for more heat isolation?

Question 2, I have read that this product needs to be protected from fire as it causes dangerous gas when exposed to flame. So is my idea of covering with ship lap a no go and I should be looking to enclose the insulation with sheetrock instead? If so would 1/2" be sufficient or do I need to go with 5/8"? That would be a bummer. Or should I use a different rigid foam product?

Thanks!

Fire rating of wood is better than you would think. How do you demonstrate the fire resistance of exposed wood members using char methods in lieu of gypsum? - WoodWorks

As I recall a 2x4 has a 60 minute rating, as used for firestops.

Typically there is an air-gap above bat insulation in cathedral ceilings, not insulation sprayed foam. That is to prevent condensation issues. I would do a bay as a test, then check it in a few months before you do it all.