My house was built in 1982 with a lean to porch. I enclosed the porch and now need to insulate the roof on the porch. The ceiling consists of 2 x 6 rafters with open perlin and a metal roof on top. I need to insulate between the rafters but the ceiling is already low and I can't drop it down enough to provide a proper vent space above the insulation.

My thought was to remove the metal roof, add decking and tar paper then 2 x 4 perlins on top of tar paper and then a metal roof. I would then add 6" of insulation between the rafters. Would this give me enough air gap between the roof and insulation? Since this is a lean to porch, there is no way to let air out at the peak of the roof. I live in central Texas and the temp. gets over 100 degrees in the summer. Any advice would be greatly appreciated.

Thanks

Your location in Texas would require about R-25 to R-30 insulation in the ceiling of this 'porch'.

All you need do is install 5" of rigid foam insulation between the rafters of the porch roof on the inside...and be done with it.

Do you really need an air gap even in Texas? In the north we usually compromise ventilation for better insulation (such as near the wall top plates) if there aren't other cost effective options. I know roofers cry about this but in reality few older homes have active soffit vents and have never had problems with moisture or condensation in the winter, but they almost always have ice damming from lack of insulation.

Good stuff about all this at: www.startinsulating.com