Metal roof in Southern AZ--Venting and foam


#1

Have an existing house in Southern AZ with a corrugated metal roof. I plan on foaming exterior of roof, either DIY or contractor. Part of the house has a crawl space between ceiling and roof. I installed R38 bat insulation on top of ceiling, but the crawl space is non-insulated and becomes VERY hot in summer, thus the reason for foaming. There isn’t any venting on this roof and the only intake of air would be the ridges of the corrugated sheets as they come off the back of house. If I do foam, should I vent the roof or totally seal it? If I do vents, wouldn’t I need more intake than what I just described? Then, how do you install vents into CMU? I do have the evap side of my heat pump in that crawl space also, so I’m suer that works overtime. It is a gentle sloped roof, one-way off the back of house, sitting between a CMU block parapet.


#2

How much of an r factor are you expecting to achieve
if you foam the exterior of your corrugated roof?
All things considered, heat pump and all, I would think that closing in the attic would cause you serious problems compared to proper ventilation. I would also think that you might gain a bigger r value if you foam under the roof up against the sheathing, and put a reflective coating on the corrugated roof to reduce solar absorption. I am sure that there are a few foam guys that come thru here that might have better information than I can give you. Have you considered a soy based foam…very environmentally friendly, and I think there’s a company in Jacksonville FL that sells a kit for the DIY market. Leave some info, and someone can contact you off-forum with that information.

hope this helps

David