Insulation, Ventilation and Purlins

Guest User
9/21/2009
I am looking to build a gambrel roof barn with living quarters. I have several questions about the assembly. What is the best method for constructing the building. My biggest concern is the moisture from the metal, do you apply spray foam directly to the underside of the metal, or do I need a separation between the metal and insulation? Do I need to vent the area below the insulation. Is it better to use 2x4 purlins, osb or plywood?
Aaron Scurlock
The Best in Polymer Roofing Soutions
9/24/2009
Applying closed cell spray foam insulation directly to the underside of the metal will eliminate consensation from forming because you are eliminating the thermal differential between the cold roof panels in the a.m. and colder months. This being said, you should fill the rafter cavities completely to the level of the ceiling in order to insulate the living space. How this is done is dependent on the construction details of the roofing/ceiling/attic, etc. Cost can be reduced by applying closed cell to the deck, which would create an air barrier, and filling the remainder of the cavity with open cell. If you're going to have attic space, then the insulation should be applied to the celings. Anymore questions? Ask away.
Todd Miller
Classic Products, Inc.
9/24/2009
Thanks. Aaron, for your input on the insulation. One drawback to installing the metal roofing over purlins and spraying insulation on the back of the metal is that, though it may be way down the road, when the roof needs to be replaced, you will be in a situation of destroying the insulation in order to replace the roof. Personally I would put decking and underlayment down before the roof and avoid that scenario. You can insulate the bottom of the decking then.
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