metal to metal: steel joists and metal roof

harry sawyer
7/6/2010
look to replace an R-PANEL roof with standing seam. The house uses steel framing - not wood. Large area of roof has not attic space due to vaulted ceilings. I can practically 'see' the heat on my ceiling. Looking to gain energy efficiency with modern product and insulation. Questions: 1) running horizontally across the joists is a 'channel' standing at most an inch off the roof joists. the existing roof is screwed to that. Does the space between the joist and the top of the channel constitute the airspace required for ventilation? Doesn't seem like it could remain maintain a constant gap with insulation stuffed between joists. 2) are there any 'special' considerations for metal on metal roofing? ie - thermal barriers or other techniques to reduce thermal transfer from roof to framing to inside room? regards, harry
Info @windowsonwashington.net
An informed customer is our best customer.
7/7/2010
Best thing to do here is to create a thermal break between inside and outside. An R-Max board (polyisocyanurate) foam insulation will have the highest R-Value per inch and is used in just such applications. Depending on the roofing system, it can be applied directly over top, or it may require a plywood topper to fasten to. You can get it in 6" thickness which will add around an R-36 to the roof deck. That is probably overkill, however the difference in price is not 6X the 1". I would recommend a minimum of 2"
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