Metal Roof components over Cathedral Ceiling

Rick Rajewski
10/24/2002
We just purchased a Los Angeles area home built in 1952. Our whole house has cathedral ceilings with no insulation. The air conditioner can't keep up with the heat on sunny days, even when set at 80 degrees. The heat is retained late into the evening. Currently, the roof is made up of 1x6 T&G boards, felt paper then one layer of wood shake shingles. According to neighbors the roof has been in place for at least 30 years. It is definitely time for a new roof. Our roofing contractor has proposed the following buildup on top of our existing T&G roof deck: 30# ASTM felt, 3" Rigid Foam, 1x4's attached vertically with foam inserts filling air space, horizontal 2x2's attached to 1x4's with foam support panels under metal roof filling air space. The foam inserts add to the R-value of the roof structure. The manufacturer states that the R-value with foam inserts is greater than their roof with air space. I have questioned him about the lack of air space for ventilation but he wants to go with the current plan. Apparently the manufacturer also stands behind the proposed plan. After reading all the threads related to insulation in this forum it seems to me we should have the following buildup on top of our existing T&G roof deck: air/vapor barrier (6 mil plastic with sealed seams), 3" Rigid Foam, radiant barrier with shiny side to sky, vertical 1x4's, 30# reinforced moisture barrier, horizontal 2x2's then metal roof attached to 2x2's. This solution allows for soffit to ridge venting above the rigid foam. Please let me know your thoughts on our current plan and my proposed plan. Thanks in advance for your response. Rick
Allan Reid
Dura-Loc Roofing Systems, Inc.
10/25/2002
Currently each City and County in California can adopt any building code. Generally it has been the Unified Building Code which ceased updates as it is being rolled into the new IBC code now being adopted universally across the USA. It is expected that it will generally be adopted over the coming year by most areas. Section 1505.3 of the UBC it requires that cathedral ceilings have ventilation to 1/150 of the area. The new IBC is clearer in that you must provide a vented 1" air chamber. The biggest bang for your dollar is having a vented air space, then insulation and couple that with a reflective and emittive roof covering. The reflective sheet is of lessor importance if you complete the first three. The poly air barrier is great. You may want to consider strapping the roof vertically with 2x4 over the rafters and filling them with R12 fiberglass, then the reflective barrier which if yo do not tape the hiorizontal laps will act as your moisture barrier and then the 2x2 and the roof. Either way you are absolutely on the right track and with the Title 24 energy law coming into effect you will be ahead of the game and enjoying a considerably more comfortable home and lower energy bills. PS we make a number of great roof systems that mount on 2x2 with 35% reflectivitey and 92% emissivity. Visit us at www.duraloc.com. Good Luck.
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