Somewhat unique roofing/insulation dilemma.

James Garza
7/10/2006
I have a Cabana style house in Austin, TX with the following specs: 1. Low pitch roof (less than a 2 inch rise for every 12 inches). 2. Currently has asphalt style shingles. We'd like to replace this with metal. 3. The entirety of the house has a cathedral ceiling. There is no attic, only a 6 inch space between the ceiling and the roof deck. 4. The six inch space if for venting. The roof is vented with soffit vents and a ridgevent at the peak. 5. There seems to ba a foil barrier on the top side of the ceiling dry wall facing into the venting space. 6. During the summer on hot days I can place may hand against the ceiling of our house and feel the heat emanating from it. I'd say at least 15 to 20 degree difference. We've talked to both metal and shingle roofing contractors and everyone seems to be leaning away from keeping the roof vented. The low pitch of the roof means that traditional venting does not move air enough to help with insulation. Beyond that no contractor that we spoke to would warrant the ridge vent from leaking due to the low pitch. We've had leak problems with the house since I purchased it (thus the new roof). A little investigation on the net led me to this article and it's recommendation regarding cathedral ceilings: http://www.applegateinsulation.com/pdf/ConsumerUpdate_14.pdf So we've decided to blow in cellulose insulation into the 6 inch vent space and then close up the ridgevent. My roofing contractor has recommended Galvalume over a radiant barrier deck. My understanding is that there will be no airspace between the deck and the roof. Questions: 1. If there is no airspace between the roof and the radiant barrier deck, does the deck really work at reflecting heat? How big an air space is needed to ensure the barrier is working? 2. Assuming that we do blow in the cellulose insulation, is the new radiant barrier deck necessary? At that point we'd have a Galvalume metal roof over a radiant barrier deck over 6 inches of cellulose insulation over the foil barrier over the ceiling. Seems like a bit of overkill. 3. Any other concerns regarding this hare brained scheme? :). Confused, James Garza
Allan Reid
Dura-Loc Roofing Systems, Inc.
7/11/2006
Well everyone is a little bit right. First is the building code determines that the split between low slope and steep slope roofs is at 2/12. It also assumes that flat roofs are used on commercial buildings (low humidity. It calls for a roof assembly with an interior air barrier, insulation, ventilation, moisture barrier and roof covering. Then on roofs 2/12 and greater on a vaulted ceiling like yours it mandates a MINIMUM 1" vented vertical air space. Tests completed at the DOE in Oakridge by the MCA showed that the vented space under a metal roof cut heat transfer over 70% as compared an asphalt shingle. If they can fill the existing 6" cavity properly, then this is a good approach. The cavity should be blocked off at the eave and ridge when completed. Then install a metal roof over. Choose a system that can accept vertical strapping to prtovide the air space and then horizontal strapping to accept the metal roof covering. No need for a radiant barrier under the new roof covering however the code calls for minimum 15 lb felt. There are vented closure strips or accessories for the eave and ridge. Look to choose a product with an Energy star rating as well. Good luck.
Guest User
7/17/2006
Thanks for the response. Everything you've indicated here makes sense and I believe should be do-able by my contractor. But I want to clarify a few things: What you are indicating is that the metal roof is installed over the vertical and horizontal strapping thus creating the minimum vented air space. The vented closure strips are then used to ensure the space between the deck and the roof is vented but is still leak proof. Is this correct? If so: 1. You indicated that the minimum 1 inch airspace is mandated by the city code for pitches GREATER than 2/12. I measured my roof and I'd say the slope is about 1.8 to every 12 inches. So is the vented area required? And is the vented area still effective in reducing heat transfer with such a low pitch? 2. Even though the closure strips are vented, do they still ensure that water will not enter the space between the metal and the decking via the ridge? Our concern is that with such a low slope, rain water may move up the roof (due to wind) and enter in via the ridge. How is this different than a traditional ridgevent? One more question: Should we go with unpainted galvalume or a white metal product such as the Kynar Snow White? The Kynar product is a siliconized polyester white painted metal roof. According to this study at the Florida Home Energy Center (http://www.fsec.ucf.edu/Bldg/pubs/cr670/), the white metal roof has a higher reflectance and much lower heat emissivities. My understanding (please correct me if I'm wrong here), is that the white metal roof absorbs less heat and retains it for a shorter amount of time. This allows the roof to cool faster in the evening. Or is this not a concern with the vented air space and the 6 inches of insulation? Thanks again, James Todd Garza
Allan Reid
Dura-Loc Roofing Systems, Inc.
7/18/2006
1) The venting is not then required by code however it is a very effective way of reducing the heat transfer into the cveiling by at least 30% over any additional advantage on a reflective roof. 2) Yes, to ensure choose a product tested to the wind driven rain test in FL. 3) The painted product could have a higher reflectivity than Galvalume if you choose the right product however the painted panel with have a higher emmissive number which is much better. Then as noted above ventilate the space to stop the heat transfer. Actually the bare Galvalume will cool faster at night however if you ventilate the system it will negate the advantage.
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