Guest User
3/4/2006
As a long time shingle and rubber roofer, I am studying to make the transiton to metal roofing. I've come the conclusion that one will never own an asphalt shingle roof sytem. One is merely paying rent on a doomed roof. My most recent observation is ventilation. The manufacturers stress it, and we contractors know it. It is an absolute must. On all systems. The irony of the asphalt shingle roof system is, while we struggle to remove excessive heat and moisture, from the attic, the roof system itself is introducing even more heat into the attic! A self-defeating cycle that is not easily, or always overcome. It seems to me, w/ any metal roof system, we would be venting the attic only- not fighting to keep the shingles cool also. As I read more about metal roofing and the energy efficiency, I see most articles stress less air-conditioning costs. As a northern roofer, I have to ask about the benefits it presents to us, as we seek the ever elusive "cold attic". I would like to know more.
Allan Reid
Dura-Loc Roofing Systems, Inc.
3/8/2006
The Metal Construction Association just completed a 1 1/2 year test at DOE Oakridge where we showed that a granular coated metal roof installed on battens over the sheathing and venting of the air space kept the attic cooler in the summer and warmer in the winter as the air space acts as a blanket and reducing the enegy required where it was 70% less than an asphalt shingle. Pretty amazing performance and the association is preparing to do another tests to try and increase it even more. Basically put on an Energy Star rated metal roof with a high emmissivity rating and install it over 1" vented air space.
Guest User
3/8/2006
Hi Mr. Reid, and thank you very much for the eagerly awaited reply. There may be something lost in the translation here, however. In my area, NW Indiana, SE of Chicago, a "cold attic" is what we strive for to minimize ice dams and to maintain a dry attic to prevent wood rot/ damage and to maximize the insulation benefits. It is important and good to know that the attics in the Oak Ridge DOE study stayed cooler in the summer. However, on the flip side, during the winter months, for energy efficiency, condensation and ice dam prevention, in my area, we need a "cold attic" during the winter months. I don't understand the benefit of holding in more heat during the winter months in a northern climate. Please advise. Thank you
Allan Reid
Dura-Loc Roofing Systems, Inc.
3/9/2006
There is a temperature point where passive ventilation slows down and stops as the air cools. I fact a number of people believe that if the attic was 30F and the outside temperature moved from 40F to 25F as the sun went down that the passive air flow would reverse but it does not. In fact a cold dense air mass forme on the underside of the roofing material or decking and acts as a buffer stopping condensation. The biggest problem is too many homes are build without an interior air barrier and warm air escapes into the attic and condensates. A metal roof up on a vented air space will assist with this but as I say it treats the symtoms but does not cure the disease and steps should be taken to correct this. With no heat loss then the passive netilation can do its job and certainly the additional air space will reduce heating load in the winter which in turn means less heat loss and less ice damning.
Guest User
3/9/2006
"A metal roof up on a vented air space will assist with this" "it treats the symptom not the disease". That is what I was wondering. Basically, it takes a load off of the ventilation sytem then, by having cooler air in the attic, then there is less heat to have to vent out. Correct? "With no heat loss, then the passive ventilation can do it's job" OK-Understood, and a very important point. It's Job- to exchange the air in the attic. " and certainly the additional air space will reduce heating load in the winter" OK- But how? "which in turn means less heat loss and less ice damming." How does the airspace in the roof sytem prevent the heat loss? Isn't that more the function of the insulation on the attic floor? Thank you again for your response(s)
Allan Reid
Dura-Loc Roofing Systems, Inc.
3/10/2006
This is getting into physics and you need to read the answers above again as you are missiing some points. 1 Install an interior air barrier to stop warm air loss and moisture from this source. 2 Properly ventilate the attic at 1/150 ratio with a balanced 50% and eave and ridge. 3 Install an Energy Star rated product with good emmisivity 4 Install a quality underlayment 5 Install the roof so that there is minimum 1" of free vertical air space with balance eave and ridge ventilation. 6 The metal roof product should have a a painted back coat. The Energy Star product reflects the visable suns rays out. The Emmisive product then emits the residual heat into the atmosphere. What heat does get through, does not transfer through the sheathing from conduction and is vented out quickly in the warm months keeping the attic closer to ambient temperatures. In the winter months there is minimal passive air venting because of the cooler temperatures. A cool dense layer of air forms under the roof covering.What residual heat that remains in the air space then is emmitted back into the air space from the backer coat. Result is less temerature swings in the attic space, generally warmer than the outside but not at the point wher it can condensate. Net result cooler home in the summer, warmer home in the winter. Remove the inerior air barrier and reduce insulation, the less chance it has to worlk and the more condensation and ice damming.
Guest User
3/10/2006
Hi Mr. Reid, I understand what you are saying. I think that the "cool dense layer of air under the roof covering" is more or less the "cold attic" I am referring to. Is this a matter of semantics here? Believe me, I've been fighting these ventilation battles a long time in the asphalt shingle trade. Simply put, there are two sources of heat in the attic: interior & exterior. As a roofing contractor I have more control over the exterior heat gain by way of installing a metal system as opposed to as asphalt shingle system. And the 3rd factor, is of course, ventilation or air-flow. "The irony of the asphalt shingle roof system is (that) while we struggle to remove excessive heat and moisture from the attic, the roof system itself is introducing even more heat into the attic! A self-defeating cycle that is not easily or always, overcome" Anyway, this is my original intent: to be able to understand, and therefore explain, in laymen's terms, the benefits of a metal roof in a cold climate, without going into physics, as it relates to a more efficient ventlation porcess. Again, it seems that with less heat in the attic in the first place, it is a win-win situation for both contractor and building owner. Thank you again Tom Williams p.s- missing the point? "In fact a number of people believe that if the attic was 30F and the outside tempature moved from 40F to 25F as the sun went down, that the passive air flow would reverse but it does not" . Why would anyone agree w/ that scenario? Would they agree because colder air "sinks" and warmer air "rises"? Air flows from top to bottom in normal conditions, and that tempature swing/ difference is not going to change that, in my opinion. What I have seen as far as different air movement due to tempatures in attics, is the prevalence of moisture migration from the south side to the northern side of an attic, (usually if the ridge is facing East/West). Top to bottom moisture migration or air flow?
Allan Reid
Dura-Loc Roofing Systems, Inc.
3/13/2006
Sorry, I have no more available time on this lik. Thanks for thinking metal.
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