| Author |
Message |
Walter James EageR
Junior Member
Username: eagerwj
Post Number: 2
Registered: 09-2007
| | Posted on Thursday, September 13, 2007 - 01:15 pm: |    |
I am designing a Solar Demonstration and Test Facility. My primary interest at this time is in minimizing the temperatures of materials that underlie standing seam metal roofing. Amorphous silicon, photovoltaic laminate will be bonded to the bottom of the pans. The efficiencies of photovoltaic systems are related inversely to their cell operating temperatures. There appears to be nothing reported on the best type of roofing underlayment material to contribute to this objective. I would like to have forum participants present their opinions and possibly applicable research results to help resolve this question.
Walt Eager, PE |
Joseph Berchenko
Senior Member
Username: josephberchenko
Post Number: 10
Registered: 08-2003
| | Posted on Thursday, September 13, 2007 - 03:26 pm: | |
I wouldn't think the underlayment would have much effect on roof temperatures. High-temperature-resisting underlayments are available but they resist the high temps generated when the panels get hot...I don't think they do much to mitigate high temps. |
John Hunter
Senior Member
Username: johnhunter
Post Number: 38
Registered: 12-2005
| | Posted on Thursday, September 13, 2007 - 04:28 pm: | |
If the issue is to reduce the temperature of the roof surface itself, it would seem that the appropriate response would be to provide a means of ventilation underneath the roof panels. |
Walter James Eager, PE
New member
Username: eagerwj
Post Number: 1
Registered: 09-2007
| | Posted on Thursday, September 13, 2007 - 05:31 pm: | |
Thank you Joseph and John,
Perhaps I am trying to guild the lily. I am specifying what appears to be the most effective continuous ridge vent available for a gable roof, fully ventilated soffits and a highly reflective metal roofing color, regal white. My thought was that 30 pound asphalt-impregnated felt as an underlayment would absorb more of the sun's and the photovoltaic laminate's infrared radiation and conduct some of this heat it back to the roofing panel and then to the laminate. Some of the lighter colored, synthetic underlayments might minimize part of this part of the heat flow. I could install patches of several different underlayment types and imbed thermocouples in an attempt to get a definitive answer to this question. The question that you raised about the underlayment withstanding the high temperature produced by the photovoltaic laminate is a good one. What types of underlayment would last at least as long as the roofing under such conditions? I would like to start out with the best choice of underlayment according to current knowledge. |
John Hunter
Senior Member
Username: johnhunter
Post Number: 39
Registered: 12-2005
| | Posted on Thursday, September 13, 2007 - 06:16 pm: | |
Walter - There are a number of high temperature underlayments on the market. We've used Grace "Vycor Ultra" with success, but there are many others. Most underlayments will require a 3# rosin paper slip sheet between the underlayment and metal panels, although one of Elk's "Shieldguard" products is supposed to be constructed in such a way that a slip sheet is not required, but I have no personal experience with the product. |
Joseph Berchenko
Senior Member
Username: josephberchenko
Post Number: 11
Registered: 08-2003
| | Posted on Friday, September 14, 2007 - 10:18 am: | |
Here's a good article about underlayment for metal panels originally published in Construction Specifier
http://www.na.graceconstruction.com/custom/toplevel_pages/featured_articles/pdf/Construction_Specifier.pdf |
Bob Woodburn
Senior Member
Username: bwoodburn
Post Number: 211
Registered: 01-2005
| | Posted on Friday, September 14, 2007 - 10:19 am: | |
Is contact with an underlying structural roof deck (e.g. plywood, etc.) necessary? I addressed this in the LEED thread, before realizing this was cross-posted here.
Since you are already providing ample eave and ridge ventilation, a simpler and more directapproach might be to use a structural standing seam roof that is self-supporting between purlins (with no need for an underlying deck) and specify that the underside of the roof panels be coil-coated with a black finish, to radiate as much heat as possible to the vented space underneath. And there would be no mass in the underlying deck or underlayment to impede the heat flow or store it for re-conduction to the roof panels.
If I remember correctly from physics class, that would make the underside of the roof panels a "black-body radiator"--the opposite of the increasingly popular white or aluminum "radiant barrier" coatings intended to prevent heat buildup in attics. |
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