vapor permeable underlayment | Ask The Experts Forum

Guest User 7/12/2007	We are considering having a standing seam roof installed on a shingle roof for rainwater harvesting and to improve energy efficiency. There is no attic ventilation over some areas that have cathedral ceilings, although there are small deadspaces under the decking before the insulation. In researching synthetic underlayments, I find that most have low vapor permeability and are recommended over vented attic space only. Would a higher perm rating be advisable for us or would putting metal directly over a permeable underlayment defeat the purpose. Or should we install battens to have a ventilation channel and let a permeable underlayment breath? Lastly, one roofer strongly recommended removing the shingles first because they would store and radiate the heat through the roof at night, whereas metal only would cool off quickly. What do you think? Gail	Share Or copy this URL: https://www.metalroofing.com/spirit/comment/9821/find/
Ken Buchinger NCI Building Systems, Inc. 7/12/2007	I assume you are harverting rainwater for use in the garden only as it would not be potable. The permeability issue relates primarily to unvented attic spaces. If you have an attic space, warm moist air in the attic needs a way to escape. Since you don't have an attic, this should not be an issue. A batten/counterbatten system would allow some ventilation under the panels if you provide air intake at the eave and air exhaust with a vented ridge. The shingles will store some heat but you could negate this by installing a 1/4" thick radiant barrier insulation under the metal roof	Share Or copy this URL: https://www.metalroofing.com/spirit/comment/9822/find/
Guest User 7/12/2007	Ken, Thanks for the very helpful response. When you refer to installing a radiant barrier under the metal roofing to negate the heat of shingles, what kind of material are you talking about - does it come in panels or rolls? Also as an aside, our interest in rainwater collection is primarily for irrigation, although in South Texas more people are looking at whole house systems with filters and UV lights for potable water. Thanks again - you are helping my husband and I work through this decision.	Share Or copy this URL: https://www.metalroofing.com/spirit/comment/9823/find/
Ken Buchinger NCI Building Systems, Inc. 7/12/2007	The radiant barrier insulation that I have used in the past comes in rolls, either 4' or 6' wide and consists of a foil backer on one or both sides with a core of foam, bubble pack or fiberglass. It is ususally about 1/4" thick. Many metal roofing manufacturers offer this type of product. All of the coil coaters that I know, and the company I work for owns three, strongly recommend that you do not drink water collected from metal roofs, either bare or painted. It is fine for irrigation though.	Share Or copy this URL: https://www.metalroofing.com/spirit/comment/9824/find/

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