I was just talking with a neighbor a few days ago, and he mentioned in passing that in doing research on spray foam, he noticed that a few sources noted that 30-pound felt is recommended under shingles where spray foam is to be used. Is this the case, or is 15-pound adequate? And while we’re at it, how impossible is it to locate leaks once a roof gets them with spray foam under it? Further, can sheathing be replaced without trashing the foam? Just curious.
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Felt shouldn't make any difference, it goes on top the sheathing, not between the foam and sheathing... And studies have shown that hot roofing does not lead to significantly elevated shingle temps, so that theory is bunk as well.
As for leaks, I don't know why this rumor persists. It is just as hard to locate leaks with a typical insulation job as it is with foam. It might make fixing the leak more complicated, but in terms of finding it, it'll be equally as frustrating. For instance, in my house, there was obvious damage from a long term roof leak and a leak that was still leaking visibly during hard rains. I before I foamed the roof, I could find no evidence of where the leak came from, even with the underside of the sheathing in full, unobstructed view. And even with the foam, it still leaked in *exactly* the same place. It wasn't until I reroofed it that I found the source of the leak... Nearly 25 feet away from where I was seeing it inside.
So don't believe the FUD, either the people telling you have never had a leak, or have some mythical leak finding 6th sense.
Z
I don't know much about roof leaks and foam. I would imagine that with foam the sheathing wouldn't have as much of a chance to dry and might rot faster, but then the foam itself wouldn't stay wet the way batts would. What has been your experience?
I have an experience, but not "experience" with any problems with the combo. Increased chance of rot would be a trade off for the system, especially on a roof where you might have to trap the sheathing between two vapor impermeable layers.Dunno, I don't know that there is a whole lot of time with the new systems to really have a good sense of the long term side effects. But yes, I think you're right that the chances of more rot is increased with a spray foam hot roof.Z
"And studies have shown that hot roofing does not lead to significantly elevated shingle temps, so that theory is bunk as well."Actually - the shingle temp IS significantly higher.The studies I've read showed that the decrease is shingle life because of that higher temp is what is almost insignificant. I forget the percentage, but I calculated it to mean 2-3 months of shingle life for a 15-20 year three tab to give it practical meaning. That might be a year of life for a 40-50 year architectural shingle is all
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Damn, I had a 50/50 chance of getting that right and blew it! Well, either way, shingle manufacturers don't have a problem with hot roofs any more... Z
To the neighbor:
Welcome to the 21st century. Here we use synthetic underlayment that is lighter, more durable, better sealing, lies flat, never wrinkles, and requires less labor to install.
Look up Titanium UDF or RoofTopGuard II.
;)
Tu stultus es
Rebuilding my home in Cypress, CA
Also a CRX fanatic!
Look, just send me to my drawer. This whole talking-to-you thing is like double punishment.
Edited 7/8/2009 5:25 pm by xxPaulCPxx
udL not "F".
Luck You!There is another way to get venting under the deck AND insulate it with foam or even cellulose. Owens Corning makes a plastic "C" channel. You place Tyvec or TypeR over your rafters and then press these channels down onto the rafters. It makes a vent channel about 1" below the roof deck of taught air/vapor/water barrier building paper. You layer the paper out to the outside of the house. Water drips on that and rolls outside again. Put a radiant barrier deck over that to cut heat transmission in summer, and spray foam on the underside or do a dense pack with cellulose.
Tu stultus esRebuilding my home in Cypress, CAAlso a CRX fanatic!
Look, just send me to my drawer. This whole talking-to-you thing is like double punishment.
And is much safer to walk on.
What is the price of Titanium UDL and Roof top guard?How many layers of 15# felt could I put on before it was worthwhile to invest in the 21st century?DC
Synthetic underlayments run about $140 for a 10sq roll.Here is something to look at:
http://www.interwrap.com/titanium-calc/index.html
Tu stultus esRebuilding my home in Cypress, CAAlso a CRX fanatic!
Look, just send me to my drawer. This whole talking-to-you thing is like double punishment.
So, what is the advantage for my clients?Most of the advantages that site gives seems to indicate that many roofs out there are sitting for weeks with just paper. Around here, most roofers paper as they shingle so it's not an issue. It is my opinion that the main purpose of roofing underlayment is to provide secondary protection against moisture intrusion; usually due to poor quality shingles, poor shingle installation, or poor roof design. I have seen many roofs out there without any underlayment and no problems, I have seen many with underlayment with lots of problems. Thus far I haven't seen any products to control what is essentially just a quality control problem.Although the zip system looks interesting.BTW: the answer is 3. I can put three layers of 15# felt on for the same material cost as using a synthetic and yes that would be three times the labor (technically).DC
Actually, since the synthetics are wider, each pass covers more. Instead of 3x the labor, it may be closer to 4x.As to your point about roofs with no underlayment being perfectly capable of staying dry - no question. But haven't we come a ways in our understanding of building practices since then?A roof is more of a system than a single layer covering. Each part of the system is designed to shed water OUT of the system, usually with overlapping layers. If one part of the system fails, there are two more that may back it up.I'm sure you can built a perfect roof that is impervious to water penetration using shingles alone. But the quality of that roof is only guaranteed as long as YOU are the only one who ever touches it again. You have to keep the utility electricians replacing the power fed, plumbers adjusting/running stacks, carpenters replacing facia damaged by termites, etc, off that roof.So what is a synthetic underlayment worth to your clients? How about the knowledge that after their shingles have worn out in 10 to 30 years, the underlayment hasn't changed a bit since it was laid up on day one. Even if their primary roof covering fails due to age or wear, that second covering will not. A covering that never rots, looses its embedded chemicals, or cracks over time might be worth something then.A roof is like paint, once applied a clock is started... sooner or later it counts down to 0 and needs to be replaced.
Tu stultus esRebuilding my home in Cypress, CAAlso a CRX fanatic!
Look, just send me to my drawer. This whole talking-to-you thing is like double punishment.
So, what is the advantage for my clients?
I can put three layers of 15# felt on for the same material cost as using a synthetic and yes that would be three times the labor (technically).
Most of my projects are re-roofs or addition roofs where we have to keep the interior dry during the project and if we're doing a copper roof, the open time is greater than with asphalt.
So the advantage is really for me. The synthetics are less likely to blow off (I don't use #15 felt anyway - just #30) and allow a leak and the economics are slightly in my favor on installation. #30 felt is about $28/2 sq roll and Titanium is $120/10 sq roll here. So I'm $14/sq vs $12/sq on material ($7 vs $12 with #15). With synthetics, one trip up the ladder brings me 10 sq vs 5 trips for #30 (1 trip vs 2.5 with #15) and the synthetics take about 1/3 less nails which translates to speed as well as material economy and less starting and stopping at the begining and end of rolls which also speeds up the process.
In my system, #15 and Titanium would probably be about a wash on raw material cost (#15 getting a slight edge), but my guys would be at the pub drinking their second one about the time your guys are nailing up the last piece of felt and then tearing holes in it on the way back down.
http://www.quittintime.com/ View Image
I can't imagine that it makes any diference whether 30 or 15 is used.
Leaks - can't say - haven't run into that one yet in roofs, just siding.
Removing sheathing? Only small sections so far, but no trouble getting it to spring free once you get a corner started to work a flat tear-off shovel in.
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