Rsipgeo
Basic Member
Posts:104
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| 16 Sep 2010 07:30 AM |
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I am thinking about using 12 1/4" SIP panels for my pitched roof. Foundations are poured and I am ready to go. I noticed that the manufacturer calls for adding felt, furring strips and another layer of plywood to vent the roof.
Is this the usual way one does a SIP roof? Would two layers of felt solve the solar vapor drive issue? Though the manufacturers warrantee might be voided would an unvented roof surface really affect shingle longevity?
Venting is not a small extra cost but I'll do what I have to do. |
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cmkavala
Veteran Member
Posts:4327
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| 16 Sep 2010 08:35 AM |
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Rsiogeo;
Elk is approved over SIPs, the furring and plywood is not worth it. Longevity is diminshed about 10%, The SIPs we use are considered the primary water barrier with the shingles applied for aesthetics.
It would not bother me even if the shingle mfg voids the warranty anyway. If you ever tried to collect on one ....good luck! In our area a 30 year shingle is good for about 15, 20 if you are lucky
Average time someone owns a home is 7 years |
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| Chris Kavala<br>[email protected]<br>1-877-321-SIPS<br /> |
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Dana1
Senior Member
Posts:6991
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| 16 Sep 2010 10:18 AM |
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Heck with shingle longevity- be more concerned about the longevity of the SIP's OSB skin! A leaking shingle/felt layup can go a very long time un-noticed on SIP, since it won't drip through to the interior. Sure, it can still dry through a layer of felt & shingle, but adding a vented nailer deck improves the drying capacity of the exterior OSB by more than an order of magnitude.
Adding TWO layers of felt would be going the wrong direction- it would slow the drying capacity while having almost zero effect on waterproofing.
OSB would be a less expensive nailer deck than ply, and with it fully vented it can dry to the vent-cavity too. It would also be protective in cold climates against imperfections in the sealing between SIP sections leading to wintertime condensation rot along the seams. See:
http://www.buildingscience.com/documents/insights/bsi-036-complex-three-dimensional-air-flow-networks?topic=doctypes/insights |
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cmkavala
Veteran Member
Posts:4327
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| 16 Sep 2010 12:29 PM |
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Dana1;
an undetected leak would still be an undetected leak for a longer period of time with a double roof, a layer of felt between the two would not ensure that the SIP would be protected with all the penetrations an nail holes.
It would be a roofers nightmare trying to find the leak with multi layers.
A simpler method would be to utilize a self seal peel-n- stick mebrane over the OSB sip for a secondary water barrier. The secondary water barrier also reduces insurance rates in hurricane prone areas |
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| Chris Kavala<br>[email protected]<br>1-877-321-SIPS<br /> |
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wes
Advanced Member
Posts:810
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| 16 Sep 2010 01:57 PM |
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I, like Chris, have, in the past, advocated the use of peel-n-stick as an underlayment on SIPS. But recently, with all the focus on vapor permiability, I have wondered if there are any significant negatives to this procedure that we should be considering. I would appreciate any thoughts on this topic. |
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| Wes Shelby<br>Design Systems Group<br>Murray KY<br>[email protected] |
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Dana1
Senior Member
Posts:6991
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| 16 Sep 2010 04:14 PM |
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Posted By cmkavala on 16 Sep 2010 12:29 PM
Dana1;
an undetected leak would still be an undetected leak for a longer period of time with a double roof, a layer of felt between the two would not ensure that the SIP would be protected with all the penetrations an nail holes.
It would be a roofers nightmare trying to find the leak with multi layers.
A simpler method would be to utilize a self seal peel-n- stick mebrane over the OSB sip for a secondary water barrier. The secondary water barrier also reduces insurance rates in hurricane prone areas
Yes, but the undetected leak would do far less damage to the SIP since it would dry quickly after wetting events. The double-deck approach to roofs is similar in many ways to rainscreens gaps in wall structures, where penetration of the outer cladding is assumed, but inconsequential to the sheathing even with imperfection in the housewrap/felt, since it's guaranteed to dry quickly. It's a more expensive way to build, no doubt, but it enhances the long term reliability of the structure by leaps & bounds. A roof is not just a tilted wall- if anything that's even MORE reason to go with the more conservative vented deck approach. Walls don't typically carry ice & snow on them for weeks/months at a time, completely eliminating the capacity to dry toward the exterior out the shingle side. But with a vented nailer deck both the nailer and the structural roof can dry into the gap. The membrane approach may have merit in cooling dominated climates, but would do nothing good for protecting against winter-condensation rot issues at air-leaky seams in cold climates. |
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cmkavala
Veteran Member
Posts:4327
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| 16 Sep 2010 05:14 PM |
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Posted By Dana1 on 16 Sep 2010 04:14 PM
Yes, but the undetected leak would do far less damage to the SIP since it would dry quickly after wetting events. Not if it gets thru the multiple felt penetrations and under felt next to SIP skin. Many leaks occur in valleys any water leaking would then find its way into the SIP valley below
The double-deck approach to roofs is similar in many ways to rainscreens gaps in wall structures, ( except walls ar vertical, not having intersecting hips and valleys) where penetration of the outer cladding is assumed, but inconsequential to the sheathing even with imperfection in the housewrap/felt, since it's guaranteed to dry quickly. It's a more expensive way to build, no doubt, but it enhances the long term reliability of the structure by leaps & bounds. There is no longterm documentation that rainscreens will be more beneficial, only time will tell A roof is not just a tilted wall- if anything that's even MORE reason to go with the more conservative vented deck approach. Walls don't typically carry ice & snow on them for weeks/months at a time, completely eliminating the capacity to dry toward the exterior out the shingle side. But with a vented nailer deck both the nailer and the structural roof can dry into the gap.
The membrane approach may have merit in cooling dominated climates, but would do nothing good for protecting against winter-condensation rot issues at air-leaky seams in cold climates. Winter condensation issues have been attributed to poor installations. If its done right to begin with there is no need for a double roof. An improperly installed second deck would be equally problematic.
if the contrator can't do a good SIP install, what would make one think they would not botch a 2nd. deck?
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| Chris Kavala<br>[email protected]<br>1-877-321-SIPS<br /> |
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Dana1
Senior Member
Posts:6991
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| 17 Sep 2010 04:46 PM |
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There are many decades (centuries?) of demonstration that rainscreens are beneficial in masonry & stucco clad assemblies. Even short-term measurements of moisture content in the structural sheathing are convincing enough to conclude that they're of benefit with other claddings as well. The evidence is clear enough that 10mm rainscreens are the law of the land for every climate zone in Canada, and clear enough that it relaxes the vapor retardency requirements of cold-climate wall assemblies according to the IBC. But I suppose time WILL tell, indeed. (Maybe I've just been drinkin' the same kool-aid that they have. :-) To be fair, I haven't measured it myself, but I believe those who have.)
A rain leak in to a ventilated SIP valley is not worse than into an unvented SIP valley, quite the reverse. Setting up the supporting furring for the ventilation gap to drain at the valley is almost assured unless pains are taken to PREVENT it.
Even a leak through a nail-point through the felt on the SIP after the secondary deck was leaking will dry much quicker, since it's drying to a dry vent space that has stayed mostly dry, not though layers of rain-retaining shingles.
To demand perfection (even on day 1, let alone for a century) for the sealing job between SIP sections is a bit much to ask. Building the structure to be resilient to some imperfection is wise. Even a less-than-perfect secondary roof to allow ventilation will offer significant resilience by making the structural roof drier. (It is absolutely NOT equally problematic.) By the same token, were siding perfect there'd be no need for felt or other drain plane layers. Sure, at some point redundant systems become a pointless expense, but for foam-insulated roof decks (SIP or stick built & foamed from below) it's pretty clear that the deck runs drier when back-ventilated. Yes, they can be made to work without the venting, but they're less resilient to leak damage. Leaks ARE harder to detect before the damage is done than in traditional stick-built roof/insulation systems. Adding the secondary deck doesn't make leak detection easier or harder, but it does make the assembly more resilient, but yes, only for a price. IMHO it's a price worth paying, but apparently you don't, and that's OK with me. It'd be a boring world if everyone's frame of reference were identical.
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vb
New Member
Posts:88
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| 20 Sep 2010 08:55 AM |
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I just completed a 12" roof. I furred up using 2x4's (lying flat) on 16" centers then 7/16 OSB. I extended the 2x's down the sub facia then nailed the facia over those which gives me my vent space. I made a bird block / bug screen under soffit with 5/4 cedar deck, and used a vented ridge. It took some time and thought for this system but it looks great and now that it's done I feel good that I did it right. Do you have to furr up? Maybe not. But most people agree that it helps on several fronts. Read "Builders guide to structural insulated panels" by Joe Lstiburek. Also keep in mind that with a 12" roof one of the big expenses is the sub facia. If you plumb cut the face you will need a 14" board for the sub facia. I used LSL instead of 2x for strength and straightness, It's nice stuff but spendy. |
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Dana1
Senior Member
Posts:6991
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| 20 Sep 2010 10:31 AM |
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Aw, what doesJoe Lstiburek know? He's one of the ones responsible for BREWING the modern versions of rainscreen kool-aid. :-) |
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Alton
Veteran Member
Posts:2164
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| 20 Sep 2010 06:06 PM |
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Dana1,
You lost me on this one. What do you mean? |
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Residential Designer &
Construction Technology Consultant -- E-mail: Alton at Auburn dot Edu Use email format with @ and period .
334 826-3979 |
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Dana1
Senior Member
Posts:6991
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| 22 Sep 2010 11:31 AM |
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Posted By Alton on 20 Sep 2010 06:06 PM
Dana1,
You lost me on this one. What do you mean?
Which part- my teasing about Joe Lstiburek , (who really DOES know his stuff, and has the data to back it up) or about the relative importance of ventilated cladding (be it roof decks or wall siding)? My basic pitch goes like this: Joe is dead-right- venting the cladding presumes that all siding/roofs leak (eventually. or periodically), and will enhance the ability of the wood to dry without damage. It's adds a large measure of fault-tolerance. With SIPs a roof leak can go on until long after the exterior skin is a mess, since it won't drip through to the interior through the foam unless the foam itself has split (or the seams aren't sealed). Discovering the leak by noticing the SIP is sagging, or having the exterior OSB peel up with the shingles when stripping them for replacement is something worth avoiding, even at some expense. A vented nailer deck holding the shingles above the SIP reduces the odds of that eventuality by orders of magnitude. An undetected roof leak might ruin the nailer deck near or below the leak (and may not even be noticed until it's time to re-shingle) but that's a far easier/cheaper repair than dealing with a compromised SIP. In the cold-climate air-leaky seam situation the presumption is that even with the best of intentions & methods a sealing job may not out-last the lifecyle of the building, and low permeabilty membrane roof would trap the moisture in the OSB, offering no drying path during cold weather. In warm climates that's not going to be an issue, and the near-perfection from leakage attainable with membrane roofs might make it a reasonable (& cheaper) alternative to a vented nailer deck above the SIP. If you live in an arid region where it rains less than 15" per year odds of deterioration of the SIP from roof leaks is probably very small though, since intervals between rewetting are long enough to dry thoroughly, (at least most of the time.) The exception might be if most of that rain typically occurs in a weeks long monsoon type season. |
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smartwall
Veteran Member
Posts:1209
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| 22 Sep 2010 07:25 PM |
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Most of the roofers in my area no longer use felt. There are quit a few roof underlayments that work better with about the same cost but easier install. Cosella-Dorken has Delta Vent S, Delta Foxx and Delta Trela, the last product works under metal roofing. |
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Rsipgeo
Basic Member
Posts:104
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| 27 Sep 2010 08:08 AM |
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I will clad the roof with Zip System sheathing. The main thing is that where I am working the roofers are not so quick to get to a job. Also over the years I have gotten less tolerant of any questions about structural longevity. I never want to wonder.
3/4" furring OK? and 5/8 ZIP roofing? |
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Dana1
Senior Member
Posts:6991
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| 27 Sep 2010 01:53 PM |
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3/4" furring is fine. (3/4" is literally TWICE the minimum rainscreen gap requirement for walls by Canadian code, and is more than sufficient for roofs from a drying point of view.) 3/4" is what Hunter uses for ventilation gap between OSB layers on their half-sip type panels: http://www.hpanels.com/2009/pages/pdfs/Lit_Prod_Color/Cool-Vent-II.pdf
Using Huber Zip roof decking is also fine, but may be a bit overkill, since it doen't need to be air-tight between the shingles & ventilation space the way it does for sheathing a studwall. Standard OSB would be OK, since it would reliably dry into the ventilation gap. |
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Rsipgeo
Basic Member
Posts:104
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| 03 Oct 2010 11:47 PM |
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Now I'm thinking of other things:
Can you get 1" coil roofing nails? If you use 3/4 furring and 7/16" OSB and 1 1/4 nails you would get all kinds of nail pricks in the felt. Maybe 7/8 rough cut furring and 5/8 OSB?
Or do 1" nails in 7/16" OSB properly adhere asphalt shingles? |
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cmkavala
Veteran Member
Posts:4327
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| 04 Oct 2010 07:08 AM |
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you did not add the thickness of the shingle , and yes you are over thinking this stuff. use 1-1/4" nails.......... in a normal shingle application the felt is riddled with holes |
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| Chris Kavala<br>[email protected]<br>1-877-321-SIPS<br /> |
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