I was looking at IRC R806.4.  If I understand it correctly for an unvented attic there are 3 options (in Zone 3):

1.  air impermeable under sheathing.

2.  air permeable under sheathing means you have to have ridgid board above the sheathing.

3.  air impermable under sheathing + air permeable directly under the air impermeable.


Am I correct that closed cell foam is air impermeable and open cell is air permeable?


I would really like to do an unvented attic.  As great as the ridgid boards are, I have a large roof and furring it out above the ridgid boards seems like a lot of work and money.  So I am looking at options 1 or 3.  Both of those involve closed cell foam.

My understanding is that closed cell foam is essentially water proof, so any roof leaks will never be noticed.  Doesn't almost every roof eventually have a leak?  If that is a true statement then how can spray foam be justified?  I understand the benefits of it for air sealing and the unvented attic.

I am assuming its not a matter of "if a roof leak," but rather "when will a roof leak"?  And if it uses closed cell foam it will go unnoticed and you may end up with some sheathing rot.

Help me understand this.  I suppose some people will say that a properly installed roof will not leak.  I suppose that is true.  But will that roof not leak 20 years from now as well?  The problem is IF it leaks, how will you know?

Both open cell and closed cell are air-impermeable, but open cell is far more VAPOR permeable (to water vapor).

The roof leak "problem" with closed cell is highly overstated, and there isn't any field evidence of an increased rates of roof deck rot from the use of closed cell on the interior due to leaks, but there IS good evidence that use of closed cell on the interior will reduce the risk of mold & rot due to winter condensation.

Closed cell foam (generally) provides a more reliable air-seal than open cell, but the installer's expertise still makes a difference. In Zone-3 even 1" flash of interior closed cell provides a non-wicking semi-permeable condensing surface that is unaffected by moisture, yet still allows the roof deck to dry toward the interior. (It also meets the R5 min prescribed by IRC. http://publicecodes.citation.com/icod/irc/2009/icod_irc_2009_8_sec006_par003.htm

The best moisture proofing of the roof deck for zone 3 is open cell foam below the roof deck with fiber to bring it up to code, but venting the exterior of the roof deck by providing a 2x4 furring-mounted nailer deck for the finish roof, and no interior side vapor retarder (but make it air-tight.) The structural roof should have only vapor-permeable weather resistant barrier (not 30# felt) facing the furring cavity to maximize outward drying. (This is the drying-equivalent of a rainscreen gap between siding & sheathing in walls.)

Foam above the roof deck improves thermal performance better than foam under the roof deck, since it provides a thermal break over the framing elements, and is worth doing whether or not you use foam on the interior. With sufficient R above the roof deck vapor-impermeable goods can be used, with only fiber between the rafter elements. It's OK to split the above and below deck foam R if that's more convenient. If using a more-permeable foam (unfaced rigid EPS) above the roof deck, there's still an advantage to using housewrap-type permeable weatherization between the foam and the structural roof, and a furring-mounted nailer deck to promote exterior drying. Leaks may go unnoticed, but the water never reaches the structural deck, and even the nailer deck dries into the cavity.

Unless it's a very complicated roof line with lots of hips valleys skylights & dormers, going with 1.5-2" of exterior unfaced EPS or foil faced iso with a furring-mounted OSB nailer deck, and using closed cell foam only for air sealing the interior would be a decent way to go. Foil facer in a 1.5" cavity will give you some cooling-season performance, even if it blocks exterior drying into to that gap. R6-R12 of thermal break over the rafters gives the assembly much better thermal performance than a code-min all-foam between the rafters treatment.

Doesn't almost every roof eventually have a leak?  If that is a true statement then how can spray foam be justified? 

Good point. Most people wait until they see a leak somewhere to replace roofing. At that point, some sheathing may need to be replaced, with or without spray foam. The spray foam makes it difficult to restore.

Posted By Dana1 on 15 Dec 2011 10:30 AM

The roof leak "problem" with closed cell is highly overstated, and there isn't any field evidence of an increased rates of roof deck rot from the use of closed cell on the interior due to leaks, but there IS good evidence that use of closed cell on the interior will reduce the risk of mold & rot due to winter condensation.

Overstated?  I still dont understand how you will discover roof leaks with the closed cell foam on the underside of the sheathing.  If there is a roof leak, then I assume it will go unnoticed with closed cell foam.  if it goes unnoticed, then I assume the moisture will remain between the sheathing and the foam and lead to rot or mold.

Is it accepted that most roofs will leak at some point during their lifetime?

Posted By Dana1 on 15 Dec 2011 10:30 AM

The best moisture proofing of the roof deck for zone 3 is open cell foam below the roof deck with fiber to bring it up to code, but venting the exterior of the roof deck by providing a 2x4 furring-mounted nailer deck for the finish roof, and no interior side vapor retarder (but make it air-tight.) The structural roof should have only vapor-permeable weather resistant barrier (not 30# felt) facing the furring cavity to maximize outward drying. (This is the drying-equivalent of a rainscreen gap between siding & sheathing in walls.)

What if I were to do away with the 2x4 furring above the roof deck.  Ideally i would like to do open cell below roof deck, fiber to bring it up to code and no 2x4 furring.

Is that acceptable per code?  Any major drawbacks.  i understand it wont be the perfect system.  Would you then use 30# felt?

Thanks

If your roof leaks with ccSPF on the interior it either runs somewhere to a void where it drips through and it's a PITA to find, or it leaks locally to be discovered only when re-roofing (and sometimes not even then, if it hasn't gotten too punky.)

From a code POV going with a flash of ccSPF from below and fattening the R with fiber works. Open cell won't necessarily meet the permeance requirements, but in general it won't likely cause an issue in zone 3. To be on the safe side there's no down-side to using a variable-permeability vapor retarder on the interior of the fiber (eg Certainteed MemBrain) if you go that route, and it'll reliably protect the roof deck from winter moisture loads, and allow ample drying capacity. Yes on the 30# felt.

But an inch of XPS above the roof deck would be comparable in cost, higher in thermal performance, and still meets the IRC prescription without an interior vapor retarder. While the nailer deck on furring would make it more resiliant, it would still perform OK without it. But long-nailing shingles through the foam to the roof deck is more susceptible to gale-force wind, and reduces the thermal break somewhat. I used 1" iso this way on a project in suburban NYC (with kraft vapor retarder on the interior), and it made it at least 20 years without issues, but I haven't seen it since the last hurricane...

Consider using ripped OSB for furring strips instead of 2x4s.

Over foam I doubt you can get the same structural integrity with ripped OSB furring unless you tighten up the fastener spacing, which would be even more expensive. With no rigid foam it may work, but a 1.5" vent cavity is still preferable, especially on lower-slope roofs. (Some of the BSC guys consider 2" even better, but I don't know what that is based on.)

Hi Guys,

  I was just contemplating doing the same thing with my ranch house. Open cell or closed cell? I was going to do 8" open cell under the roof deck. On the roof deck, I was going to double up 4x8 sheets for better strength (24" centers").  I was then going to ice/water shield the entire roof, then put 1x4 furing strips and do an exposed fastner metal roof. Does this sound like good practice? My concern was with 8" open cell under the roof deck, will I have moisture issues? Will I be better off sandwiching 2" rigid foam between 4x8 plywood, then ice/water shield with furred metal roof? I have a 4.8kw DC solar array, this is why I original was swayed off 2" rigid foam.. Suggestions?

Tom

Posted By tomtoth on 04 Jan 2012 05:33 PM

Hi Guys,

  I was just contemplating doing the same thing with my ranch house. Open cell or closed cell? I was going to do 8" open cell under the roof deck. On the roof deck, I was going to double up 4x8 sheets for better strength (24" centers").  I was then going to ice/water shield the entire roof, then put 1x4 furing strips and do an exposed fastner metal roof. Does this sound like good practice? My concern was with 8" open cell under the roof deck, will I have moisture issues? Will I be better off sandwiching 2" rigid foam between 4x8 plywood, then ice/water shield with furred metal roof? I have a 4.8kw DC solar array, this is why I original was swayed off 2" rigid foam.. Suggestions?

Tom

Ice & Water shield over the entire roof is a bad idea unless the roof deck is vented from below (any climate).  It's a very low permeance vapor barrier, and effectively blocks all drying toward the exterior.

If you put 2" of rigid foam above the roof deck, and leave a ventilation gap between the foam and the nailer deck so the nailer deck can dry it can work.  Whether that's enough exterior foam-R to be able to then use air or vapor permeable on the interior of the roof deck depends on your local climate and the type of foam.  Where are you?

Thanks for your input Dana1,

I'm in Northern Virginia
What if I used the plastic staple up stuff, all the way to the ridge. (my soffit is already vented.) This way I get venting under the roof deck, then spray foam on top of this? I thought of rigid foam ON top of the roof deck, but then this makes for interesting on how to attach metal roof/Solar array.

What if I take 2" rigid foam, furr that off the existing roof deck, the take 3" screws, screw through another furring through the foam into the existing roof deck?

One of my big concerns is existing deflection on roof deck.. if foam is added on top, there is a good chance I Can break foam when walking on roof?

What about, furring off existing roof deck, add new 4x8 plywood on top of the furring, giving me venting on top and bottom deck, then add rigid foam on top of new roof deck, apply ice/water directly over rigid foam, then fur metal roof on top of that?

Trying to get best bang for buck..

thanks again, Tom

Think about it for a minute - if you vent a space on the house side of the insulation your insulation isn't going to do a thing for you. You'd be letting cold air into a space that is supposed to be the warm side of the insulation. I'm guessing - and Dana would actually know - but I would think 2" of rigid outside (but tight to) the sheathing would be enough in Northern VA - although it somewhat depends on how much other insulation you have on the underside of the sheathing. The goal is to keep the underside of the sheathing warm enough to stay above the dewpoint. I would think a good application would be sheathing, rigid foam 2" taped sealed), a permeable membrane like vaproshield or commercial Tyvek (check for heat ratings), furring, metal roofing. I would run the furring at 45 degrees to the eave and "break" the runs to allow moisture to drain down and air to flow through. Vent the furring space at eave and ridge. Most metal roofing "likes" to be on furring as having it solidly backed tends to keep the condensation that forms trapped by capillary action and that leads to corrosion.

Its kinda obvious now, yeah thats a waste. My big thing is, that I have a 24" OC roof rafter. All of my plywood on the deck have some sort of deflection if you step in the middle of it. (Installed solar PV and water panels exacerbated the issue). So I don't want to have to replace the entire roof deck.. Rigid foam on top of this roof deck may crack.. and have new issues. I was thinking perhaps vent under the roof deck with the --\___/-- pieces between rafters from soffit to gable, spray foam on that..

Its why I originally wanted to put another layer of 4x8 sheets staggerd to keep deflection down.

If deflection between the trusses is an issue why not up the strapping on top of the foam to 2x3's or 2x4's on the flat to "span" the 24" spacing. The deflection would only be from wind load and occaisional walking up there for maintenance. If most of the roof is covered with panels just make sure the raking system supports align with the trusses and that wherever one will be "walking" the strapping is beefed up appropriately. Design the different areas of the roof for the function they are to serve.....

If you glue foam to existing plywood and then glue another piece of plywood onto that, you will have created a SIP. Quite strong.

I was under the impresson that SIP roof panels become "gooey" after a while...

For the panels, I crossmounted 4x4 between 2 rafters, and mounted thru asphalt shingles and attached my unirac anchors to that.

Birdman/Jonh,

What if I did a hybrid of this, glue the rigid foam to top of roof deck, ice/water shield on top of that foam, and us 2x4pt lumber staping/furring on top for the metal roof to attach to. Now if I did that, would the 2" rigid foam on top, allow me to directly apply spray foam under the roof deck?

I'm still trying to figure how to appropriately vent the wood structure.. on a non-vented/warm roof application..


so..
Metal Roof
Furing 2 x4
Ice/water Shield
2" Rigid Foam
Glue
1/2" Roof Deck
8" Open cell Spray Foam..

OR
Metal Roof
Furing 2 x4
Ice/water Shield
1/2" Roof Deck
Glue
2" Rigid Foam
Glue
1/2" Roof Deck
8" Open cell Spray Foam..

Thanks again,
Tom

I would be wary of the ice and water shield over the whole roof. This product is highly impermeable and will essentially prevent all drying to the exterior. IF you do that, make CERTAIN that you have some really adequate drying potential to the interior. Installing 6 mil poly behind the drywall for example would be a near fatal error..... I think the combo of I&W Shield over the whole roof and OC foam might be a problem (although I would think CC foam would be worse).


I&W Shield over the whole roof is too much of a good thing. If some is good, more is not necessarily better.....

I'm ice/water shield crazy because I've never lived in a house where the roof has NOT leaked...

Looking back at what Dana1 Said:

The best moisture proofing of the roof deck for zone 3 is open cell foam below the roof deck with fiber to bring it up to code, but venting the exterior of the roof deck by providing a 2x4 furring-mounted nailer deck for the finish roof, and no interior side vapor retarder (but make it air-tight.) The structural roof should have only vapor-permeable weather resistant barrier (not 30# felt) facing the furring cavity to maximize outward drying. (This is the drying-equivalent of a rainscreen gap between siding & sheathing in walls.)

Metal Roof
2x4 furring mounted to roof deck
vapor-permeable weather resistant barrier
Roof deck
8" spray foam

Would this work? I do not want to use Fiberglass in attic as its low pitch roof with engineered roof trusses, so trying to attach it succesfully is nearly impossible.

Would I then need to add some sort of fire/vapr barrier on inside of spray foam? Drywall is not option.. Attic is open cavity in which I hope to install HVAV unit.

Check with the manufacturer of the metal roofing you're using before buying PT lumber - copper arsenates and aluminum don't get along too well - steel may not either. With metal you need to consider the movement of the material due to temperature and also the effects of condensation on the back side (this is normal for metal roofing and is not to be confused with the possible condensation of vapor coming from the interior of the house). 8" of OC foam will be big $$$. Usually it's more cost effective to spray foam the thickness needed to control condensation then use a less costly insulation like FG or cellulose to get the rest of the required (or desired) R value. You WILL need an ignition barrier over the foam.

Maybe:

Metal Roof
2x4 furring or other venting
4" rigid foam
vapor-permeable weather resistant barrier
Roof deck
4" spray foam

Or a vented attic with cellulose. Then you can use I&W Shield.