Designing a Class A unvented roof assembly with metal covering in WUI area

[flightcrazed]

Hi all, working on a project that was originally designed to be an unvented attic with asphalt shingles.  Plan is to change to metal, however the underlayment (Titanium UDL50) has already been laid and is only Class A under asphalt.  I've found that Versashield can bring metal to Class A but not intended for unvented assembly. 

 

Anyone know of another product that could be laid on top of UDL50 to reach Class A for metal?  I've found Atlas FR10 but all the documentation seem to support a commercial roofing system.  

 

Much appreciated!

[Alaskan_Son]

Because of its low perm rating, I don't believe Titanium UDL50 is designed for use over an unvented attic either.  If the builder was okay with that, then I would think Versashield would be just fine as well--if not better.  I think the real problem is the perm rating of any product going over an unvented assembly and the manufacturers resistance to taking on any liability whether actual or perceived.  I believe UDL50 has a perm rating of around 0.05 whereas Versashield has a perm rating of 3 at a minimum.  Major difference and a major improvement in breathabiliy over the UDL50. 

 

Either way, if you really want to do it by the book and decide to go with DensDeck or some other similar fire resistant paneling/sheathing, make sure not to defeat the whole purpose by putting another non breathable roofing layer over the top of it. 

[flightcrazed]

Thanks for the reply @Alaskan_Son.  Sorry, I didn't share full context, but there is a 2" layer of polyiso above the structural OSB followed by another 7/16 osb as a covering attachment layer.  It is this attachment layer that has the UDL50 on top.  The design principles for the unvented roof is how i arrived here...but seems like perhaps a metal roof shouldn't be installed above an unvented assembly (all of the "recommended" underlayments are low perm in nature, i.e. Titanium FR, Polystick XFR) if i'm understanding you right?

[Alaskan_Son]

36 minutes ago, flightcrazed said:

Thanks for the reply @Alaskan_Son.  Sorry, I didn't share full context, but there is a 2" layer of polyiso above the structural OSB followed by another 7/16 osb as a covering attachment layer.  It is this attachment layer that has the UDL50 on top.  The design principles for the unvented roof is how i arrived here...but seems like perhaps a metal roof shouldn't be installed above an unvented assembly (all of the "recommended" underlayments are low perm in nature, i.e. Titanium FR, Polystick XFR) if i'm understanding you right?

I wasn't necessarily trying to put forth a specific solution or conclusion, just presenting you with some things to consider.  The simple fact is that unvented roof assemblies with any sort of wood product in them are going to be problematic PERIOD.  Once you go down that road though, I think you just do the best you can.  In your particualr case:

• You have 2 layers of OSB, both of which can be less than 1 perm
• Your polyiso layer might have a perm rating of as low as zero depending on what backing it has
• Your current roofing underlayment (UDL50) has a perm rating of around 0.05
• I don't know whats happening underneath the original layer of OSB, but at best you already have plywood sandwiched between 2 vapor retarders
• If I'm not mistaken, at 3 perms or higher, Versashield would actually be the most breathable and least problematic part of that whole assembly, so I think this is the least of your worries.  If you are at peace with the rest, I'm not sure I would lose any sleep over using Versashield on top of it...although I might consider removing the UDL50 first.

 

Just my 2 cents.

[flightcrazed]

Thanks Michael, appreciate that.  Honestly, i don't really find peace in either scenario: venting in a high fire area, or now unventing and having water problems.   Though pulling the UDL50 and giving a little breather to the top doesn't sound too bad at this point. 

 

Underneath the roof deck, i'm a bit undecided on insulating at roof plane or ceiling plane now.  Original insulation plan was 6" polyiso on top (none underneath) to keep the unvented design but since that has changed to 2" on top, I am thinking R30+ fiberglass batts in either roof or ceiling plane.  The struct OSB does have a perforated reflective barrier (ceiling side).  I think if i put in ceiling plane I'd have to ventilate.  

[Gawdzira]

I just spoke with a tech at Benjamin Obdyke. Both the Vapordry SA and the Cedar Breather products are class A rated. The documentation for Vapordry is forthcoming but it just passed the class A rating. 

 

https://benjaminobdyke.com/product-systems/system-metal-roof/

[flightcrazed]

1 hour ago, Gawdzira said:

I just spoke with a tech at Benjamin Obdyke. Both the Vapordry SA and the Cedar Breather products are class A rated. The documentation for Vapordry is forthcoming but it just passed the class A rating. 

 

https://benjaminobdyke.com/product-systems/system-metal-roof/

awesome, thanks Alan. I'll check in with those guys. Do you have good experience with their products? 

[Gawdzira]

I have used Benjamin Obdyke Slicker for wall rain screen. Easy to order direct through them. The products are very good. I have also used the Hydro gap product on a few projects. That WRB is great if you are not using a full peel and stick or liquid flash product.