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TOPIC: Ventilation

SIP roof

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I am installing a metal roof with a 4/12 pitch SIP roof on 1 x 4 strapping. I would like to provide ventilation between the metal roofing and the OSB of the structural insulated panels. The manufacturer tell me that I should be using foam closure strips in order to keep critters out, but that would effectively cut off any incoming air. At the top they have provided me with a ridge cap, a coarse mesh and foam closure strips. Ventilation there would exit out the gable ends of the ridge only. Is there any way to effectively vent the underside of the metal panels? The manufacturer does not seem familiar with SIP roofing. Thanks in advance.
Guest User

4/5/2007
You could put down vertical battens first followed by horizontal and then have an eave detail and ridge vent that allow for air flow. You may also want to look at a product called Profilevent. Make sure that you have a complete vapor barrier on the inside of the panels and that the panels are assembled and the joints sealed properly. Todd Miller
Todd Miller
Isaiah Industries, Inc.

4/5/2007
Reading with interest your advice on metal over SIPs. I have installed Premier SIPs on my timber frame house being built on the BC coast and was advised by Premier rep and Premier tech support to apply peel & stick roofing membrane to SIPs then screw metal directly to SIPs without any strapping. I have also talked with tech support from Insulspan here in Calgary and was advised to cross strap without peel & stick. The Premier and Insulspan SIPs are identical I believe, so why the difference in advice? Or do both systems prevent moisture build up via different means? If no warm moisture laden air gets through the SIPs to cold underside of peel & stick, (due to vapor barrier on underside of SIP) there will be no moisture on underside of peel & stick, and if moisture condenses on exterior of peel & stick in raised metal panel ridges, the ventilation through same ridges should be enough to clear it and peel & stick will prevent any moisture from getting through to SIP OSB. I agree that adding cross strapping on top of peel & stick will provide better ventilation under the metal, but dont see how it will reduce moisture reaching the SIP OSB. Your comments please? Thanks, Leon
Guest User

12/5/2007
The VB on the warm side of the assembly, and in these cases on the interior, are 'blocking' the moisture that is held within the interior environment, which is generally introduced by the occupants, from migrating into the roof assembly and condensing when it reaches a temperature where the colder air (or surface) can no longer support the moisture content that the warmer air is supporting in the interior. Leon’s comment about a self-healing non vapor permeable membrane (like a self-healing product like Grace Ice-Water Shield) being installed outboard of the VB in Calgary will not trap the moisture that the VB is dealing with in the roof assembly as the VB is already doing that job… so having the non-vapor permeable membrane on the outboard of the surface of the SIP panel isn’t an issue if you install that VB. If you didn’t install the VB you should be more concerned as you’d potentially trap that interior generated moisture exactly where you wouldn’t want to. Ventilating a roof assembly is generally addressing ‘other’ issues unrelated to moisture migration that the VB is dealing with. Specially ‘air’ is an insulator and ‘r’ and ‘u’ values of assemblies take into consideration both still and moving air in vertical and horizontal positions. Ventilating a ‘cathedral’ roof and / or an ‘attic’ basically creates a cavity that allows the exterior roof surface and finishes to remain at a temperature much closer to each other, hence allowing the insulting assemblies to be more ‘efficient’. In application this can’t be achieved without adequate ventilation as the trapped air will impact the temperatures of the materials and surfaces. The typical scenario to think of is a cold climate condition (like Calgary) where snow build-up on your roof is melted when a not so efficient roof assembly allows interior heated air to melt snow trapped close to the roof… then as it melts, drains and reaches an overhang that is colder (since there is no heated space below it) it freezes (now it’s ice)… and you’ve effectively created an ice damn trapping any additional water behind that may find it’s way into the roof assembly. All of this can be alleviated if you keep the roof finish as close to the outside air temperatures as possible to minimize any thawing of snow from the inside out. Just remember that ALL of this is impacted by the details of a given installation… however SIP systems are generally more efficient with less thermal bridging, more efficient insulation per thickness than batts and thicker assemblies can create ‘super’ insulated scenarios… thus if you address the vapor migration with a VB, ventilating the roof isn’t necessarily ‘required’… as the assembly should already be pretty darn efficient (hence why you’re probably getting two answers to your question from manufacturers). But, if you truly want a belt and suspenders systems (which is never a bad thing in my opinion)… ventilating the space below the metal roof is a good idea. If it wasn’t a SIP roof system I would say definitely ventilate a metal roof installed over a conventionally framed ‘cathedral’ roof… and there’s never a reason not to in an attic application as it’s simply easy detailing. Also give a look into ventilated wall systems as well (Rain-screen) as both roof and wall systems benefit from these applications. Canada has done a much better job than the US in progressing these systems and contractors in those areas are ‘’generally’ more familiar with their ‘correct’ use. City’s like Vancouver, BC actually (generally) require the installation of ventilated envelope systems in new construction…. But that’s a long detailed topic in itself. Sorry for the long detailed response… the thread is fairly old but hopefully this is still useful.
Guest User

12/30/2007
Thanks. Todd Miller
Todd Miller
Isaiah Industries, Inc.

1/5/2008
cor-a-vent is the correct solution to your problem
Guest User

5/13/2009
It sounds like you are using a Delta-rib or similar product. If this is the case ventilation is the least of your worries. I would NEVER recommend this over any sheathing. Delta rib is intended to be applied to metal perlins running perpendicular to the primary direction of thermal expansion. Without the flex/wobble allowed these perlins your roofing screws will 'pop' because the metal roof is tightly bound to a material with radically dissimilar thermal expansion/contraction values. In my experience you will have water infiltration issues from bad fasteners within 15 years (likely 10 if your observant). Find a product with at least slotted holes, it should have sliding clips. The only screws that should be driven directly though the panel and into sheathing are the 1 or 2 which keep the panel from slipping down the roof. Concerning the eave detail; a standing seam roof should have the bottom turned under and attached at the eave with a continuous cleat.
Guest User

12/2/2009