Tech Talk: SIPs Roofed The Right Way - Sloped Roof Deck Assemblies

Structural Insulated Panels, SIPS, are among the most advanced High-Performance Building Envelope (HPBE) technologies available to construct walls, floors, and roofs.  A SIP HPBE delivers energy savings, superior strength, and long-term durability.  When SIPs are used as the roof structure, they are ceiling, insulation, and exterior roof deck, all in one component.  SIPs, with their solid foam insulation core, create a high-performance non-ventilated roof deck that stops energy-robbing air movement at the roof line of the structure.  Due to this unique energy-saving performance attribute of SIPs , SIPS must be covered with roof cladding systems in a thoughtful building science manner that matches their non-ventilated roof deck benefit.

Foundation For Long-Term Roofing Success

The foundation layer that sets the stage for long-term roofing success over SIPs, is specifying the right underlayment for Sloped Roofing Decks (2” of vertical rise for every 12”  horizontally of the roof deck - or greater).  Both mechanically adhered and self-adhered underlayments can be used over SIPs roof decks. Still, in both cases, the underlayment MUST BE VAPOR PERMEABLE with a recommended Per Rating of 14 or greater.  Additionally, Self-Adhered underlayments made with synthetic material and having non-VOC adhesive for attachment to the SIP roof deck, are required. The reason for vapor permeable underlayments over SIPs is to ensure that any vapor emanating from the interior of the building and goes through micro openings at SIP joints, can pass through the vapor permeable underlayment and dissipate any condensate at the underside of the roof cladding material.  Using vapor permeable underlayments supports good building science applications for Sloped Roofing assemblies.  In response to more demand for vapor permeable underlayments in the marketplace, numerous roofing materials manufacturers offer both mechanically and self-adhered underlayments.

Special Note: Underlayments that are non-permeable (made usually with modified bitumen technology and adhered to roof decks with VOC primers and adhesives)  and describe themselves as a “vapor and air barrier”, must NEVER be applied over SIP roof decks.  Non-permeable underlayments, by the instruction of the manufacturers of these products, limit their use to “ventilated” attics and “vented spaces” below the non-permeable underlayment layer.  VOC-based primers and adhesive will volatilize in hot weather, causing damage to the SIP’s foam core, and are NEVER to be applied over a SIP roof deck.

Mechanically attached vapor permeable underlayments and roof claddings are installed as specified by the underlayment and roof cladding material manufacturers and must meet building code requirements.  Self-Adhered underlayments must also be installed per the specification of the underlayment manufacturer.  As stated in the Special Note above, VOC-based primers and adhesives are prohibited for use over SIP roof decks.  Primers and adhesive used to attach the Self-Adhered vapor permeable underlayment must be low VOC and/or water base and compatible with EPS foam.  A very important condition must be met after applying a self-adhered vapor permeable underlayment to a SIP roof deck:  The vapor permeable self-adhered underlayment must NEVER be removed from the SIP roof deck.  Attempting to remove the underlayment may cause physical damage to the OSB facing of the SIP.  Therefore, when future reroofing is done, the vapor permeable self-adhered underlayment is to remain in place, and a second layer of vapor permeable underlayment is to be attached over it.

Roof Cladding systems applied over a SIP roof deck having a vapor permeable underlayment below must consider these potential concerns and solutions:

Asphalt Shingles

Life Durability and fire codes of the shingles – solution:  Use architectural 2-ply Class A grade shingles for greatest durability and to meet fire regulations for flame propagation of the roof assembly.

Metal Roofing Systems

Moisture condensate forming under the metal roofing – solution: Add a thin layer of synthetic ventilating fabric creating a venting space below the metal roof that will allow active moisture dissipation.

Elevated heat to the underlayment due to the dark color of the metal roof or the use of Zinc or Copper – solution: Add a thin layer of synthetic ventilating fabric creating a venting space below the metal roof that will allow active heat dissipation and the protection of the underlayment.

Noise transfer from heavy rains and hail hitting the metal roof – solution:  Add a thin layer of synthetic ventilating fabric creating a disconnect space below the metal roof that will reduce impact sound transfer to the SIP.

When SIPs technology is used to construct a home or commercial structure, energy is saved, durability and strength are achieved.  Applying Sloped Roofing Systems properly to the SIPs roof deck will add years low maintenance and  service life to the SIP structure.  We hope that you have found this Tech Talk to be useful, but if you have any questions about the roofing of SIP roof decks, please contact your Premier SIPS representative here.