Good – Better – Best
By: Patrick Sughrue
Buildings can go to heck in two ways:
- Water and vapor trapped on the inside
- Unmanaged Water on the outside
Luckily there are some simple solutions to use when using Structural Insulated Panels for framing the building envelope.
Water on the inside.
On the inside of walls it’s easy to design assemblies to avoid future problems by simply building it tight and ventilating it right. With SIPs, the interior and exterior skins are made of OSB, which are rated at 40 Ng (2/3rds of a perm) and qualify as a vapor retarder. The OSB interior skin of a SIP wall can be used as this vapor retarder on the interior.
When the SIP seams are sealed per the SIP manufactures details on the inside, the SIP structure will achieve an ACH of < 1.0 and the interior is taken care of. When assembling SIPs, always follow the manufacturer’s instructions and construction details for sealant and tape application. (View here on Premier’s YouTube channel.)
Including appropriate ventilation for the interior as part of the HVAC system design is the final key to managing interior moisture issues. An ERV and/or HRV system should be included depending on the geographical location of the structure.
Water on the outside.
What you need to do depends on how much rain you receive in your region. SIPs really do not need an air barrier on the outside. What is needed is a moisture barrier covering the SIP exterior that will keep any bulk moisture on the outside where it can dry out.
The three options below also require QUALITY installation of the moisture barrier, claddings, windows and doors.
- Good option: In dryer areas a vapor permeable weather resistant barrier (WRB) building wrap should be sufficient. Consider products like Solitex, Tyvek, etc. see Benjamin Obdyke, and Protecto Wrap.
- Better option: In moderately rainy areas at least a drainable house wrap (WRB) is preferred. Explore products like HydroGap, and Sliker Max. Cedar breather.
- Best option: In marine coastal areas, vapor permeable roll on / spray on WRB or self-adhered WRB and battens, or a product like Sliker Max, create a true rain screen drainage plane.
OSB is an amazing product and the structural grade used for SIPs skins comes in 8’ x 24’ sheets. (Bonus: These large factory manufactured structural insulated panels sections also offer considerable labor saving for contractors.) Exposure 1 SIPs grade
OSB can withstand wetting during a structure’s building phase, but after being covered with WRB’s and cladding, must stay dry to be durable and long lasting for the structure’s life.
Always remember the tenants of building science: If an assembly is sealed on the inside, it should be vapor permeable on the outside. SIPs are sealed on the inside.
All roofing materials have a range of roof pitches for where they are best suited. A 3-tab asphalt shingle is designed for 3/12 – 15-degree roof pitch or higher. The same is true for snap lock standing seam metal roofing. Can you go to a lower pitch? Sure, but you better double check your detailing with the specific manufacturer for the roofing material you intend to use.
With SIPs being sealed on the inside, when a three-tab asphalt roofing shingle is used over SIPs, a good quality vapor permeable (breathable) roofing underlayment included in the roof assembly is usually sufficient.
Standing seam metal roofing materials offers a few options:
- Good option: A vapor permeable (breathable) roofing underlayment is sufficient in dryer climates.
- Better option: A thin batten ¼” batten creating a ventilating space over a breathable synthetic underlayment.
- Best option: A product like a thin cedar breather or a full ¾ moisture resistant batten system over a breathable synthetic underlayment.
- On lower sloped roof pitches, a product like SlopeShield plus, a self-adhered vapor permeable underlayment is your best choice.
BUILDING OWNERS, PLAN AHEAD
If you are the designer or owner of your project, it is your responsibility to specify and demand the best level of protection for your project’s long term weather durability resistance. Stick to your decision, since the contractor or sub-contractor that you’ve hired may not be familiar with the building science tenants that I have presented in this blog.
You may have to purchase the material and have on hand for the installers. PLAN AHEAD! Some of the solutions presented may cost a few dollars more, but the durability of your building envelope should take maximum priority over the interior transitional choices you will make for interior finishes.
I hope that the recommendations I have discussed in the article enable you to build your best possible structures. My long experience, and my companies’ committed market involvement in supplying a High Performance Building Envelopes that I have presented here, will lead you to design and build the highest performing structures you have ever experienced.
Patrick Sughrue is Premier SIPS Oregon & Eastern Washington SIPs Expert. With more than 25+ years experience building his own structures, as a general contractor for clients and representing Premier in multiple West Coast markets, Patrick offers real life solutions for building optimal structures with SIPs.