Creating an air barrier in a SIPs roof
SIPs (Structural Insulated Panels) most commonly include a styrofoam core. The panels themselves are generally airtight, but the joints often are not. This can lead to a lot of damage to homes for lack of a proper air barrier.
The installation of a SIPs roof
Any time warm interior air can find a path to the cold outside, it will take it. As warm air cools (still inside your wall or roof) it leaves condensation on any available surface.
SIPs panels themselves are most likely air tight; the potential problem lies in how the joints are managed. Having assembled several of these systems, I have seen first hand a few different designs that attempted to create an air seal at the joints, and seen the failure of some of those attempts as well.
Worth noting from the beginning, is that houses move.They shift, they creak, and they settle. What always has to be considered, is how that movement may affect the systems of your wall. By systems I mean vapour barriers, air barriers and thermal barriers.
Despite how often they are spoken about on a building site, vapour barriers are relatively unimportant within a wall assembly. Air barriers, however, are extremely important and should be designed to withstand any stresses they might experience. The problem stems from the misconception that they are one and the same, when they are not.
Any leak in your air barrier will allow warm moist air to find its way into your building envelope and potentially damage your thermal barrier (insulation) as well as your structure.
In order to prevent air leakage, you should ideally do some troubleshooting when you are still in the design phase and find where a potential breach in your air barrier could occur, and have a solution in place that is strong enough to withstand the normal movement a home will experience. This applies to any type of wall system you choose, not just SIPs.
Brown stains on the drywall where air leakage caused condensation
The house in question here performed fairly well (though there were a few very minor signs of moisture damage in the ceiling), until being jostled by a moderate sized earthquake, after which there was significant moisture damage. What was counted on to be an air barrier was not the least bit flexible, making it completely unreliable.
Moisture damage on the exterior OSB of the SIPs roof
As mentioned above, the panels themselves aren't usually a problem, but the joints certainly can be. The more successfully sealed SIPs I have worked with included panels with either a tongue and groove design or lap joints on the edges, and some sort of flexible sealant. I cannot attest to how flexible those sealants will remain with the passing of time, but they certainly stand a better chance than rigid ones.
This brings us to one of the bigger offenders, panels that were assembled relying on only spray foam to seal the joints. Having worked on several such panel systems in the past (with crews that had the best of intentions but lacked an undertanding of air barriers) I have seen too many failures to trust them.
The following chronicle is about the 'better late than never' installation of an air barrier on a SIPs roof, and an effective technique for any new building made with SIPs. It is a simple solution that with a little foresight and an understanding of air barriers can save a lot of headaches and unnecessary repair expenses. It involves the installation of a flexible airtight membrane over the joint of every panel, that in inconjunction with the OSB amounts to being a continuous and impermeable air barrier over the entire building envelope.
A timber frame house with air leakage through the roof SIPs
© Stripping a SIPs roof system to repair the air barrier
Ten years ago after raising this frame, we covered the walls and roof with SIPs panels, an exterior envelope that left the entire timber frame structure visible from the inside. Still wet behind the ears when it came to air barriers, we put spray foam in all the joints, then patted ourselves on the back for a job well done.
Over the course of a couple of years, we experienced a varying degree of success (or failure if you will) depending on circumstances. The less complicated roofs took longer to show damage, the most complicated and poorly foamed roof was a disaster within a few years, with the OSB so rotted in places it was coming apart in our hands.
Given that we tried multiple designs and manufacturers, there is no doubt there are countless other houses out there experiencing similar issues, which is the reason for documenting this process.
Our step by step SIPs repair process:
1. Strip the roof to it's original OSB, removing the metal, strapping and tar paper drainage plane. Be sure to carefully map out where the strapping was so you can re-install it in the same place so it lines up with the screw holes in the metal.
The installation of a Resisto flexible membrane along the seam of a SIP roof will stop air leaks.
2. Apply a liquid primer to all joints and 6 inch wide flexible membrane over all the joints of the OSB, and all fastening penetrations.
3. For the ridge we took a 36 inch membrane and cut it down the middle to make an 18 inch membrane, installing 9 inches on the first pitch, the other half ready to flip down on the other side when the second pitch was completed.
4. We installed an additional 2 inches of Polyisocyanurate foam for good measure since the roof was open anyway, bringing it to approximately R57. We chose polyisocyanurate because it has the highest R value per inch of any foam insulation and the lowest environmental impact.
5. Polyisocyanurate comes with a foil membrane on either side, but is still quite prone to water damage, so we chose to cover it with a top of the line breathable waterproof membrane as a drainage plane. The breathable aspect to this product was not needed in our particular case, but a tough waterproof membrane was essential.
AirOutShield waterproof drainage plane from SRP Canada with vertical strapping.
6. Vertical strapping to allow a clear path for any water that finds its way through the metal, then horizontal strapping for re-attaching the metal roofing. Another mistake was originally made here - only horizontal strapping was installed, so any water that penetrated the metal would pool up instead of draining away.
7. New roofing screws - metal roofing screws have rubber gaskets, which can break down after years of exposure to weather and UV rays. It is generally recommended that you replace the screws every 25 years or so to be safe, but if you remove them the gaskets are often damaged, so we went ahead and replaced them now for good measure.
8. With an additional 5/8 of an inch of strapping and 2 inches of foam, we installed a new drip edge at the bottom to account for the shortfall in our existing fascia.
Repairing the air barrier on a SIP roof © Ecohome
With all that done, we are now quite confident that this new air barrier will withstand the normal movements this home will experience, along with any other earthquakes. The vapour barrier in this case (completely unrelated to the air barrier) is fiberglass drywall with vapour retarder primer.
The repair we did here is specific to a roof; wall cladding is rarely as easily removed. Fortunately in the cases I have witnessed, the walls were not a problem. That could have to do with the natural chimney effect of a home, as there is always more air pressure at the highest points making ceilings more vulnerable to air leakage, also the wall SIPs were well-fastened with a plywood spline that likely helps stop air movement.
What we did here could also have been accomplished by covering the entire roof with an ice and water shield membrane, but it seemed a more affordable option to just do the joints, particularly since there was additional insulation to follow, and an additional drainage plane.
There are also some high performance solvent-free building tapes available that would probably do the job just fine, but I wouldn't even consider it with an average off-the-shelf building tape, they break down and lose their seal in very short order.
For building with any SIPs system I would highly recommend some kind of flexible membrane such as this for all joints, walls and roof alike. Even if a good interior seal is in place, a little redundency is a good thing when the stakes are high, like the performance and durability of your house. Pay particular attention to the eaves, corners and any dormers, those are notoriously tricky areas to deal with when building with SIPs.