Before 2001, the biggest construction project I had ever taken on was a bookcase. My most complicated project was probably the mahogany lamp table I built in tenth grade shop class. (I still have it!) Then I decided to build a house. After weighing all my options, I decided to build with structural insulated panels (SIPs). With help from a SIP consultant and an experienced builder, I got the house framed and then finished it out over the next 21 months.
What are SIPs?
Structural insulated panels are sandwiches of oriented strand board (OSB) on each side and expanded polystyrene (EPS) rigid foam in the middle. You can see the ones we built with laid out on the floor on our first day of erecting the SIP walls in the photo below. After getting the basement done and a framed floor on top of it, we built all of the above-grade walls and the roof with SIPs.
Different companies have different methods for connecting the panels. Some eliminate all thermal bridging where the panels join by using connecting pieces (called splines) with foam in the middle. The house I built used solid wood splines between each two panels.
The standard width for the panels is 4 feet, though, so there’s a lot less thermal bridging with “studs” 4 feet apart rather than 16 inches apart. Using structural insulated panels for the roof requires wood splines for structural support, but again, they’re 4 feet apart. You could always put exterior continuous insulation on the outside of the walls or roof to reduce the thermal bridging from the wood splines.
The advantages of SIPs
The advantages of SIPs are reduced thermal bridging and easier control of heat, air, and moisture. Solid insulation embedded in panels means that air sealing should be easier. I had never seen or done a blower door test before building with SIPs, and I measured an air leakage rate of 1.7 air changes per hour at 50 Pascals (ACH50) when I tested the house upon completion.
Controlling liquid water is no different than doing so for the other types of structures. You can use house wrap, fluid-applied membranes, felt, peel-and-stick membranes, or another type of liquid water control layer.
The panels themselves have a low vapor permeability so you must make sure that the walls can dry from the panel to the indoors and also from the panel to the outdoors. The interior OSB can dry only to the inside of the house, so don’t put plastic under the drywall. The exterior OSB can dry only to the outside, so it really should have a gap to aid drying (i.e., a rainscreen).
SIP homes are generally stronger than many stick-built homes. They have survived hurricanes in neighborhoods where all the other houses were destroyed.
The flaw to avoid
The big caution for building with SIPs is that you must make sure all the seams, joints, and penetrations are air-sealed to the hilt. This is especially true at the top of the house because the stack effect will put pressure on any weakness in the air barrier. Some SIP houses have required extensive repairs to the sheathing after only a few years because of air leakage.My friend John Semmelhack of The Comfort Squad in Charlottesville, Virginia built a SIP house in 2008. As his family grew, he added onto the house in 2015 and discovered that some of the OSB on the roof and the upper part of the walls was damaged. The problem was not the overall airtightness of the whole building enclosure. He was almost Passive House tight at a little over 0.6 ACH50.
The problem was the concentrated air leakage through seams at the top of the house because of the stack effect. In winter, that put humid air from inside the house in contact with cold surfaces, and you know what happens then: accidental dehumidification. You can see one of the moisture-damaged seams on his roof in the photo above.
How to keep your SIP house from rotting
Semmelhack repaired his roof, but what could you do differently to avoid this problem from the start? One simple change could make the difference between needing repairs in less than 10 years and a SIP house that lasts for decades. The basic principle is to keep humid air away from cool surfaces.
When John and I built our SIP houses, the air barrier was the whole panel. The weak part was at the connections between panels. Before we connected two panels, we sprayed can foam on both sides to stop air leakage. As Semmelhack found out, though, even a few small areas of air leakage can damage the enclosure.One step in solving the problem is to put a continuous air barrier on the outside of the panels. Peel-and-stick or fluid-applied membranes work well as long as they’re vapor permeable. But that’s not be enough. Warm, humid air inside the house can cause problems through convective looping in the seams between panels (diagram above). Thus, the other important step in ensuring your SIP house won’t rot is to seal the inside.
Some suitable products for this are:
If I had it to do over again, I’d still spray can foam into the connections between panels. But I’d also make sure each seam on the inside and the outside of every wall and roof section was sealed with a good liquid flashing membrane or high-quality air-sealing tape like the ones listed above. The key is finding a good one that works with OSB.
Since John’s SIP house had moisture problems and mine used the same methods and materials, you may be wondering how mine has fared. It’s been 21 years since we framed and sealed it after all. You might think that mine has fared better because Carrollton, Georgia (IECC climate zone 3) is a warmer climate than Charlottesville, Virginia (IECC climate zone 4).
Like you, though, I also wonder if the house has had those problems. I got divorced and moved out in 2006, so I don’t know what problems the new owner may have had with it. But I know what to tell him if he ever calls to ask for advice.
Allison A. Bailes III, PhD is a speaker, writer, building science consultant, and the founder of Energy Vanguard in Decatur, Georgia. He has a doctorate in physics and is the author of a popular book on building science. He also writes the Energy Vanguard Blog. You can follow him on Twitter at @EnergyVanguard.
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