8/14/2023 0 Comments Sip panels for roof![]() Convective loops can cause moisture damage in SIP construction even if the top is airtight. As Semmelhack found out, though, even a few small areas of air leakage can damage the enclosure. Before we connected two panels, we sprayed can foam on both sides to stop air leakage. The weak part was at the connections between panels. When John and I built our SIP houses, the air barrier was the whole panel. The basic principle is to keep humid air away from cool surfaces. 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. You can see one of the moisture-damaged seams on his roof in the photo above. In winter, that put humid air from inside the house in contact with cold surfaces, and you know what happens then: accidental dehumidification. The problem was the concentrated air leakage through seams at the top of the house because of the stack effect. He was almost Passive House tight at a little over 0.6 ACH50. The problem was not the overall airtightness of the whole building enclosure. ![]() 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. Moisture damage to structural insulated panel joint on roof My friend John Semmelhack of The Comfort Squad in Charlottesville, Virginia built a SIP house in 2008. Some SIP houses have required extensive repairs to the sheathing after only a few years because of air leakage. This is especially true at the top of the house because the stack effect will put pressure on any weakness in the air barrier. 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. They have survived hurricanes in neighborhoods where all the other houses were destroyed. SIP homes are generally stronger than many stick-built homes. The exterior OSB can dry only to the outside, so it really should have a gap to aid drying ( i.e., a rainscreen). The interior OSB can dry only to the inside of the house, so don’t put plastic under the drywall. 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. You can use house wrap, fluid-applied membranes, felt, peel-and-stick membranes, or another type of liquid water control layer. 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.Ĭontrolling liquid water is no different than doing so for the other types of structures. Solid insulation embedded in panels means that air sealing should be easier. The advantages of SIPs are reduced thermal bridging and easier control of heat, air, and moisture. You could always put exterior continuous insulation on the outside of the walls or roof to reduce the thermal bridging from the wood splines. ![]() Using structural insulated panels for the roof requires wood splines for structural support, but again, they’re 4 feet apart. 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. ![]() The house I built used solid wood splines between each two panels. Some eliminate all thermal bridging where the panels join by using connecting pieces (called splines) with foam in the middle. First day of installing structural insulated panelsĭifferent companies have different methods for connecting the panels. 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. ![]() 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. Structural insulated panels are sandwiches of oriented strand board (OSB) on each side and expanded polystyrene (EPS) rigid foam in the middle. 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. After weighing all my options, I decided to build with structural insulated panels (SIPs). (I still have it!) Then I decided to build a house. My most complicated project was probably the mahogany lamp table I built in tenth grade shop class. Before 2001, the biggest construction project I had ever taken on was a bookcase. ![]()
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