Just recently we became aware that some home builders are now field testing the use of foam insulation as an exterior sheathing. Thats right some builders are now replacing the osb and plywood sheathing on homes with foam insulation. It has become standard practice to insulate older homes from the outside with foam insulation but in this case the foam material completely replaces the plywood.
The the potential dangers of such use is fourfold:
First Foam insulation provides no structural advantage.
When installing Plywood or OSB as a sheathing material you must follow proper recommendations as to its orientation on the wall and nail placement. This is because the Plywood provides Lateral Strength that is completely absent in a standard stud wall. Why is this important? Think of your home as a house of cards if a strong wind blows against the walls then the house will collapse. This is also the case in Earthquakes, the potential for damage would be extreme in an earthquake zone if a home has had a large portion of its lateral strength removed just to cut costs.
Secondly Foam provides zero or very little impact protection in storms.
If a 2×4 or a brick from your neighbors house comes lose in a storm and hits your home you can only imagine that it will travel directly through your wall into the interior of your home. How scary is that? Now as we have said wood products do not protect as well as brick or concrete walls but foam gives no protection at all.
Third a home that has foam insulation used as sheathing will deteriorate and require total replacement.
Everyone knows that foams and plastics break down over time just like other products however where sheathing made of wood boards, plywood or even the worst of the worst OSB will break down over many decades based on water contact and other factors the potential for Foam used as sheathing to break down uniformly across the whole structure is extreme. No matter how well you protect the foam it will constantly breakdown at a uniform rate. In addition outside conditions such as heat from sun and cold temperatures will accelerate its breakdown. This means one day you will need to rip off all of the siding on your home including any stone or brick work and replace all of the foam. This expense will be enormous to home owners and will most likely come at a time when they are senior citizens.
Fourth Fire Damage.
Every year homes in California and also around the country are subjected to flash field fires or fires that are close to their structures. If your home is made of wood you may not see instant melting of your interior walls. Unfortunately Foam has both a flash and melting point. This means even if the heat of the fire is not hot enough to cause ignition it could well be high enough to melt your walls into a puddle of goo.
YouRepair.com is not a Testing Lab and we have not hired one to come to these conclusions about the use of Foam as a sheathing product. However we have used common sense and more then a couple years of home building experience to deduce that Foam as a Sheathing Material only makes sense to the manufacturer and the builder so they can cut costs. In residential construction its use should be limited to insulation because the time that we spend in our homes is often consumed most by sleeping and the precautions needed to inhabit a structure of this material require constant monitoring. In other words we will never suggest such use when it could lead to sleeping residents being subjected to potential danger.