Interior rigid foam
wjrobinson
| Posted in Energy Efficiency and Durability on
My gut feeling is rigid foam no matter what thickness is prone to less moisture caused problems than exterior. And less costly.
Would like Martin Peter Joe and all to discuss. The rim joist cause for concern to me is not a cause for any concern.
Seems to me a better lesser cost less dangerous assembly whether new construction or a renovation.
I have noticed many contractors in my area are now insulating exteriors but only with an inch or less of for example Dow Styrofoam. Are these homes going to have problems? Seems likely to me. Zone 6a.
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Replies
AJ,
It's possible to do a good job installing interior foam. It's also possible to do a good job installing exterior foam.
On the other hand, it's certainly possible to screw up either approach, and end up with a disaster.
As long as your foam is thick enough, the exterior approach tends to be safer, because it keeps your plywood or OSB wall sheathing warm and dry. However, if you mess up the flashing details, it's always possible to get a water intrusion problem. That likelihood is greatly reduced if you include a ventilated rainscreen gap.
The risk with interior foam is due to the fact that it leaves your exterior sheathing cold and wet during the winter. Again, this risk is greatly reduced by the inclusion of a ventilated rainscreen gap.
The worst scenario with interior foam occurs when the builder does a poor job of air sealing. If air leaks through electrical boxes, you can definitely get moisture accumulation in the cold wall sheathing. The result is mold and rot. So, if you insist on using interior foam, you need to do a really good job of air sealing, especially at electrical boxes, and you need to include a ventilated rainscreen gap to help your damp sheathing dry out every summer.
Martin, your thoughts make sense to me. My purpose for posting this is to try to get a handle on what I have seen in the past versus what we say we should see and taking this discussion forward as to what actually works in regards to what builders are actually doing.
I have seen many builds that use 1/2"-1" of foil faced foam under drywall with no special sealing of the electric boxes and no rainscreening. In fact I have never seen anyone rainscreen until of late one homeowner on his own house who also comes to GBA.
Anyway... the rainscreen idea is smart for interior foam so as to give moisture an exterior path out.
Back to my thinking. I am postulating that keeping the exterior more cold than no interior foam is a good thing just like keeping it more warm with exterior foam is. Also I am thinking that too little exterior foam is the worst case scenario of all. This postulate is based on the idea that rot has a temperature band a wall will be in more if some but not enough foam is on the exterior. And we know from all the existing homes before rigid foam use do not have wall rot issues generally.
Any studies relating? Are their past posts where interior foam under drywall has led to rot in walls or vented cathedral assemblies or vented attics with attic floor insulation?
AJ,
Yes, there are plenty of studies of exterior sheathing temperatures and moisture accumulation. John Straube has done research on this issue in Canada, and the Building Science team has an ongoing study of these issues in Coquitlam, BC (the Coquitlam test huts).
In an upcoming blog, I'll be reporting on two or three current studies of sheathing moisture content on double-stud walls insulated with cellulose.
Your observation that "too little exterior foam is the worst case scenario" is a building science truism -- one that many experts have been preaching for years. (For more on this topic, see Calculating the Minimum Thickness of Rigid Foam Sheathing.)
I have no idea what you mean by "I am postulating that keeping the exterior more cold than no interior foam is a good thing just like keeping it more warm with exterior foam is." In general, warm sheathing is always safer than cold sheathing, because warm sheathing is dryer. That truth is the basis of the PERSIST approach.