Thermal expansion of exterior rigid foam
CaseyJahner
| Posted in General Questions on
I’m looking at build a new house with 2×6 FG and around R20 exterior insulation. I’m debating between EPS,XPS, mineral wool. I have concerns over the thermal expansion of the foam. In my current house I have 6″ XPS and osb over an unvented cathedral ceiling , when the sun heats the roof up it makes a popping sounds. I’m scared a whole house covered in 4″ of XPS would do the same.
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Replies
User-6869481,
First of all, can you tell us your name? (I'm Martin.)
Does your house have metal roofing? I'm guessing that it does. If my guess is correct, that's the source of the popping sounds -- it's the metal roofing expanding.
Exterior rigid foam does not contribute to popping sounds related to thermal changes.
There is some limited evidence that older versions of XPS and polyiso had problems with shrinkage -- but this wasn't thermal contraction, it was a change related to aging. (For more information on this issue, see the end of the following article: "Using Rigid Foam As a Water-Resistive Barrier." (Scroll down until you see the subhead "Do rigid foam panels shrink?")
Green builders try to avoid the use of XPS. Either EPS or polyiso is preferable to XPS from an environmental perspective. For more information on this issue, see "Choosing Rigid Foam."
Thanks Martin, My name is Casey. My current roof has plywood, 6” of XPS, 7/16” osb, ice and water, and asphalt shingles.
The reason I was leaning towards xps on my walls is just based on price. In my area (north dakota) mineral wool is $3000 more to do the exterior.
One other question, my garage will be three stalls and double depth that has room for a workshop. Do you think it is worth it to do r35 exterior mineral wool walls in a space that will leak so much air due to the garage doors and also will loose so much heat due to driving vehicles in? I plan to use hydronic radiant heat.
Casey - did you ever figure out the reason for your roof popping? I have the same type roof and same maddening popping/creaking sounds...
>>>6″ XPS and osb over an unvented cathedral ceiling , when the sun heats the roof up it makes a popping sounds.<<<
I have had 2 houses with thick T&G and ISO foam, and they both did that
I don't think a wall will because of the shorter span
Hi Keith, if you are still active on this forum and receive this, can you email me at [email protected]? I have same issue with my house (polyiso+T&G causing a lot of popping and creaking noise). Have posted about it on the forum in last month or so. Still looking for answers.
Hi Casey -
Everything contracts and expands with changes in temperature and moisture content. I have done building investigations on about a half dozen SIP nailbase-wrapped homes (walls and roofs) and in all these cases after about 20 years or so, the adhesive bond of the materials at the nailbase panel joints cracks and then warm moist air travels up the cracks, condenses, and moisture problems arise/become noticeable. Most often the panel joint detail involves spray foam of some sort, and while it has elasticity, it apparently eventually, gives.
So, I strongly recommend that whatever panel material and joint detail you choose make sure that you back up the actual butt joint configuration with an at least 6-inch wide high performance PSA tape over the joint which WILL maintain the air seal over time and lots of contraction and expansion.
Peter
Hi Casey, Sadly, through a project of my own I can confirm that this indeed quite an issue. Sheets of XPS against XPS create quite a lot of noise during times of significant heating and cooling. One thing I did on a later part of my project was to apply vertically striped beads of Foamboard adhesive between the layers which seems to have almost completely solved the problem. I wish I hadn't had to learn this myself on my own home and am quite surprised that there isn't more discussion on the topic. I would recommend a different form of rigid insulation than XPS for this reason alone. I'm curious, did you proceed with your project? If so, what materials did you choose, and how did it turn out? - Jared
Went in my attic today to inspect, what I’ve assumed is, some wildlife in my walls. I noticed that a wall, which is covered with 2” of XPS (Foamular), was making lots of popping noises. When I pressed on a board and let go, I could hear it move/pop back into place slowly, and it sounded the same as the other pops. My house is wrapped with .5” of XPS (Styrofoam). Wondering now if the exterior insulation is the reason for the noises in my walls.