Mixed Humid Climate – Closed Cell vs Open Cell Foam


I posed a question this week: In a mixed humid climate, do you use open cell foam exclusively or can you in fact use closed cell in some limited places. In speaking to a very experienced architect on the subject, he hands down believes that closed cell in the answer for the typical wall system design. My problem – I can only find science that refutes his claim. Closed cell foam is a vapor barrier that will not let moisture flow through the wall system. In our climate, where we do have two seasons, we cannot have a vapor barrier, or we will produce moisture issues. The only way to prevent moisture issues is to include a dehumidification system as part of the HVAC design, hope that dew point is not reached in the wall, and cross your fingers. While I do want a dehumidification system in every project, we know they don’t always get installed due to budget cuts. However, the bigger issue is when the building is shut down for a few days, people go on vacation and cut back the thermostat, schools are out for the summer, then the system is not running as designed. When this happens, you are going to have a mold problem. I cannot find any science to back up the claim that closed cell foam can be used here in our climate. If you want to give me some data, I would love to be able to achieve the higher R values that the product offers.

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2 Comments Add yours

  1. Can you expand on this issue a bit, please? My understanding has always been that open cell insulation causes moisture damage in roofs/foundations & that closed cell does not. I thought that, if sweating does occur, closed cell systems keep any moisture to the far outside & far inside of the home where it can evaporate…. however, open cell insulation can hold water inside the insulation & cause rot. Sounds like I may be wrong?

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  2. Hot to cold and cold to hot, temperature and pressure is always trying to find equilibrium. In our climate in the winter warm moist air is pushing from the inside to the outside and in the summer it reverses. So if you use closed cell foam you are creating a vapor barrier that stops the movement through the wall system. This on the surface is a good thing. We don’t want moisture in our wall system so a vapor barrier on both sides would keep the wall dry right? Well, no it does not work that way unless the vapor barrier is on the outside of the wall assembly. So think of this scenario for the summer months, morning dew on the siding, sun comes up, 90 degree 100% humidity day, not a rare scenario here in our climate. The sun pushes the surface moisture into the siding, through the housewrap and into the sheathing. If there is closed cell on the inside of the sheathing, that is where it will stop. So now you need the vapor to dry back to the outside. On a 100% humidity day, this is not going to happen. Maybe it is only 80% humidity, might happen, but not likely. So will it dry at night, only if the humidity gets really low which rarely happens. So now you are forcing moisture into your sheathing on a daily basis. What happens over time, rot or insects get in there and destroy your home. So in the winter months, you are looking at the opposite side of the wall. Moisture goes through any drywall that does not have wallpaper or high gloss paint. Will the warm heat system you use inside the home ever pull moisture back out of the wall system. Not until summer when you turn on the AC or those rare days when you can open up the windows for the dry air – not a common action in the winter. So this will lead to mold on the back side of the drywall and rot on the bottom plate.

    So what about open cell foam. It allows moisture to move through to the side that will allow moisture to dry out in the winter and summer. The vapor is not stopped as open cell lets vapor move through. This eliminates, or at least reduces, the chance that you will trap moisture in your wall assembly.

    As for the roof, all the same issues exists as walls, but this one is easier for me to make the decision. If you have a roof leak with closed cell the water will run to the first hole you have – probably near the outside wall. There is a good chance you will never see this leak as it will probably be outside your thermal envelope. Not a bad thing except your roof will rot away without you knowing it. Open cell foam will allow bulk water to move through it hopefully in an area where you will see the leak and be able to fix it before you have a bigger problem.

    Let me know what you think of my argument. I would love to be able to embrace closed cell for something other than very limited areas where I don’t want any vapor movement like under slab insulation. I just don’t see it as a wise move for high performance houses.

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