I'm building a brick home in a humid climate. I'm confused as to which foam insulation to use for walls/attic: closed or open cell?
I've heard advantages and disadvantages of each but I'm still confused. I want to be as energy efficient as possible.
From an energy efficiency standpoint, they will perform equally well: both are air barriers and both have a decent R-value. The real difference between the two is the amount of moisture the foam allows to pass through. Open cell will let moisture pass fairly easily while closed cell is more of a vapor retardant/barrier. Of course the brand of foam you use will vary slightly on the perm rating, R-value, etc., so you will need to do some research on the different brands.
The main thing to consider when choosing between open and closed cell foam is your climate. You mentioned a humid climate -- is it hot humid or mixed humid? Are you near the coast? Your choice will also depend on your wall design -- what is your exterior sheathing, what type of roofing felt and housewrap are you using? All of these questions can help you determine the best insulation to use.
There is still much debate within the industry about open versus closed cell foam. There have been a few good articles in Fine Homebuilding recently; their website would be a good resource to review. The building code in your area will also dictate a few things. Some areas will require a vapor barrier on the roof if you put foam on the roof deck.
Here are some basic rules of thumb. In a mixed humid climate (where you heat part of the year and cool part of the year, like North Carolina) you will want to use an open cell foam. You want moisture to be able to pass through and you want your wall to be able to dry in both directions. The exception is if you are near the coast -- then you should consider closed cell. There is a lot of wind-driven moisture in the air and you will want to keep it out of the wall. If you are in a hot climate and you cool all year long, then moisture typically only works one way -- from the outside in -- so either foam should be fine and closed may be preferred.
I would ask the installers/salesmen from each of the companies the same questions. Ask them how the wall will dry if moisture gets in and see what they say. I would also consider bringing in a third-party consultant, a building scientist or maybe a HERS rater, to get some advice. It can be a very expensive mistake if a product is used in the wrong climate.
Finally, I would not rule out dense-packed cellulose or fiberglass. At the right density, they too are air barriers, which is the greatest benefit of the foam.
For more information:
You should read Chris Benedict's Q&A "Which foam insulation is best for the south coast Atlantic region: closed cell or open cell?"
The U.S. EPA has recently published comprehensive safety information for installers and building occupants.