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Closed Cell Vs Open Cell Foam Insulation


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I have a friend that is considering having a closed cell foam insulation product installed directly against his roof decking in his finished attic. The plan is to then install drywall with a sealer, and 2-coats of low-perm paint as a vapor retarder over this. (He lives in Northern New York)

Some time has passed since this topic was discussed on this forum, and perhaps new experience-based opinions are now available?

What's the current recommendation on this - an open cell product like icynene or a closed cell product? He's being swayed by the higher R-value of the closed cell foam, but I seem to remember cautions against closed cell for this application due to its potential to trap moisture.

I told him to hold off until I was able to post a query on this to this site. I seem to recall that the opinion was that an open-cell foam was recommended due to its diffusion characteristics, but my recall isn't what it used to be.

Any current thoughts and advice on this would be greatly appreciated.


Dave Tontarski

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If he's really hopped to get the R value up around 60 (which I think is right nowadays), he needs to consider composite applications, which essentially negates any open cell option.

Polyiso board covered with closed cell is a good process. Read the Building Science Corp. document on their most recent retrofit.

There would be a lot of variables to consider in any specific application. I'd have to see the property to know for sure what the appropriate approach would be.

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Thanks for directing me to Fine Homebuilding. I subscribed to finehomebuilding.com and searched through their insulation related articles. The article that I found that came closest to the one you referenced is one by Betsy Pettit that was published in April/May of this year.

The article illustrated 3-different roof insulation systems that were utilized while updating 3-older homes. These systems included both open cell and closed cell sprayed foams in combination with XPS foam board, but they never addressed my initial inquiry about closed cell versus open cell and the opinion of many that closed cell foams can trap moisture, thereby making roof leaks and related damage hard to detect.

I'd still be interested and appreciative of any opinions out there for or against one system over the other.

Is the concern over "potential" moisture trapment a legitimate reason not to apply closed cell against the sheathing?

As Betsy is partner (and wife) of Joe Lstiburek, does anyone know if they have discussed this topic at their Building Science Corporation website?


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Hi Don,

Not to be a prig; but trying to explain all of that in a post in here would require a treatise that would rival one of my own long-winded tomes.

I just went to buildingscience.com, clicked on "information" and then searched for "closed-cell versus open-cell foam insulation" and got one document back. It was a profile done for a home in a very cold climate - your's qualifies - from there, every section of the diagram in question has extensive links to it that have links to those. If you follow those links as I did, in about five minutes you'll find what you're looking for.

I guess I could post the last link here but if I did that you'd completely bypass some good relavent information that, working in the cold northeast, you're gonna need.

Happy Hunting!



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Yeah, that's the article. As O' said, there's so much to consider, I've held off from even starting to write about it.

Honestly, if the roof is sound and functional, the entrapped water argument never made much sense to me. It's not hard to install a roof so it doesn't leak. If you're working from that starting point, I think either foam is worthwhile and satisfactory.

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Couple thoughts about how it (composite insulation) might work......

The largest issue is finding folks that are competent installers. There is no industry training for this. Zero. If you're doing a lot yourself, it's relatively simple jig saw cutting and fitting. If you're trying to find folks to do it for you, you will spend more time trying to find them and explain what you want than simply doing the job.

The spray foam guys are a mixed bag of competencies. There's not enough folks doing this (basically, no one, it's a Joe Only thing so far) so we know little about how the real time application plays out economically and logistically on a large scale.

I'm a total believer though. It's the future, hopefully.

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Thanks again for the recommended resources to delve into this further. Simply googling this topic I found lots of (theory-based) opinions against closed-cell for this application, but no facts to back these opinions up.

Per my original inquiry...I was looking for "experience-based opinions" - I thought that maybe someone participating in this forum may have actually seen problems with one or the other during a home inspection. I wasn't looking to skirt doing my own investigation, just looking to take advantage of the collective experience of the inspectors participating on this forum.

I've spent considerable time at both the buildingscience.com site and the finehomebuilding.com site since my initial inquiry. I highly recommend both of these sites for anyone looking to broaden their knowledge.

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  • 2 months later...
  • 1 year later...

I live in Bluffton, SC and it get extremely hot in the attic during the summer (it hit 152 on my min/max thermometer last summer). Our air handlers are in the attic, so needless to say we get significant blasts of hot air coming out of the ductwork during the summer, and very cold air coming out in the winter. There is 10 inches of insulation on the attic floor but no insulation anywhere else (ductwork is flexible - r6).

I have been looking at installing foam insulation to condition the attic so that I don't get the extreme temperatures coming through the ductwork. I have talked to a number of foam insulation installers in the area and can't seem to get a straight answer as to which way to go; open-cell or closed-cell.

It gets extremely humid here during the summer. If we use closed-cell, where does the humidity in the attic already there go? Several of the contractors suggested using closed-cell because of the better r-rating. I also spoke to someone who pointed out that in the mornings when the roof tiles are moist from the nightly dew, vapor pressure from the sun on the moisture could push the moisture through the paper and roof into the attic. If we used closed-cell, where would the moisture go except against the roof. Could this cause the plywood to rot over time?

If we used closed-cell, and sealed the ridge vent, how would the attic breathe?

If we use open-cell is there a greater risk of mold and mildew during the summer months?

I know it's a lot of questions, but I can't seem to get a straight answer from the contractors I have spoken to.

Any help would be gratefully appreciated...


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I am not going to comment on closed versus open cell foam but I would like to comment on using foam insulation to retrofit a home. When insulation is the only factor you consider during your retrofit you are forgetting that the house is a system. If you change one thing it has an impact somewhere else. For example if you foam the rafters and do nothing to the central air conditioning unit (which is probably oversized in the first place) you can run the risk of not getting the humidity/moisture out of your house. While I like foaming the rafters in new construction where the A/C system is sized properly I would be wary of during only one thing and not the other.

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Thank you for commenting. Your point is well taken. When we were building the house, I contracted with an engineering firm to size the heat pump system that was installed. So I am reasonably certain that it is sized properly for the size home we have. My intention was not to condition the space for use. Rather, to eliminate the extremes in temperature we have experienced coming out of our ductwork for the first minute or two until the heatpump has a chance to do its work.

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South Carolina isn't all that hot. You should not be having a significant problem with short periods of hot air coming from the ceiling registers. Perhaps your attic doesn't have sufficient ventilation.

You might also install a radiant barrier on the underside of the rafters. It's quite effective and not expensive.


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Thank you for your comments. I had a fast-read thermometer attached to the register in my office last summer and in the afternoon, the temperature of the air coming into my office was initially at 136 degrees for the first few seconds. It took over a minute for the air to drop below a 100 degrees.

One of the first things I checked when i moved into my house was to make sure that the soffit vents were unobstructed by insulation (and they weren't). We have a ridge vent, so I would have thought that we would have had proper ventilation.

One of the complicating factors in the heat accumulation is that we have a very steep roofline facing south.

I readily concede that we are not as hot as other parts of the south. All I am trying to do with the foam is to lower the temperature differential between the attic and the rest of the house.


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  • 5 months later...

Well, I live in Bluffton, Work for a Commercial Roofing Company.

My Experience indicates that it is as hot in Bluffton as Orlando, Fl.

I have encountered the same problems that you are speaking of.

Having a roof vent and having a working roof vent is two different worlds. My attic was 145 when I moved in, it is now 110-105 most summer days. A huge difference just by ripping off the "ridgeVent" and replacing with a proper system.

Over ninety percent of the roofing companies down here, have very little experience in roofing let alone venting systems. The actual possession of a hammer and nails seems to qualify people to be contractors in this area. You have to do your homework before, you hire a contractor.

The shingle industry has been around for well over 100 years and it is pretty simple for a Qualified Contractor to follow their instructions in placing a roof system on you home.

A thirty year roof will last 30 years if done according to their instructions and it will vent properly.

We do it every day, however, not at the lowest bid, which is how the vast majority of General Contractors build homes today.

We are qualified to do any roof system, we have over 35 years experience in Roofing. So should your roofer.


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