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Open VS Closed Cell New Contruction


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I am sold on foam instulation and intend to use it in my new house. The outside of the house is completed and the wiring and plumbing are finished. I am ready for insulation. I have two estimates and one company has experence over the other and both quotes are in line. I have discussed with contractor I intend to use. He recommends open cell for the attic and if I decide to use closed cell I will be required to provide a vent or sign a wavier. I have ask if the vent is for humidity control or fresh air for humans and will have his answer monday.

The attic is about 1500 sf under a metal roof which has tar paper plus 1/4 in of styrefoam under tin. The attic is a sell contained apartment with its own AC unit not installed yet, new contruction. My intent and goal was to condition this space, incapulate it with foam and condition the air to abour 80 degrees year around until someone stays there. There is also a 300 sq cupola/sun room above the attic with 24 windows which will have foam in roof and walls below the windows. My thoughts is I can always crack a window in the sunroom for any addition fresh air that may be needed.

I live in the N Alabama and we do have hot humid summers (10 100 degree day with 80-90 % RH) and mild winters average 32 * but ocasional 5* for 24-48 hour. The house is 6500 sf so this in the reason to make it as tight as possible but can it be too tight ?

In addition, the downstairs (main floor) wall will be foamed, no vents here either, allot of windows and doors. The house is 54 ft square with wrap around porch which provides allot of shade from windows etc.

Quotes from contractor.

1) "No, you wont receive as much R-value with the open-cell but i

personally do not think that will matter much...R-Value does not apply

to foam the way it applies to fiberglass/cellulose. In the attic with

closed-cell, you are looking at 23R-Value. With open you are looking

at 18R-value...Not a major difference, but to answer your question:

Yes, closed will give more of an R-value."

2) "In my opinion- In your particular attic space- I wouldn't gamble with

closed cell foam. Like you said, you have no vents in the attic! The

closed cell seals so tight, you could potentially have problems in the

future. We are required by the manufacturer to vent in closed-cell

apps. The open-cell is designed for a ventless install. I think this

is more inline with your particular home. As a dealer, i have nothing

to gain either way!! Note that. I just want you to be happy with your

insulation......Meaning, i don't want you to call me out there in a

year and say: "We have problems"!!!! LETS AVOID PROBLEMS!!!"

Actually, he would gain/make more from closed cell by about 2k and cosed cell is less labor for him since he will have to trim open cell to 3 1/2 inch on the main floor application....

Again, open vs closed ??? Humidity, Fresh air, moisture between tin and roof, vent vs ventless ? Thoughts, Issues, References, Thanks, Jackson

I posted a pic for visuals,woops,,Would let me post pic maybe next time ,,Thanks Again for any info or recommendations you may provide.

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It's not just about the choice of open cell or closed cell. Insulation is just one of several components of the wall structure. Each component affects temperature and moisture conditions across the entire cross section of wall. With so many different choices of vapor barriers, insulations, cladding, etc, the equation can become quite complex. An incorrect design in the wrong climate might lead to condensation at a point in it's cross section where it isn't prepared to handle it. Rather than guess at it, look to established designs for your climate. Even better, look to established building methods that have the qualities that you're interested in, before you even begin to spend your money.


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