There you have it.
Open or closed cell foam for attic.
Others like david butler see his comment below say you need to do it only if a moisture problem develops.
Closed cell foam can also increase the racking strength of attic walls which.
Closed cell foam has a higher r value than open cell foam usually about 6 0 per inch.
This type of spray polyurethane foam can be classified into two different types.
Open cell should be at least 6 inches in depth and closed cell 4 inches or more of each depending on local code.
Open cell foam weighs 1 2 pound per cubic foot and closed cell weighs 2 lbs per cubic foot.
There is 300 difference there but the cost they give you is about a third difference and open cell foam at 12 inches will not give you the performance of 2 inches of closed.
The myth that open cell is more economical is bullshit.
Both low density open cell and medium density closed cell spray foam can be used in an unvented attic to provide the needed thermal resistance and airtightness.
Light density open cell spray foam insulation and medium density closed cell spray foam insulation.
Open cell foam has an r value of about 3 5 to 3 7 per inch while closed cell foam has an r value of about 6 per inch.
Most open cell foam has a density of about 5 pounds per cubic foot.
Lstiburek says to do it for every spray foam attic.
A foams r value is its resistance to heat flow or in other words how well they insulate.
Closed cell foam can be over three times that with a density of 1 75 pounds per cubic foot or more.
Open cell spray foam is a perfectly acceptable insulation product to use in attics.
Do it right and your roof will not rot.
Using spray foam insulation in an unvented attic application.
While the denser closed cell variety does not readily absorb moisture open cell foam can draw it in like a sponge.
Closed cell foam is much denser than open cell foam.