Open Cell vs. Closed Cell

[Jerry Simon]

Any disadvantages to using less-costly open cell foam insulation for the attic/roof framing for a home in South Carolina? With the climate in mind, I don't know if the moisture-permeable open cell insulation will promote problems.

Here, moisture migrates from inside-out/warm-cool. There, moisture migrates outside-in. So, even though open cell is moisture-permeable, will it still act as an effective vapor barrier (as I believe more costly closed cell would)?

[Tom Raymond]

You need a minimum depth of 6" to get that kind of vapor performance out of open. That said, I filled 2x8 rafters with open on an addition at my house. 4 winters and no issues.

[palmettoinspect]

Typically it's open cell that's used around here. You do see closed cell used in attics from time to time and it's being used more often, but mostly open cell. I think it's mainly cost. Closed cell works the best to encapsulate and stop air travel though. Many who are using spray foam in the attics are using it to encapsulate the attics more to reduce excessive humidity rather than the insulation value. The spray foam is used along with dehumidifiers to block the ventilation and reduce humidity.

If you have a beach or water front home here it's a must do thing.

Click to Enlarge

43.85 KB

[Jerry Simon]

Typically it's open cell that's used around here. You do see closed cell used in attics from time to time and it's being used more often, but mostly open cell. I think it's mainly cost. Closed cell works the best to encapsulate and stop air travel though. Many who are using spray foam in the attics are using it to encapsulate the attics more to reduce excessive humidity rather than the insulation value. The spray foam is used along with dehumidifiers to block the ventilation and reduce humidity.

If you have a beach or water front home here it's a must do thing.

Click to Enlarge

43.85 KB

Thanks, Kiel. I'm still having trouble wrapping my head around your climate and its effects on a house, especially with the high humidity. Your info helps. (Any more would be appreciated.)

[BADAIR]

http://www.greenbuildingadvisor.com/blo ... -sheathing

http://www.greenbuildingadvisor.com/blo ... -rot-roofs

there's more from Lstiburek @ building science & gba

what i see most problematic w/oc are retrofits that don't take into account all the leaking ceiling penetrations, ducts, hvac lines & equipment installed in attics a typical snafu down here all the way to the gulf coast & south to the border wall

[Jerry Simon]

http://www.greenbuildingadvisor.com/blogs/dept/musings/open-cell-spray-foam-and-damp-roof-sheathing

http://www.greenbuildingadvisor.com/blo ... -rot-roofs

Just what I needed. Mille Grazie!

[palmettoinspect]

Great reads!

In my experience sealing that attic with spray foam alone doesn't help much and doesn't create a conditioned attic space. You must mechanically condition the attic. Whether it's with the heating and air ducts or dehumidifiers. I've seen attics that used both the heating and air and dehumidifiers and one or the other.

The problem with using the heating and air equipment and ducts is, you must install both a supply and returns. Supplies only dumping into the attic only make things worse. There must be a return to extract the moisture. The problem with many of the beach homes that truly need a condition attic is they're second homes and when vacant and not in regular use the AC isn't on and there's no dehumidification happening.

In my experience conditioned attics that are sealed and use dehumidifiers, whether it's spray foam or some other type of material used to block the attic ventilation works the best. The dehumidifier being key. The dehumidifier also needs to be sized properly for the space and in some cases of chopped up attics need multiple units or ducts for the dehumidifiers are needed.

I'm not a huge fan of spray foam, but will agree it's the best way to get a pretty air tight space. Conditioned attic spaces can be archived without using spray foam by just blocking the ventilation and using a dehumidifier.