My contractor is recommending using open Cell insulation against the roof sheathing, then Closed Cell insulation for a vapor barrier from the inside. Has anyone ever heard of this?
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Everything I've seen recommends against open cell in roof rafters. Checkout the Green Building forum and they''ll give good advice on this,
I've read that too but I've never read anything about open Cell directly under the roof followed by closed cell for the vapor barrier. I can't think of a reason why this wouldn't work.
If water or moisture does get through the roof sheathing into the open cell I don't think you want it trapped there which is what the closed cell would do underneath the open cell.
That's totally backward. Open cell is porous and if it gets wet will rot your roof. What else is your contractor totally wrong about?
Seems to me that if the roof sheathing gets wet from above, it wouldn't matter what order the closed cell spray foam goes in. Either way, it can only dry towards the outside. and either way, vapor from below is prevented from contact with the roof sheathing. The open cell foam agaist the sheathing, however, will act like a sponge/ reservoir should there be a roof leak. Either way, no one will know about it until a roofer puts his foot through rotted sheathing. Structurally, I would think that the closed cell against the sheathing/ rafter interface would contribute more resistance to flexural movement than the open cell. I would defer to the spray foam manufacturers recommendations.
You need to use closed cell on the sheathing, but have to make sure you spray at right depth. There are guidelines in building code according to the climate region you live in. In my region it is required to be at a 3” depth. Building Science indicates when dew point occurs within the foam it will not condense. It’s very important that you spray at the right depth. I’ve found the science to be true, but I also inspect my insulators work to make sure they sprayed at right depth. I typically use the flash and batt method. Have used it multiple times and have not seen any issues on any of my projects in some of the worst weather conditions in years. Flash and batt basically means you spray foam at right depth and then pack the rest of the cavity with fiberglass. The only reason to choose foam over other materials is for the moisture and air resistance qualities. Once you achieve that with proper depth all you need to do is bring up R value, might as well use a more affordable material for that (I wouldn’t bother with open cell). Also, never have more than one moisture barrier in wall, so need to make sure it’s unfaced fiberglass. It’s important that the home interior maintains proper moisture levels as well. Want to avoid creating a ton of moisture in the home especially during winter.
I haven't heard of open Cell insulation before.So finally what did you choose? We used to work with MR Roof company and as I remember they also recommended it but we didn't agree