Flash and batt closed cell with initial flash of open cell
Ok. I’m building an addition. This is in Cape Ann MA. North of Boston. North facing roof 4 pitch that ends in a sidewall. It is a hot roof. it was flashed with 3″ of closed cell foam this past october. The foam failed and is now being removed. It pulled away from the roof sheathing and at the rafter tails. Not everywhere, but there Is no Pattern to where it shrank. Some places it shrank 1/8″ others 3/4″. The installers want to now flash the rafter tails with open cell because they think the sheathing is cold and the open cell will adhere. They will then go over the open cell with closed cell. They just want to do this at the rafter tail and no where else. I attempted air sealing at the outside with caulking and vycor on the sheathing to rafter tails. Is flashing with open cell and then covering with closed cell ok? Just seems weird to me. The owner claimed this is common practice with cold surfaces. Thanks, Nick.
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Replies
Nick,
1. I have never heard of open-cell foam being used for a flash-and-batt job.
2. If the sheathing is too cold, heat the building or wait for the weather to change.
Flashing open cell as a bonding base for closed cell is something new to me too- never heard of it! (But I'm not in that biz.) Most foam vendors have formulations & specs for application at cool temps down to about 25F or so, but it's probably better to take Martin's advice and wait a few weeks for nice spring-thaw day, even though the 80F+ stuff seen last March was probably a fluke, at least a few 50F days in March is all but guaranteed in your neighborhood.
Open cell flash + 3" of closed cell wouldn't have the same structural capacity & stiffening effect that it would otherwise be adding to the roof assembly too, since the open cell layer would be mechanically weaker than closed cell foam at the critical junctures. You don't want to just hang the foam you want it to have a strong mechanical bond that holds up to shear stresses as well.
If the initial closed cell foam was applied in a single 3" lift it likely violated the installation specs of the manufacturer. The typical recommendation is a max of 5cm/2" per pass, with a specified minimum wait time for cooling between passes, but those instructions are all too often ignored. When you go too thick in one shot the risk of shrinkage and bonding failure goes up. If you go 4-5" at a time with closed cell foam it's a fire hazard, since the heat buildup inside the insulating material as it cures exceeding the ignition temperature of the material.
This can happen with open cell foam applied in lifts deeper than the typical recommendation of 5" per pass too:
https://www.greenbuildingadvisor.com/blogs/dept/green-building-news/three-massachusetts-home-fires-linked-spray-foam-installation
That totally sounds like a botched initial job. Too much per
pass like Dana says, and/or wrong A/B component mix to the gun.
Find a different vendor? Or failing that, train this one how
to do their jobs..
_H*
It's hard to tell what caused the failure. My feeling is they switched barrels and something wasn't right, and they were rushing as it was the last room of the day. Everywhere else looks ok. (knock on wood). I haven't given everywhere else a thorough look over yet. anyway, I talked them out of the open cell flash. It didn't really make sense to me to do it in one spot if everywhere failed. They have plenty of time to do a small room tomorrow so hopefully it will go well. In regards to temperature of the substrate getting foam applied to it- It's not exactly warm around here right now. I have two electric heaters that I can run in the room. I also have a propane heater that can be run. Is there an issue with the propane heater because it puts out some moisture. I would actually think the moisture would be a good thing for the foam. Polyurethane glue recommends wetting the surfaces before gluing as does my foam gun with pur1 foam if low humidity. I haven't gotten a straight answer from the installer on that one. Regardless, the warmer the room the better it seems. Any thoughts on the moisture? Many thanks. Nick.
It appears you CC foam was not installed correctly. Your proposed idea of installing OC foam under the sheathing and then CC foam is not code. Read 2009 R806.4. If open cell is to be installed below the sheathing, you most istall closed cell above the roof sheathing. You can install all closed cell foam, but the installation instructions is to be applied 2" at a time, otherwise you could set you house on fire.
They just wanted to spray open cell at the rafter tails. Not the entire rafter length. In any case, that is settled. Any thoughts on moisture from a propane space heater?